Perl Primer
Python Primer
Patrice Koehl
Modified by Xin Liu in Apr., 2011
Department of Computer Sciences, University of California,
Davis.
Acknowledgments:
This primer is mostly a compilation of information found in
books / web resources that I highly recommend:
· http://docs.python.org/reference/introduction.html ; this is a
reference manual, and not a tutorial, but provides invaluable
information about the language: do not hesitate to consult it!
· http://wiki.python.org/moin/BeginnersGuide ; a Beginner’s
guide with many links to resources for writing and running Python
programs
· Michael Dawson, “Python programming for the absolute
beginner”, 3rd edition, Thomson Course Technology, ISBN:
978-1435455009; this is the class textbook; a great resource book
on Python
Introduction
1. Why Python?
“Python” is an interpreted computer language developed in the
1980s and first released in 1991. Its design philosophy emphasizes
programmer productivity and code readability.
It is important to understand that there is always more than one
way to solve a problem. In programming, Python focuses on getting
the job done. One Python program may be faster than another, or
more concise, or easier to understand, but if both do the same
things, there won’t be a judgment that defines which one is
“better”. This also means that you do not need to know every detail
about the language to do what you want with it.
Python has strength that makes it an ideal language to learn and
use:
· It is completely free, and available on all operating
systems
· It is very easy to learn
· Python was designed to be easy for humans to write, rather
than easy for computers to understand. Python syntax is more like
English than many other programming languages
· Python “talks” texts. It works with words and sentences,
instead of characters. Files are series of lines, instead of
individual bytes.
· Python is very portable. Python programs can be run on any
computers, as long as Python is installed on it.
· Python is a “high-level” language: you do not have to worry
about the computer’s operation (such as allocation and
de-allocation of memory, …).
It is only fair to mention that these strengths can also
translate into weaknesses:
· Python takes care for you of all “low-level” operations: this
may not always lead to efficient code
· Python is interpreted, and loses the efficiency of compiled
languages.
· Python users then to write programs for small, specific jobs.
These programs are usually for the programmer’s eye only, and as
such are often incomprehensible to everyone but the original
programmer. In that respect, I can only emphasize the need for
clarity, as well as for useful comments in your source files!
· Python was designed to be easy for humans. As a consequence,
it is relatively lenient on the style you use. This can lead to bad
programming habits. As an analogy, think of what would happen to
your English writing style if nobody had ever cared about how you
write as long as they understand what you have written. To avoid
this, the key is to develop first a method to solve your problem
that is independent of Python (or any other language), and then to
adapt this method to Python.
2. What is Python used for?
Python has been successfully implemented in many software
applications as a scripting language.
Python is a very useful programming language for web
applications.
Python is used widely for game development, for 3D animation
packages, in the information security industry,…
3. How do I get Python?
Python has been ported to many platforms, and will certainly run
on the standard operating systems such as UNIX, Linux, Solaris,
FreeBSD, all flavors of Windows, and Apple MacOS.
Python 2 versus Python 3
In December 2008, the Python consortium released a completely
new version of Python, Python 3.0, that is not backward compatible:
this means that programs written with Python 1 or Python 2 may not
run under Python 3.0. At this stage, we move to Python 3.1.3. If
you have Python 2 code, a tool, 2to3, will help you convert most of
Python 2.x code to Python 3.x code by handling most of the
incompatibilities.
Where to get Python:
· You can get the source to the latest stable release of Python
from
http://www.python.org. Remember that you want Python 3.
· Binary distributions for some ports are available at the same
address
· You can get binary packages of Python for Linux, Solaris, Mac
OS and Windows from ActiveState at
http://www.activestate.com/ActivePython (free for download)
Installing Python on Linux/UNIX
Python is freely available, and usually comes packaged with most
Linux/UNIX distribution. Type python from a shell prompt to check
this. If you see something that starts with the text Python, then
you already have python. If you don’t, check the install media (CD
or DVD from which you installed Linux): the Python package should
be available on it. Otherwise, get the binaries from the site
mentioned above.
Installing Python on Mac Os
Again, Python comes packaged with the different flavors of Mac
OS X, and you probably have nothing to do! Check it using python
from a terminal. If you do not have it, I would advise getting it
from ActiveState.
Installing Python on Windows
Installing ActivePython is quite straightforward. Download
ActivePython’s Python installer for Windows from the activestate
web site (again, it is free for download). Choose the appropriate
version for the operating system you have (32 bit, or 64 bit). I
would strongly advise using the MSI installer, in which case you
will need Windows Installer 2.0+ (which you probably already use).
It should work under Vista, but I have not tried it.
4. Getting an IDE for Python
IDEs (integrated Development Environment) are great tools for
learning a computer language and use it efficiently. There are many
IDEs available for Python: see
http://wiki.python.org/moin/IntegratedDevelopmentEnvironments for a
list of such IDEs.
I strongly recommend IDLE that is available for nearly all
platforms. See http://www.python.org/idle/doc/idlemain.html ; it
should come by default when you install Python on Windows; you have
to install it however on Linux and MacOS platforms. For the latter,
please check: http://challenge.ncss.edu.au/gsg-osx/ .
5. Using Python
You have two main ways to use Python:
· Use it directly through a Control Window: again, I strongly
advise using the IDLE interface
· Write the Python program (module) using a text editor, and
then execute this program through the python interpreter. Again,
this can be done using IDLE; alternatively, you can use standard
text editors for this task (see below)
Editing a Python program
Python source code is just plain test and should be written with
a plain text editor, rather than a word processor. If you are using
Windows, you can use Notepad, despite its annoying tendency to
rename file extension to .txt. You may also use Word, as long as
you save the file as text, with line breaks.
I would really recommend getting a good programmer’s editor. For
Windows and Mac, I can recommend jEdit (http://www.jedit.org/ ): it
is free (open source), runs under Windows, Mac OS X, Unix and
Linux. It is easy to use, highly customizable, with many useful
plugins. Another option for Mac user is TextWrangler
(http://www.barebones.com/products/textwrangler/ ).
Naming a Python program
Traditionally, UNIX programs take no extension, while Windows
files take a three-letter extension to indicate their type (.exe
for an executable, .doc for a document –usually Word file-, .xls
for a spreadsheet, …); the standard extension for Python program is
py.
Obviously, the choice of the name in front of the extension is
entirely yours!
Using Python in an IDE
If you are mainly using your computer in a graphical environment
like Windows or X, you may not be familiar with using the command
line interface, or “shell”. The “shell” is the program that gets
input from you through the keyboard. The “shell prompt” or just
“prompt” refers to the text that prompts you to enter a command.
The standard prompt in IDLE is:
>>>
i.e. 3 chevrons.
In this primer, I will use a prompt that looks like:
>>>
I will show the text that you would type in bold and the text
the computer generates in italic:
>>> print (“Hello world!”)
Hello World!
6. Your first Python program
Traditionally, the first program anyone writes in a new language
is called “Hello World!”, where you make the program prints that
statement. Python allows us to do so using the print statement. The
simplest form of the print statement takes a single argument and
writes it to the standard output, i.e. the command window you have
open. So your program consists of the single statement:
print( “Hello World!”)
You can execute this command directly in the IDLE main window,
or you can incorporate it into a Python module, hello.py. The file
hello.py contains:
Scalar variable:
One storage box
Array:
Ņ
chest of drawers
Ó
1
2
3
4
0
The different elements of a Python script:
· Documenting the program: any line (except the first) starting
with a sharp (#) is treated as a command line and ignored. This
allows you to provide comments on what your program is doing: this
is extremely useful, so use it! More generally, a line in a Python
script may contain some Python code, and be followed by a comment.
This means that we can document the program “inline”.
· Keywords: Instructions that Python recognizes and understands.
The word print in the program above is one example. There are two
types of keywords:
· functions (such as the print keyword); these are the verbs of
the programming language and they tell python what to do.
· Control keywords, such as if and else.
The number of Python keywords is small:
and
del
from
not
while
as
elif
global or
with
assert
else
if
pass
yield
break
except
import
print
class
exec
in raise
continuefinally
is
return
def
for
lambda
try
It is a good idea to respect keywords, and not use them as names
in your programs!
· Modules: Pythons come with a large list of modules that
increases its functionality; these modules add keywords to the
small list provided above, but are only available when the module
has been specifically called. For example, adding:
use numpy
adds the modules of numerical functions “numpy” that are now
accessible to the programmer.
· Statements: Statements are the sentences of the program.
Python is lenient however, and does not need a full stop to end a
statement. The indentation levels of consecutive lines are used to
generate INDENT and DEDENT, which in turn are used to determine the
grouping of statements.
· White space: White space is the name given to tabs, spaces,
and new lines. Python is quite strict about where you put white
space in your program. For example, we have seen that we use
indentation to help show the block structure of statements.
· Escape sequences: Python provides a mechanism called “escape
sequences” to output special characters/actions: the sequence \n in
the program above tells Python to start a new line. Here is a list
of the more common escape sequences (also called
“metacharacters”):
Escape Sequence
Meaning
\t
Tab
\n
Start a new line
\r
Carriage return
\’
Single quote
\”
Double quote
\\
Backslash
\b
Back up one character (‘backspace’)
\a
Alarm (rings the system bell)
Simple exercises:
1) Write a program printline.py, that prints the sentence “This
is my second program”:
a. As a single line
b. With a single word on each line.
2) Find an online manual for Python
3) Which of the following statements are likely to cause
problems:
a. print (“This is a valid statement\n”)
b. print (“This is a valid statement”\n)
c. print (“This is a ”valid” statement”)
d. printx (“This is a valid statement\n”)
Chapter 1: Scalar Variables and Data Types
1. Python as a calculator
The Python interpreter acts as a simple calculator: you can type
an expression at it and it will write the value. Expression syntax
is straightforward: the operators +, -, * and / work just like on
your regular calculator; parentheses can be used for grouping. For
example:
Key
Value
Remember that, by default, Python only has a limited set of
keywords. For example, it only knows how to do the basic
mathematical operations (+,-,/,x). If you want a more scientific
calculator, you need to first import the math functions included in
the module “math”:
Block of instructions 1
Block of instructions 2
Block of instructions 3
Condition
True
False
For each item
Block of instructions
Sequential
Conditional
Repeating
instructions 1
instructions
2
2. Python Variables
A variable is a name reference to a memory location. Variables
provide an easy handle to keep track of data stored in memory. Most
often, we do not know the exact value of what is in a particular
memory location; rather we know the type of data that is stored
there.
Python has three main types of variables:
· Scalar variables hold the basic building blocks of data:
numbers, and characters.
· Array variables hold lists referenced by numbers (indices)
· Dictionary variables hold lists references by labels.
The name of a variable can be practically any combination of
characters and of arbitrary length. Note that the type of a
variable cannot usually not be guessed from its name: I strongly
advise you to choose a name for a variable that makes this type
explicit. For example you can use names like X, X_list, X_dic to
define a scalar, a list, and a dictionary, respectively.
There are a few rules regarding variable names that you need to
be aware of:
· The first character of the name of a variable cannot be a
digit
· Spaces are one type of characters that are not allowed: use
underscore instead.
· Variables are case sensitive: this means that abc refers to a
different location in memory than ABC.
Creating a variable is as simple as making up a variable name
and assigning a value to it.
Assigning a value to a variable is easy: all you have to do is
write an equation, with the variable name on the left, an = sign,
and the value on the left. The = sign is called the assignment
operator:
3. Special variable
Python has one special variable, _, that points to the place in
memory that stores the more recent result:
This special variable “_” should be considered as “read-only”,
i.e. I strongly advise against assigning a value to it!!
4. Scalar variables
Python has two types of scalar values: numbers and strings. Both
types ca be assigned to a scalar variable.
4.1 Numbers
Numbers are specified in any of the common integer or floating
point format:
Numbers can also be represented using binary or hexadecimal
notations, but we will not need that.
Table of the most common number operators in Python:
Operator
Meaning
=
Assign
+
Add
-
Subtract
*
Multiply
/
//
Divide
Integer divide
**
Exponentiation
%
Modulus
abs(x)
Absolute value of x
int(x)
x converted to integer
float(x)
x converted to float
+=
Assign add
-=
Assign subtract
*=
Assign multiply
/=
Assign divide
Python allows us to use all standard arithmetic operators on
numbers, plus a few others. The mathematical operations are
performed in the standard order of precedence: power comes first,
then multiplication has a higher precedence than addition and
subtraction: 2+3*4 is equal to 14, and not 20. If we want the
multiplication to be performed on 2+3, we need to include
parenthesis: (2+3)*4. These are exactly the rules used by
Python.
Some of the operators listed in the table above are unusual, and
require more explanation:
The modulo operator:
In the example given above, the variable k holds the remainder
of the division of 52 by 3, i.e. 1.
Operating and assigning at once:
Operations that fetch a value from memory, modify it and store
it back in memory are very common: Python has introduced a special
syntax for those. Generally:
i = i b;
can be written as:
i = b;
Let us see an example:
In this example, “a” takes successively the values 20, 24 and
8.
This works for +=, -=, *=, /=, **= and %=.
4.2 Strings
A string is a group of characters attached together, enclosed by
quotation marks. For now, we will only consider double quotes.
Just like with numbers, many operations can be performed on
strings: the most common ones are listed in the table below.
String operator
Meaning
a+b
Concatenates strings a and b
a*i
Repeats string a i times
a[i:j:k]
Returns a string containing all characters of a between position
i and j, with step k; if k is negative, starts from the right
a[::-1]
Returns a string that is the reverse of a
a.split(sep)
Split string a into a list, using sep to decide where to cut
a.strip()
Returns a string equal to a, but that has been stripped of any
“white” characters at the beginning and end of a (space, tab,
CR,…)
a.upper()
Returns a string equal to a, but with all letters uppercase
a.lower()
Returns a string equal to a, but with all letters lowercase
a.capitalize()
Returns a string equal to a, but with the first word
capitalized
a.count(‘sub’)
Counts the number of instances of the substring ‘sub’ in the
string a
a.replace(‘sub1’,’sub2’,n)
Returns a string equal to a, but with n instances of substring
sub1 replaced with substring sub2; if n is not given, all instances
are returned
Concatenating strings:
The + operator, when placed between two strings, creates a
single string that concatenates the two original strings. In the
following example:
The variable C contains the string “ATTGCGGCCT”. Note that the
concatenation operator can be attached to an assignment:
C+=”G”;
Adds a “G” at the end of the string contained in C.
Repeating a string
The operator “*” repeats a string a given number of times:
Indexing and slicing strings
Characters within a string can be accessed both front and
backwards. Frontways, a string starts at position 0 and the
character desired is found via an offset value:
String[i] is the character at position i (starting from 0) from
the left side of the string.
You can also find the same character by using a negative offset
value from the end of the string:
String[-i] is the character at position i from the right side of
the string.
Slicing is a very useful and heavily used function in Python: it
allows you to extract specific substrings of a string. The syntax
for slicing is:
b = S[i:j:k]
b collects characters between positions i and j (j not
included), starting at I, every k characters.
Note that you do not have to specify i, j and/or k:
· if you do not specify i, you start at the first character of
the string
· if you do not specify j, you go up to the last character of
the string
· if you do not specify k, it is set by default to 1
Note also that k can be negative, in which case you start from
the right end of the string. For example,
b = S[::-1]
reverses the string S and stores it in b.
Examples:
The other string manipulations described below apply a function
on the string. The syntax is:
string.function(argument)
where string is the string considered, function is the function
applied, and argument are parameters for the function, if any.
Breaking a string into a list
A string can be broken down into a list using the function
split. The syntax is:
A.split(sep)
where A is the string, and sep the separator. If sep is not
provided, Python uses the white space.
Examples:
Striping a string
A string may have leading or lagging white characters, such as
blanks, tabs, or carriage return. It is a good idea to remove
those, using the function strip().
Changing case
· Setting the whole string as upper case: apply function
upper()
· Setting the whole string as lower case: apply function
lower()
· Capitalizing the string:
apply function capitalize()
Counting occurrence of substrings
Count is a function that finds and counts the number of
occurrence of a substring in a string:
Replace
Replace is a function that substitutes a string for another:
String.replace(‘sub1’,’sub2’,n)
String is the string on which replace is applied; n instances of
‘sub1’ are replaced with ‘sub2’; if n is not provided, all
instances of ‘sub1’ are replaced.
5. Input data in a Python program
Often when we write a Python script, we need to be able to ask
the user for additional data when he/she runs the program. This is
done using the function input (raw_input() in Python 2.x is
replaced with input() in Python 3:
answer = input(“Question :”)
where:
· “Question” is the string printed on the screen to let the user
know what he/she needs to input
· answer is a string that stores the answer of the user.
Note that the result of input is always a string. If you expect
an integer or a float from the user, you need to change the
type:
age is now an integer that contains the age entered by the
user.
Exercises:
1. Without the aid of a computer, work out the order in which
each of the following expressions would be computed and their
value.
i. 2 + 6/4-3*5+1
ii. 17 + -3**3/2
iii. 26+3**4*2
iv. 2*2**2+2
Verify your answer using Python.
2. Without the aid of a computer, work out these successive
expressions and give the values of a, b, c and d upon completion.
Then check your answer using a Python script:
3. Write a Python program that:
i. Reads a sentence from standard input
ii. Writes this sentence on standard output all in lower
case
iii. Writes this sentence on standard output with all vowels in
upper case and all consonants in lower case
iv. Writes the sentence in reverse order
4. Write a Python program that:
i. Reads a sentence from standard input
ii. Counts the number of words and the number of characters, not
included space
iii. Counts the number of vowels.
5. Write a Python program that reads from standard input the
amount of a restaurant bill and outputs two options for tipping,
one based on 15% of the bill, the other based on 20% of the
bill.
6. Write a Python program that:
i. Reads a sentence
ii. Remove all vowels
iii. Replaces all v and b in the original sentence with b and v,
respectively (i.e. for example string ‘cvvbt’ becomes ‘cbbvt’
iv. Count number of letters in the modified sentence
v. Writes the resulting sentence and number of letters on
standard output
Chapter 2: Lists, Arrays and Dictionaries
6. Higher order organization of data
In the previous chapter, we have seen the concept of scalar
variables that define memory space in which we store a scalar, i.e.
single numbers or strings. Scalar values however are usually
insufficient to deal with current data. Imagine writing a program
that analyzes the whole human genome. Since it contains approx.
30,000 genes, we would have to create 30,000 variables to store
their sequences! Initializing these variables would already be a
daunting task, but imagine that you wanted to count the number of
times the sequence ATG appears in each gene, you would have to
write one line of code for each gene, hence 30,000 lines, changing
only the variable name for the gene!
Fortunately, Python thinks that laziness is a virtue, and would
never tolerate that you have to write 30,000 lines of code. Two
special types of variables exist to help managing long lists of
items, namely arrays and dictionaries. These variables store lists
of data, and each piece of data is referred to as an element. In
this chapter, we will look in details at what are lists, and how
they are stored and manipulated within arrays and dictionaries.
We are all familiar with lists: think about a shopping list, a
soccer team roster, all integers between 1 and 10, genes in the
human genomes,…A list can be ordered (increasing or decreasing
values for numbers, lexicographic order for strings), or unordered.
Python has two types of lists, tuples and lists. Tuples are
immutable, i.e. they cannot be modified once created, while lists
are mutable, i.e. they can be modified once created.
7. Tuples
2.1 Tuples in Python
By definition, a tuple is a set of comma-separated values
enclosed in parentheses.
Examples:
· (1,2,3,4,5,6,7,8,9,10) is the tuple of integers between 1 and
10
·
(‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’,’Sunday’)
is the tuple containing the days in the week.
2.2 Accessing tuple values
We have seen how to build a tuple. Another thing that is useful
is to be able to access a specific element or set of elements from
a tuple. The way to do this is to place the number or numbers of
the element (s) we want in square brackets after the tuple. Let us
look at an example:
Which day do you think will be printed? If you try it, you will
get:
‘Wednesday’
Why didn’t we get “Tuesday”, the second element of the list?
This is because Python starts counting from 0 and not 1!! This is
something important to remember.
The element you want does not have to be literal: it can be a
variable as well. As an exercise, write a small program that reads
in a number between 1 and 7, and outputs the corresponding day of
the week. The answer is on the next page.
The examples above show you how to single out one element of a
tuple; if you wanted more than one element, you can “splice” the
tuple, the same way we splice strings. For example,
will print
(‘Monday’,’Tuesday’)
Remember that the range i:j means from position i to position j,
j not included.
8. Lists
A list in Python is created by enclosing its elements in
brackets:
Elements in a list are accessed the same way elements are
accessed in tuples.
Special lists: ranges
Often the lists we use have a simple structure: the numbers from
0 to 9, or the numbers from 10 to 20. We do not need to write these
lists explicitly: Python has the option to specify a range of
numbers. The two examples cited would be written in Python as:
Note that lists (and tuples) in Python can be mixed: you can
include strings, numbers, scalar variables and even lists in a
single list!
9. Arrays
There is not much we can do with lists and tuples, except print
them. Even when you print them, the statements can become
cumbersome. Also, there is no way to manipulate directly a list: if
we wanted to create a new list from an existing list by removing
its last element, we could not. The solution offered by Python is
to store lists and tuples into arrays.
9.1 Assigning arrays
Names for arrays follow the same rules as those defined for
scalar variables. We store a list into an array the same way we
store a scalar into a scalar variable, by assigning it with =:
for a tuple, or
for a list.
Note: the name of an array does not indicate if it contains a
list or a tuple: try to use names that are explicit enough that
there are no ambiguities.
Once we have assigned a list to an array, we can use it where we
would use a list. For example,
will print:
[‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’,’Sunday’]
9.2 Accessing one element in an array
We can access one element in an array by using the index of the
drawers it has been assigned to.
Remember how we access an element in a list:
[‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’,’Sunday’][0]
gives us the first element in the list, i.e. ‘Monday’. We could
do:
days[0];
to access the first element of the array days.
Accessing an element in an array works both ways: we can either
retrieve the value contained in the position considered, or assign
a value to that position. For example,
Creates an array names numbers that contains the list [0,1,5].
This list can then be modified:
The array numbers now contains the list [3,4,5].
Important: you can change elements in a list, but you will get
an error message if you try the same thing in a tuple.
9.3 Array manipulation
Python provides a list of functions that manipulates list. Let A
be a list:
Type
Notation
Function
Adding values
A.append(obj)
Adds obj at the end of list A
A.extend(list)
Adds list at the end of list A
A.insert(index,item)
Adds item at position index in A, and move the remaining items
to the right
Remove values
del A[i]
Removes element at position i in the list A
Item=A.pop(index)
Removes object at position index in A, and stores it in variable
item
A.remove(item)
Search for item in A, and remove first instance
Reverse
A.reverse()
Reverses list A
Sorting
A.sort()
Sorts list A in place, in increasing order
Searching
I=A.index(item)
Search for item in list A, and puts index of first occurrence in
i
Counting
N=A.count(item)
Counts the number of occurrence of item in A
Length
N=len(A)
Finds number of items in A, and stores in N
Important again: you can manipulate elements in a list, but you
will get an error message if you try the same thing in a tuple.
9.4 From arrays to string and back.
join: from array to string:
You can concatenate all elements of a list into a single string
using the operator join. The syntax is:
It concatenates all elements of LIST and stores them in the
string A. Not the presence of ‘’ before join.
Split and list: from string to array
We have seen in chapter 1 that the function split will break
down a string into a list, using a specified delimiter to mark the
different elements. If the delimiter is omitted, split separates
each word in the string. For example, if A=”This is a test”,
A.split() will generate the list [“This”,”is”,”a”,”test”]. Split
however cannot break down a word into characters: to do that, we
use the function list.
For example, if we want to break a paragraph into sentences, a
sentence into words, and a word into characters, we use:
In all three cases, the result is a list and the input was a
string.
10. Dictionary
5.1 Definition
Arrays are very good for maintaining and manipulating lists.
They have one limitations however: each element in an ARRAY is
indexed with an integer value, varying from 0 to its last index,
len(ARRAY). You can think about it as the array representing a
street of houses, with each house defined by its number. There are
instances however in which it would be more convenient to refer to
the different houses using the name of its inhabitants: this is
exactly what a dictionary variable does (see figure 2.2).
Dictionaries are also referred to as associative arrays or
hashes.
5.2 Assigning values to dictionaries
A dictionary is a set of pairs of value, with each pair
containing a key and an item. Dictionaries are enclosed in curly
brackets. For example:
Creates a dictionary of countries with their capitals, where the
capitals serve as keys.
Note that keys must be unique. If you try to add a new entry
with the same key as an existing entry, the old one will be
overwritten. Dictionary items on the other hand need not be
unique.
Dictionaries can also be created by zipping two tuples:
5.2 Accessing elements in dictionaries
This is similar to looking inside an array, except that the
positions in the dictionaries are indexed by their keys, and not by
an integer index. Using the dictionary d defined above,
If you give a key however that is not in the dictionary, Python
will output an error message.
To circumvent this problem, it is often better to use the
function get: if key does not exist, Python will output “None”, or
an error message that you have defined:
5.3 Manipulating dictionaries
Adding new key-value pairs to a dictionary is simply done by
assignment. For example, we could have added Germany and Mexico in
our dictionary d using:
We can also change the entries in a dictionary just by
reassigning them.
To remove an entry in a dictionary, we need to use the function
del. For example, to remove the key-value (“Paris”:”France”) from
our dictionary d:
Other useful functions on dictionaries are illustrated in the
example below:
Exercises:
7. Write a program that prints all the numbers from 1 to 100.
Your program should have much fewer than 100 lines of code!
8. Starting with the word GENE1=”ATGTTGATGTG”, write a Python
program that creates the new words GENE2, GENE3, GENE4 and GENE5
such that:
i. GENE2 only contains the last two letters of GENE1
ii. GENE3 only contains the first two letters of GENE1
iii. GENE4 only contains the letters at positions 2,4,6,8 and 10
in GENE1
iv. GENE5 only contains the first 3 and last 3 letters of
GENE1
9. Suppose you have a Python program that read in a whole page
from a book into an array PAGE, with each item of the array
corresponding to a line. Add code to this program to create a new
array SENTENCES that contains the same text, but now with each
element in SENTENCES being one sentence.
10. Let d be a dictionary whose pairs key:value are
country:capital. Write a Python program that prints the keys and
values of d, with the keys sorted in alphabetical order. Test your
program on
d = {“France”:”Paris”,”Belgium”:”Brussels”,”Mexico”:”Mexico
City”,”Argentina”:”Buenos Aires”,”China”:”Beijing”}
Chapter 3: Control Structures
11. Higher order organization of Python instructions
In the previous chapters, we have introduced the different types
of variables known by Python, as well as the operators that
manipulate these variables. The programs we have studied so far
have all been sequential, with each line corresponding to one
instruction: this is definitely not optimal. For example, we have
introduced in the previous chapter the concept of lists and arrays,
to avoid having to use many scalar variables to store data
(remember that if we were to store the whole human genome, we would
need either 30,000 scalar variables, one for each gene, or a single
array, whose items are the individual genes); if we wanted to
perform the same operation on each of these genes, we would still
have to write one line for each gene. In addition, the programs we
have written so far would attempt to perform all their
instructions, once given the input. Again, this is not always
desired: we may want to perform some instructions only if a certain
condition is satisfied.
Again, Python has thought about these issues, and offers
solutions in the form of control structures: the if structure that
allows to control if a block of instruction need to be executed,
and the for structure (and equivalent), that repeats a set of
instructions for a preset number of times. In this chapter, we will
look in details on the syntax and usage of these two
structures.
Figure 3.1: The three main types of flow in a computer program:
sequential, in which instructions are executed successively,
conditional, in which the blocks “instructions 1” and “instructions
2” are executed if the Condition is True or False, respectively,
and repeating, in which instructions are repeated over a whole
list.
12. Logical operators
Most of the control structure we will see in this chapter test
if a condition is true or false. For programmers, “truth” is easier
to define in terms of what is not truth! In Python, there is a
short, specific list of false values:
· An empty string, “ “, is false
· The number zero and the string “0” are both false.
· An empty list, (), is false.
· The singleton None (i.e. no value) is false.
Everything else is true.
2.3 Comparing numbers and strings
We can test whether a number is bigger, smaller, or the same as
another. Similarly, we can test if a string comes before or after
another string, based on the alphabetical order. All the results of
these tests are TRUE or FALSE. Table 3.1 lists the common
comparison operators available in Python.
Notice that the numeric operators look a little different from
what we have learned in Math: this is because Python does not use
the fancy fonts available in text editors, so symbols like (, (, (
do not exist. Notice also that the numeric comparison for equality
uses two = symbols (==): this is because the single = is reserved
for assignment.
Table 3.1 : Python comparison operators
comparison
Corresponding question
a == b
Is a equal to b ?
a != b
Is a not equal to b ?
a > b
Is a greater than b ?
a >= b
Is a greater than or equal to b ?
a < b
Is a less than b ?
a <= b
Is a less than or equal to b ?
a in b
Is the value a in the list (or tuple) b?
a not in b
Is the value a not in the list (or tuple) b?
These comparisons apply both to numeric value and to strings.
Note that you can compare numbers to strings, but the result can be
arbitrary: I would strongly advise to make sure that the types of
the variables that are compared are the same!
2.4 Combining logical operators
We can join together several tests into one, by the use of the
logical operator and and or
13. Conditional structures
13.1 If
The most fundamental control structure is the if structure. It
is used to protect a block of code that only needs to be executed
if a prior condition is met (i.e. is TRUE). The generate format of
an if statement is:
Note about the format:
· the : indicates the end of the condition, and flags the
beginning of the end structure
· Notice that the code block is indented with respect to the
rest of the code: this is required; it allows you to clearly
identify which part of the code is conditional to the current
condition.
The condition is one of the logical expressions we have seen in
the previous section. The code block is a set of instructions
grouped together. The code block is only executed if the condition
is TRUE.
if statements are very useful to check inputs from users, to
check if a variable contains 0 before it is used in a
division,….
As example, let us write a small program that asks the user to
enter a number, reads it in, and checks if it is divisible by n,
where n is also read in:
13.2 Else
When making a choice, sometimes you have two different things
you want to do, depending upon the outcome of the conditional. This
is done using an if …else structure that has the following
format:
Block code 1 is executed if the condition is true, and block
code 2 is executed otherwise.
Here is an example of a program asking for a password, and
comparing it with a pre-stored string:
Python also provides a control structure when there are more
than two choices: the elif structure is a combination of else and
if. It is written as:
Note that any numbers of elif can follow an if.
14. Loops
One of the most obvious things to do with an array is to apply a
code block to every item in the array: loops allow you to do
that.
Every loop has three main parts:
· An entry condition that starts the loop
· The code block that serves as the “body” of the loop
· An exit condition
Obviously, all three are important. Without the entry condition,
the loop won’t be executed; a loop without body won’t do any thing;
and finally, without a proper exit condition, the program will
never exit the loop (this leads to what is referred to an infinite
loop, and often results from a bug in the exit loop).
There are two types of loops: determinate and indeterminate.
Determinate loops carry their end condition with them from the
beginning, and repeat its code block an exact number of times.
Indeterminate loops rely upon code within the body of the loop to
alter the exit condition so the loop can exit. We will see one
determinate loop structure, for, and one indeterminate loop
structure, while.
4.1 For loop
The most basic type of determinate loop is the for loop. Its
basic structure is:
Note the syntax similar to the syntax of an if statement: the :
at the end of the condition, and the indentation of the code
block.
A for loop is simple: it loops over all possible values of
variable as found in the list listA, executing each time the code
block.
For example, the Python program:
Will print out:
Note that if the list is empty the loop is not executed at
all.
The for loop is very useful for iterating over the elements of
an array. It can also be used to loop over a set of integer values:
remember that you can create a list of integers using the function
range. Here is an example program that computes the sum of the
squares of the numbers from 0 to N, where N in read as input:
4.2 While loop
Sometimes, we face a situation where neither Python nor we know
in advance how many times a loop will need to execute. This is the
case for example when reading a file: we do not know in advance how
many lines it has. Python has a structure for that: the while
loop:
The while structure executes the code block as long as the TEST
expression evaluates as TRUE. For example, here is a program that
prints the number between 0 and N, where N is input:
Note that it is important to make sure that the code block
includes a modification of the test: if we had forgotten the line
i+=1 in the example above, the while loop would have become an
infinite loop.
Note that any for loop can be written as a while loop. In
practice however, it is better to use a for loop, as Python
executes them faster
4.3 Break points in loops
Python provides two functions that can be used to control loops
from inside its code block: break allows you to exit the loop,
while continue skips the following step in the loop.
Here is an example of a program that counts and prints number
from 1 to N (given as input), skipping numbers that are multiples
of 5 and stopping the count if the number reached squared is equal
to N:
Exercises:
1. Write a program that reads in an integer value n and outputs
n! (Reminder: n!=1x2x3x4….xn).
2. Write a program that reads in a word, and writes it out with
the letters at even positions in uppercase, and the letters at odd
positions in lower case.
3. In cryptography, the Ceasar cipher is a type of substitution
cipher in which each letter in the plaintext is replaced by a
letter some fixed number of positions down the alphabet. For
example, with a shift of 3, A would be replaced by D, B would
become E, …, X would become A, Y would become B, and Z would become
C.
a. Write a program that reads in a sentence, and substitutes it
using the Ceasar cipher with a shift of 3.
b. Write a program that reads in sentence that has been
encrypted with the Ceasar cipher with a shift of 3, and decrypts
it.
c. Repeat a and b above for a Ceasar cipher with a shift N,
where N is given as input, N between 0 and 10
#
print(‘Hello World!\n’)
#
>>> 1+3
4
>>> # This is a comment
>>> 2+2 # and a comment on the same line as code
4
>>> (60-5*6)/3
10
>>> 7//3 # Integer division returns the floor:
2
>>> 7//-3
-3
From math import *
>>>Width=4# Note that the value of an assignment is not
written
>>>Height=3*12
>>>Area=Width*Height
>>>print (Area)
144
>>>x=y=z=0# Python allows multiple assignments: x, y
and z are
set to 0
>>>DNA=’aattgcg’# assign a string variable
>>>Name_list=[‘John’,’David’]# set up a list of
names
>>> 4+5
9
>>>print( _)
9
>>>x = 1# Integer
>>>y = 5.14# Floating point
>>>z = 3.25E-7# Scientific notation
i=52
j=3
k=i%j
#
a = 5*4
print( “5 times four is”, a, end=’\n’)
a =a+4
print (“Plus three is “,a)
a/=3
print (“Divided by three is “,a)
#
>>>A==”ATTGC”
>>>B=”GGCCT”
>>>C=A+B
>>> text=”No! “
>>>newtext=text*5
>>> print (newtext)
No! No! No! No! No!
>>> S = ‘Index’
>>> S[0]
I
>>> S[3]
e
>>> S[-1]
x
>>> S[-3]
d
>>> S = ‘This is a string’
>>> b = S[1:3]# Select substring from position 1 to 3,
3 not included
>>> print( b)
‘hi’
>>> S[5:12:3]# Select every third character, between
position 5 and 10
‘iat’
>>> S[1:5:-1]# Starts from the end of the string; but
order 1:5 is wrong
get nothing:
‘’
>>> S[5:1:-1]# correct syntax
‘i si’
>>> S[10::]# all characters from position 10 till the
end
‘string’
>>> S[::-1]# reverse the whole string
‘gnirts a si sihT’
>>>text=”This is a test case; it has two parts”
>>>text.split()
[‘This’,’is’,’a’,’test’,’case;’,’it’,’has’,’two’,’parts’]
>>> text.split(‘;’)
[‘This is a test case’,’ it has two parts’]
>>> text.split(‘a’)
[‘This is ‘,’ test c’,’se; it h’,’s two p’,’rts’]
>>> S = ‘This Is A Test’
>>> S.upper()# All upper case
‘THIS IS A TEST’
>>> S.lower()# All lower case
‘this is a test’
>>> S.lower().capitalize()# Set proper case
‘This is a test’
>>> S = ‘This is a test‘# Remove leading space
>>> S.lstrip()
‘This is a test’
>>> S=’aattggccttaa’
>>> S.count(‘a’)# Number of character ‘a’ in the
string
4
>>> S.count(‘A’)
0# Remember that python is case sensitive
>>> S.count(‘at’)# Number of ‘at’ in the string
1
>>> S.count(‘Gc’)
0
>>> S=’This is a test case’
>>> S.replace(‘is’,’was’)# replaces all instances of
‘is’
‘Thwas was a test case’
>>> S.replace(‘is’,’was’,1)# replaces only first
instance
‘Thwas is a test case’
age = int(input(“What is your age :”))
a=4
b=9
c=5
d= a*2+b*3
b%=a
a=b-1;
>>>
week=(‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’,’Sunday’)
>>> print(y[2])
>>> # Read in day considered
>>> day=int(input(“Enter a number from 1 to 7 : “))
>>> #
>>> # print element ‘day’ of the list of days of the
week:
>>> #
>>> print (week[day])
>>>print (week[0:2])
>>>
[“Monday”,”Tuesday”,”Wednesday”,”Thursday”,”Friday”,”Saturday”,”Sunday”]
>>> list(range(10))
[0,1,2,3,4,5,6,7,8,9]
>>> list(range(10,21))
[10,11,12,13,14,15,16,17,18,19,20]
>>> list(range(1,10,2))
[1,3,5,7,9]
>>>
days=(‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’,’Sunday’)
>>>
days=[‘Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’,’Sunday’]
�
Figure 2.1: Scalar variables and arrays.
A scalar variable is like a single box, while an array behaves
like a chest of drawers. Each of the drawers is assigned a number,
or index, which starts at 0.
>>> print( days)
numbers = [0,1,5];
numbers[0]=3;
numbers[1]=4;
numbers[2]=5;
>>> A=’’.join(LIST)
>>> sentences = paragraph.split(‘.‘)
>>> words=sentence.split(‘ ‘)
>>> characters=list(word)
�
Figure 2.2: The dictionary variable
A dictionary is a special array for which each element is
indexed by a string instead of an integer. The string is referred
to as a key, while the corresponding item is the value. Each
element of a dictionary is therefore a pair (key,value).
>>> country = { “Paris”:”France”, “Washington”: “USA”,
“London”:”England”, “Ottawa”:”Canada”,“Beijing”:”China”}
>>>
seq1=(“Paris”,”Washington”,”London”,”Ottawa”,”Beijing”)
>>>
seq2=(“France”,”USA”,”England”,”Canada”,”China”)
>>> d = dict(zip(seq1,seq2))
>>> d
{ “Paris”:”France”, “Washington”: “USA”, “London”:”England”,
“Ottawa”:”Canada”, “Beijing”:”China”}
>>> d[“Paris”]
“France”
>>> d[“Beijing”]
“China”
>>> d.get(“Paris”)
“France”
>>> d.get(“Mexico City”)
>>> d.get(“Mexico City”,”This key is not defined”)
‘This key is not defined’
>>> d[“Berlin”]=”Germany”
>>> d[“Mexico City”]=”Mexico”
del d[“Paris”]
>>> d =
{“A”:”California”,”B”:”Nevada”,”C”:”Oregon”}
>>>
>>> d.keys()# list all keys of d
[‘A’,’B’,’C’]
>>>
>>> d.values()# list all values of d
[‘California’,’Nevada’,’Oregon’]
>>>
>>> d.has_key(‘A’)# check if a given key is known in
d
True
>>>d.has_key(‘D’)
False
�
a and b True if both a and b are true.
a or bTrue if either a, or b, or both are true.
not aTrue if a is false.
>>> if condition:
code block
number=int(input(“Enter your number --> “))
n=int(input(“Test divisibility by --> “))
i=number%n
if i != 0:
print (“The number “,number,” is not divisible by “,n,”\n”)
if condition:
block code 1
else:
block code 2
hidden=”Mypasscode”
password=input(“Enter your password : “)
if password == hidden:
print (“You entered the right password\n”)
else:
print (“Wrong password !!\n”)
if CONDITION1:
block code 1
elif CONDITION2:
block code 2
else :
block code 3
for variable in listA:
code block
>>> names=[“John”,”Jane”,”Smith”]
>>> j=0
>>> for name in names:
j+=1
print “The name number “,j,” in the list is “,name
The name number 1 in the list is ‘John’
The name number 2 in the list is ‘Jane’
The name number 3 in the list is ‘Smith’
>>> N=int(input(“Enter the last integer considered
--> “))
>>> Sum=0
>>> for i in range(0,N+1,1):
Sum+=i**2
>>> print (“The sum of the squares between 0 and “,N,”
is “,Sum)
while TEST:
code block;
>>> N=int(input(“Enter N --> “))
>>> print (“Counting numbers from 0 to “,N,”\n”)
i=0
while i < N+1:
print (i)
i+=1
>>> N = int(input(“Enter N --> “))# Input N
>>> for i in range(0,N+1,1):# Start loop from 0 to N (N
included)
if i**2 == N:# test if i**2 is equal to N…
break# if it is stop counting
else:
if i%5==0:# Test if i is a multiple of 5
continue# if it is, move to next value
print (i)