Python 3 Language Basics - veritas.ucd.ie

Python 3 Language Basics

John Quinn

1

Useful Cheat Sheet

• https://perso.limsi.fr/pointal/_media/python:cours:mementopython3-english.pdf

2

Numbers• Numbers:

• integer numbers (base 2, 8, 10 (default), 16), e.g.

• x=15

• x=0b1111 # binary = 15 in decimal

• x=0o17 # octal = 15 in decimal

• x=0xF # hexadecimal = 15 in decimal

• floating point numbers, e.g. y=11.2, y=3.0e8

• complex numbers, e.g. z=1+2j

• The real, imaginary and conjugate of a complex number can be accessed:

• z.real, z.imag, z.conjugate()

• Note: you can use the python function type(variable) to get the type of a variable, e.g. type(z)

3

3

Boolean

• There is a Boolean type (Python type: bool) that can have the value of either “True” or “False”

• Normal logic rules apply to the bools.

4

• The following also get evaluated as False in Python: • numerical zero values (0, 0L, 0.0, 0.0+0.0j), • empty strings, • empty lists and empty tuples, • empty dictionaries. • plus the special value None.

4

Common Number OperatorsOperator Description Example

+,- Addition, Subtraction

*,% Multiplication, Modulo 27 % 7Result: 6

/ Division (Note: Python 2 does floor division)

>>> 10 / 33.3333333333333335

// Truncation/floor division 27 // 7 Result: 3

** Power 10 ** 3Result: 1000

or, and, not Boolean (a or b) and c

<, <=, >, >=, !=, ==

Comparison, returns a Boolean type 2 <= 3

%=, =, /=, //=, -=, +=, *=, **=

assignment x+=2 # add 2x-=3 # subtract 3 5

5

Operator Precedent

https://www.tutorialspoint.com/python/python_basic_operators.htm

Note: == checks if two variables have the same values while is checks of they are the same object.

6

Variable type and reassignments• Variables may be changed to store other types than what they were created with:

>>> x = 3 # x has value 3, stored in some memory location>>> y = x # y now points to the same memory as x>>> y = 2 # y now points to a new memory location, x is unchanged

i = 42 # data type is implicitly set to integeri = 42 + 0.11 # data type is changed to floati = "forty" # and now it will be a string

• Python automatically tidies up the memory left behind.

• Assignment of variables in Python might be a little different to what you’d expect:

7

Test two objects the same with id(var) or is

7

Mutable -vs- Immutable

• Mutable objects are objects that can have their size and/or values changed (e.g. lists, dictionaries)

• Immutable objects cannot be changed.

• If two variables refer to the same mutable object then a change to one is seen by both.

• Note: this does not apply to reassignment.

• Examples:….

8

Containers

• The most common Python containers (e.g. see https://docs.python.org/3.4/tutorial/datastructures.html) are listed below with the ones in blue the ones that we will use:

• strings: ordered sequence of characters, fast access, repeatable values, immutable

• tuples: ordered sequence, fast access, repeatable values, immutable, faster than lists, use if data is not to be changed

• lists: ordered sequence, fast access, repeatable values, mutable

• dictionaries: no a priori order, unique key, fast key access, mutable.

• sets: unordered collection with no duplicate elements, mutable.

9

9

Strings• Strings:

• strings are sequences of characters.

• strings are not changeable (“immutable”)

• strings are made with:

• single quotes (') 'This is a string with single quotes'

• double quotes (") "Obama's dog is called Bo"

• triple quotes, both single (''') and (""") '''String in triple quotes can extend over multiple lines, like this on, and can contain 'single' and "double" quotes.'''

10

10

Lists & Strings

• Indexing and slicing strings (& lists, tuples, sets….):

• s=”Python”

• s[0] is “P”

• s[-1] is “n” (-1 is last character)

• s[1:3] is “yt”

• s[3:5] is “ho”

• s[1:] is “ython”

• s[:3] is “Pyt”

• s[:] is “Python”

• s[0:5:2] is “Pto”Note: slicing strings always returns a copy of the elements in question.

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11

Lists & Strings• Lists & Strings can be concatenated with “+”

• s='Hello ' + 'World'

• Lists & Strings can be repeated with “*”

• >>> 'hello '*5

• 'hello hello hello hello hello'

• Strings have lots of functions that act on them: e.g. ⇒

>>> s="Hello World">>> s.lower()'hello world'

>>> s.upper()'HELLO WORLD'

>>> s.count('o')2

>>> s.find('w')-1

>>> s.find('W')6

• Note: functions like upper() return a new string and do not change the original one.

https://docs.python.org/3/library/stdtypes.html#string-methods

• Note: strings can be compared with the “==“ operator.

12

12

Joining and Splitting Strings

Sequences indexing

Base Types

Python 3 Cheat Sheet©2012-2013 - Laurent PointalLicence Creative Commons Attribution 2

Official Python documentation on

http://docs.python.org/py3k

0783 -192int

9.23 -1.7e-60.0float

True Falsebool

"One\nTwo" 'I\'m'str

"""X\tY\tZ1\t2\t3"""

10-6

tab char

new line

multiline

Container Types

list [1,5,9] ["x",11,8.9] ["word"] []tuple (1,5,9) 11,"y",7.4 ("word",) ()

dict{1:"one",3:"three",2:"two",3.14:"π"}{"key":"value"}

set

{}

{1,9,3,0}

◾ ordered sequence, fast index access, repeatable values

set()

◾ no a priori order, unique key, fast key access ; keys = base types or tuples

{"key1","key2"}

immutable

Variables assignment

x = 1.2+8+sin(0)

y,z,r = 9.2,-7.6,"bad"

value or computed expression

variable name (identifier)

a‥zA‥Z_ followed by a‥zA‥Z_0‥9◽ diacritics allowed but should be avoided◽ language keywords forbidden◽ lower/UPPER case discrimination

expression with just comas

immutable,ordered sequence of chars

dictionary

integer, float, boolean, string

container with severalvalues (here a tuple)

variablesnames

Identifiers

☺ a toto x7 y_max BigOne☹ 8y and

x+=3 x-=2incrementdecrement

Conversions

for lists, tuples, strings, …

int("15")

float("-11.24e8")

bool

str(78.3) repr("Text")

can specify integer number base in 2nd parameter

int(15.56) truncate decimal part (round(15.56) for rounded integer)

and for litteral representation

type(expression)

use comparators (with ==, !=, <, >, …), logical boolean result

see other side for string formating allowing finer control

":".join(['toto','12','pswd']) 'toto:12:pswd'joining string sequence of strings

"words with spaces".split() ['words','with','spaces']

"1,4,8,2".split(",") ['1','4','8','2']splitting string

dict([(3,"three"),(1,"one")]) {1:'one',3:'three'}

list("abc") ['a','b','c']use each element

from sequence

set(["one","two"]) {'one','two'}use each element

from sequence

lst=[11, 67, "abc", 3.14, 42, 1968] lst[1]→67lst[-2]→42

0 1 2 3 54

-6 -5 -4 -3 -1-2

individual access to items via [index]positive index

negative index

0 1 2 3 54 6

-6 -5 -4 -3 -1-2negative slice

positive slice

access to sub-sequences via [start slice:end slice:step]

len(lst) 6

lst[1:3]→[67,"abc"]

lst[::2]→[11,"abc",42]lst[-3:-1]→[3.14,42]lst[:3]→[11,67,"abc"]

lst[:-1]→[11,67,"abc",3.14,42]

lst[4:]→[42,1968]

lst[1:-1]→[67,"abc",3.14,42]

lst[:]→[11,67,"abc",3.14,42,1968]Missing slice indication → from start / up to end.

Conditional Statement

if x==42: # block if logical expression x==42 is true print("real truth")elif x>0: # else block if logical expression x>0 is true print("be positive")elif bFinished: # else block if boolean variable bFinished is true print("how, finished")else: # else block for other cases print("when it's not")

Boolean Logic Statements Blocks

parent statement: statements block 1… ⁝ parent statement: statements block 2… ⁝

next statement after block 1

ind

enta

tio

n !

Comparators: < > <= >= == !=≠=≥≤

a and b

a or b

not a

logical and

logical or

logical notone or other or both

both simultaneously

if logical expression: statements block

statements block executed

only if a condition is true

TrueFalse

true constant value

false constant value

can go with several elif, elif... and only one final else,example :

lst[-1]→1968

lst[0]→11

last one

first one

x=None « undefined » constant value

Maths

Operators: + - * / // % **× ÷

integer ÷ ÷ remainder ab

from math import sin,pi…

' escaped

abs(-3.2)→3.2

round(3.57,1)→3.6

☝ floating point numbers… approximated values!

sin(pi/4)→0.707…cos(2*pi/3)→-0.4999…

sqrt(81)→9.0 √log(e**2)→2.0

angles in radians

acos(0.5)→1.0471…

etc. (cf doc)

(1+5.3)*2→12.6

for variables, functions,

modules, classes… names

Mémento v1.2.2

str as an ordered sequence of chars

On mutable sequences, usable to remove del lst[3:5] and to modify with assignment lst[1:4]=['hop',9]

key/value associations

Sequences indexing

Base Types

Python 3 Cheat Sheet©2012-2013 - Laurent PointalLicence Creative Commons Attribution 2

Official Python documentation on

http://docs.python.org/py3k

0783 -192int

9.23 -1.7e-60.0float

True Falsebool

"One\nTwo" 'I\'m'str

"""X\tY\tZ1\t2\t3"""

10-6

tab char

new line

multiline

Container Types

list [1,5,9] ["x",11,8.9] ["word"] []tuple (1,5,9) 11,"y",7.4 ("word",) ()

dict{1:"one",3:"three",2:"two",3.14:"π"}{"key":"value"}

set

{}

{1,9,3,0}

◾ ordered sequence, fast index access, repeatable values

set()

◾ no a priori order, unique key, fast key access ; keys = base types or tuples

{"key1","key2"}

immutable

Variables assignment

x = 1.2+8+sin(0)

y,z,r = 9.2,-7.6,"bad"

value or computed expression

variable name (identifier)

a‥zA‥Z_ followed by a‥zA‥Z_0‥9◽ diacritics allowed but should be avoided◽ language keywords forbidden◽ lower/UPPER case discrimination

expression with just comas

immutable,ordered sequence of chars

dictionary

integer, float, boolean, string

container with severalvalues (here a tuple)

variablesnames

Identifiers

☺ a toto x7 y_max BigOne☹ 8y and

x+=3 x-=2incrementdecrement

Conversions

for lists, tuples, strings, …

int("15")

float("-11.24e8")

bool

str(78.3) repr("Text")

can specify integer number base in 2nd parameter

int(15.56) truncate decimal part (round(15.56) for rounded integer)

and for litteral representation

type(expression)

use comparators (with ==, !=, <, >, …), logical boolean result

see other side for string formating allowing finer control

":".join(['toto','12','pswd']) 'toto:12:pswd'joining string sequence of strings

"words with spaces".split() ['words','with','spaces']

"1,4,8,2".split(",") ['1','4','8','2']splitting string

dict([(3,"three"),(1,"one")]) {1:'one',3:'three'}

list("abc") ['a','b','c']use each element

from sequence

set(["one","two"]) {'one','two'}use each element

from sequence

lst=[11, 67, "abc", 3.14, 42, 1968] lst[1]→67lst[-2]→42

0 1 2 3 54

-6 -5 -4 -3 -1-2

individual access to items via [index]positive index

negative index

0 1 2 3 54 6

-6 -5 -4 -3 -1-2negative slice

positive slice

access to sub-sequences via [start slice:end slice:step]

len(lst) 6

lst[1:3]→[67,"abc"]

lst[::2]→[11,"abc",42]lst[-3:-1]→[3.14,42]lst[:3]→[11,67,"abc"]

lst[:-1]→[11,67,"abc",3.14,42]

lst[4:]→[42,1968]

lst[1:-1]→[67,"abc",3.14,42]

lst[:]→[11,67,"abc",3.14,42,1968]Missing slice indication → from start / up to end.

Conditional Statement

if x==42: # block if logical expression x==42 is true print("real truth")elif x>0: # else block if logical expression x>0 is true print("be positive")elif bFinished: # else block if boolean variable bFinished is true print("how, finished")else: # else block for other cases print("when it's not")

Boolean Logic Statements Blocks

parent statement: statements block 1… ⁝ parent statement: statements block 2… ⁝

next statement after block 1

ind

enta

tio

n !

Comparators: < > <= >= == !=≠=≥≤

a and b

a or b

not a

logical and

logical or

logical notone or other or both

both simultaneously

if logical expression: statements block

statements block executed

only if a condition is true

TrueFalse

true constant value

false constant value

can go with several elif, elif... and only one final else,example :

lst[-1]→1968

lst[0]→11

last one

first one

x=None « undefined » constant value

Maths

Operators: + - * / // % **× ÷

integer ÷ ÷ remainder ab

from math import sin,pi…

' escaped

abs(-3.2)→3.2

round(3.57,1)→3.6

☝ floating point numbers… approximated values!

sin(pi/4)→0.707…cos(2*pi/3)→-0.4999…

sqrt(81)→9.0 √log(e**2)→2.0

angles in radians

acos(0.5)→1.0471…

etc. (cf doc)

(1+5.3)*2→12.6

for variables, functions,

modules, classes… names

Mémento v1.2.2

str as an ordered sequence of chars

On mutable sequences, usable to remove del lst[3:5] and to modify with assignment lst[1:4]=['hop',9]

key/value associations

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13

Read from the keyboard

• To read a string from the keyboard use input(‘Prompt: ‘), e.g.

• s=input('Please enter something: ')

• To convert use the functions: float(), int(), complex(), e.g.

• s=float(input('Please enter a number: ‘))

• Or one can split the string and convert different parts of the list individually.

• If you try to convert something that is incompatible (e.g. characters to a float) an error will occur.

• Catching and handling errors (try, except) is beyond the scope of this tutorial!

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Tuples• Tuples are immutable lists of Python objects.

• They are created with (), e.g.

• tup1 = ('physics', 'chemistry', 1997, 2000);

• tup2 = (1, 2, 3, 4, 5, 6, 7 );

• The are indexed using the [] notation, e.g.

• tup1[0]: physics

• tup2[1:5]: [2, 3, 4, 5]

• They cannot be changed. i.e. (immutable) elements cannot be changed, nor can elements be deleted or appended) but mutable elements may be modified.

15

Lists

• Lists:

• Store ordered sequences of elements and may be accessed via indices using the [] notation

• Create with “[]”

>>> l = ["London", "Paris", "Strasbourg", "Zurich"]

>>> print(l[1], l[2])Paris Strasbourg

• Lists may store arbitrary types, e.g. x=[1, "hello", 3.4]

• Lists may be nested.

• Lists are mutable - items can be removed, added, changed., e.g. 16

16

Lists>>> cubes = [1, 8, 27, 65, 125][1, 8, 27, 65, 125]

>>> cubes[3]=64[1, 8, 27, 64, 125]

>>> cubes=cubes+[216, 343, 512][1, 8, 27, 64, 125, 216, 343, 512]

>>> cubes.append(729)[1, 8, 27, 64, 125, 216, 343, 512, 729]

>>> cubes[2:4]=[] # removing elements[1, 8, 125, 216, 343, 512, 729]

>>> cubes[2:2]=[27,64] # inserting elements[1, 8, 27, 64, 125, 216, 343, 512, 729]

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Dictionaries• Dictionaries contain (key: value) pairs.

• Dictionaries are created with {} and indexed using [key] notation.

• Items are access via their key and not their position (as withs strings and lists)

• They can be easily changes and shrink and grow as needed….

• Example: city: populations

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>>> city = {"New York City”: 8175133, “Los Angeles": 3792621, ”Washington”: 632323, "Chicago": 2695598, “Toronto”: 2615060, “Montreal": 11854442, “Ottawa": 883391, “Boston": 626000}

18

Dictionaries>>> city = {"New York City":8175133, "Los Angeles": 3792621, "Washington":632323, "Chicago": 2695598, "Toronto":2615060, "Montreal":11854442, "Ottawa":883391, "Boston":62600}

>>> city["New York City"]8175133

>>> city["Toronto"]2615060

>>> city["Halifax"] = 390096

>>> city{'Toronto': 2615060, 'Ottawa': 883391, 'Los Angeles': 3792621, 'Chicago': 2695598, 'New York City': 8175133, 'Halifax': 390096, 'Boston': 62600, 'Washington': 632323, 'Montreal': 11854442}

>>> city = {}>>> city{}

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19

Operations on Containers

Display / Inputprint("v=",3,"cm :",x,",",y+4)

print options:

◽ sep=" " (items separator, default space)

◽ end="\n" (end of print, default new line)

◽ file=f (print to file, default standard output)

items to display: litteral values, variables, expressions

loop on dict/set = loop on sequence of keys

Conditional loop statementstatements block executed as long

as condition is true

while logical expression: statements block

s = 0i = 1

while i <= 100: # statement executed as long as i ≤ 100 s = s + i**2 i = i + 1

print("sum:",s)

initializations before the loop

condition with at least one variable value (here i)

s= ∑i=1

i=100

i2

☝ make condition variable change

Iterative loop statementstatements block executed for each

item of a container or iterator

for variable in sequence: statements block

s = "Some text"cnt = 0

for c in s: if c == "e": cnt = cnt + 1print("found",cnt,"'e'")

Go over sequence's values

Count number of

e in the string

Go over sequence's index

◽ modify item at index

◽ access items around index (before/after)

lst = [11,18,9,12,23,4,17]lost = []for idx in range(len(lst)): val = lst[idx] if val > 15: lost.append(val) lst[idx] = 15print("modif:",lst,"-lost:",lost)

Limit values greater

than 15, memorization

of lost values.

☝ be careful of inifinite loops !


loop variable, value managed by for statement

use slices to go over a subset of the sequence

computed result after the loop

Generator of int sequences

Files

s = input("Instructions:")☝ input always returns a string, convert it to required type

(cf boxed Conversions on on ther side).

frequently used in

for iterative loops

range returns a « generator », converts it to list to see

the values, example:

print(list(range(4)))

range(5) 0 1 2 3 4range(3,8) 3 4 5 6 7range(2,12,3) 2 5 8 11

range([start,]stop [,step])

f = open("fil.txt","w",encoding="utf8")storing data on disk, and reading it back

opening mode

◽ 'r' read

◽ 'w' write

◽ 'a' append…

encoding of

chars for text

files:

utf8 ascii

latin1 …

name of file

on disk

(+path…)

file variable

for operations

f.write("hello")writing

☝ text file → read /write only

strings, convert from/to required

type.

reading

s = f.read(4)

for line in f : # line processing block

cf functions in modules os and os.path

if char count not

specified, read

whole file

s = f.readline()

read next

line

f.close() ☝ don't forget to close file after use

Pythonic automatic close : with open(…) as f:

very common: iterative loop reading lines of a text file

Function definition

def fctname(p_x,p_y,p_z): """documentation""" # statements block, res computation, etc. return res

function name (identifier)

result value of the call.

if no computed result to

return: return None☝ parameters and all of this bloc

only exist in the block and during

the function call ("black box")

named parameters

Function callr = fctname(3,i+2,2*i)

one argument per parameter

retrieve returned result (if necessary)

empty string if end of file

not includeddefault 0

"model {} {} {}".format(x,y,r)"{selection:formating!conversion}"◽ Selection :

2 x 0.nom 4[key] 0[2]

str

Strings formatingvalues to formatformating directives

"{:+2.3f}".format(45.7273)→'+45.727'"{1:>10s}".format(8,"toto")→' toto'"{!r}".format("I'm")→'"I\'m"'

◽ Conversion : s (readable text) or r (litteral representation)

< > ^ = 0 at start for filling with 0

integer: b binary, c char, d decimal (default), o octal, x or X hexa…

float: e or E exponential, f or F fixed point, g or G appropriate (default),

% percent

string : s …

◽ Formating :

fillchar alignment sign minwidth.precision~maxwidth type

+ - space

Ex

am

ple

s

Operations on containerslen(c)

min(c) max(c) sum(c)sorted(c)

reversed(c)

☝ modify original list

lst.append(item)

lst.pop(idx)lst.sort() lst.reverse()

c.index(val) c.count(val)

→ items count

→ sorted copy

→ reverse iterator

→ position → events count

lst.extend(seq) add item at end

add sequence of items at end

lst.insert(idx,val) insert item at index

lst.remove(val) remove first item with value

remove item at index and return its value

sort / reverse list in place

Operations on dictionaries Operations on setsOperators:

| → union (vertical bar char)

& → intersection

- ^ → difference/symetric diff

< <= > >= → inclusion relations

d.update(d2) update/add

associations

Note: For dictionaries and set, these

operations use keys.

Special for sequence containeurs (lists, tuples, strings) :

val in c → boolean, membersihp operator in (absence not in)

Operations on lists

d[key]→value del d[clé]d[key]=value

d.keys()

d.clear()

d.items()d.values() views on keys, values

associations

d.pop(clé)

s.update(s2)s.add(key) s.remove(key)s.discard(key)

c*5 → duplicate c+c2→ concatenate

Loop control

break

continue

immediate exit

next iteration

Go simultaneously over sequence's index and values:for idx,val in enumerate(lst):

enumerate(c)→ iterator on (index,value)

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Operations on Lists & Dictionaries


print options:









s = 0i = 1


print("sum:",s)



s= ∑i=1

i=100

i2








Count number of

e in the string







of lost values.







Files



frequently used in

for iterative loops







opening mode

◽ 'r' read

◽ 'w' write

◽ 'a' append…

encoding of

chars for text

files:

utf8 ascii

latin1 …

name of file

on disk

(+path…)

file variable

for operations




type.

reading

s = f.read(4)



if char count not

specified, read

whole file

s = f.readline()

read next

line




Function definition








named parameters







2 x 0.nom 4[key] 0[2]

str







% percent

string : s …

◽ Formating :


+ - space

Ex

am

ple

s



reversed(c)


lst.append(item)



→ items count

→ sorted copy











& → intersection




associations





Operations on lists


d.keys()

d.clear()


associations

d.pop(clé)



Loop control

break

continue

immediate exit

next iteration



21


print options:









s = 0i = 1


print("sum:",s)



s= ∑i=1

i=100

i2








Count number of

e in the string







of lost values.







Files



frequently used in

for iterative loops







opening mode

◽ 'r' read

◽ 'w' write

◽ 'a' append…

encoding of

chars for text

files:

utf8 ascii

latin1 …

name of file

on disk

(+path…)

file variable

for operations




type.

reading

s = f.read(4)



if char count not

specified, read

whole file

s = f.readline()

read next

line




Function definition








named parameters







2 x 0.nom 4[key] 0[2]

str







% percent

string : s …

◽ Formating :


+ - space

Ex

am

ple

s



reversed(c)


lst.append(item)



→ items count

→ sorted copy











& → intersection




associations





Operations on lists


d.keys()

d.clear()


associations

d.pop(clé)



Loop control

break

continue

immediate exit

next iteration



21

Check it an item is in a container

• To check if an item is in a container use in or not in:

>>> abc = ["a","b","c","d","e"]

>>> "a" in abcTrue

>>> "a" not in abcFalse

>>> "e" not in abcFalse

>>> "f" not in abcTrue

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22

Conditional Statements• The general form of the if statement is:

if condition_1: statement_block_1elif condition_2: statement_block_2else: statement_block_3

• The following get evaluated as False in Python: • numerical zero values (0, 0L, 0.0, 0.0+0.0j), • the Boolean value False, • empty strings, • empty lists and empty tuples, • empty dictionaries. • plus the special value None.

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23

Conditional Statements

• Equivalent common short form of if statement in Python:

if (a > b) max=aelse max=b

• Consider calculation of maximum of two numbers a, b:

max = a if (a > b) else b

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24

Conditional Statements

# Program to convert dog years to human years:

age = int(input("Age of the dog: "))print()

if age < 0:print("This can hardly be true!")

elif age == 1:print("about 14 human years")

elif age == 2:print("about 22 human years")

elif age > 2:human = 22 + (age -2)*5print("Human years: ", human)

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while Loops

• A while loop is executed while some condition is True, execution ceases and the program continues after the while loop when the condition becomes False.

• It is also possible to

• break out of while loop with the break command or to • skip the rest of the code block and jump straight to testing the expression with

the continue command.

n = 100sum = 0counter = 0

while counter < n: sum = sum + counter counter += 1

print("Sum=",sum)

while True: value=int(input("Guess: ")) if value==42 break

print("Congratulations! ")

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for Loops

• The general syntax for a for loop is: for <variable> in <sequence>: <statements>

>>> languages = ["C", "C++", "Perl", "Python"] >>> for x in languages:... print(x)... CC++PerlPython>>>

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• It is also possible to

• break out of for loop with the break command or to

• skip the rest of the code block and jump straight to testing the expression with the continue command.

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for and while Loops• for and while loops both have optional else statements, that get executed

when the loop has not terminated by a break.

• See documentation on line at:

• https://docs.python.org/3/reference/compound_stmts.html#while

• https://docs.python.org/3/reference/compound_stmts.html#for

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for Loops• The range() function is very useful for for loops.

• range() is a generator - it produces a different integer each time it is called.

• range() takes at least 1 but up to 3 arguments:

• range(begin=0, end, step=1)

for i in range(5): print(i) Produces numbers 0, 1, 2, 3, 4

for i in range(4,8): Produces numbers 4, 5, 6, 7

for i in range(0,11,2): Produces numbers 0, 2, 4, 6, 8,10

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for Loops: range and enumerate

• A range can be converted to a list: >>> list(range(0,11,2)[0, 2, 4, 6, 8, 10]

• When looping over a list it is dangerous to modify the list - instead loop over a copy of the list.

>>> for x in a[:]: if x < 0: a.remove(x)

• To loop over a list: >>> for i in range(len(mylist)); print(i,mylist[i])

>>> for i,value in enumerate(mylist): print(i,value)

or

• The step can be negative.

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for Loops: zip

• Use zip to loop over two (or more) lists simultaneously:

In [13]: x=list(range(10))

In [14]: y=list(range(10,20))

In [15]: for a,b in zip(x,y): print(a,b)0 101 112 123 134 145 156 167 178 189 19

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List Comprehensions

• Consider the code: numbers=list(range(10))xsq=[]for x in numbers: xsq.append(x**2)

• Python has an elegant alternative approach:

numbers=list(range(10))xsq=[x**2 for x in numbers]

• Logical expressions can be included:

xsq=[x**2 for x in numbers if x>5]

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string formatting

• There are a few different ways for produce formatted output - we will use the Python 3 function the string .format() function.

• str.format()is very powerful and we will only touch on the basics here.

• Learn by examples:

>>> x=10>>> y=20

>>> print("x={}, y={}".format(x,y))x=10, y=20

>>> print("x={0}, y={1}".format(x,y))x=10, y=20

>>> print("x={a}, y={b}".format(a=10,b=11))x=10, y=11

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string formatting• After the identifier in the {} we can specify formatting option by using a “:”

followed by the options.>>> print("x={0:10.5f}".format(2.35))x= 2.35000

>>> print("x={0:+010.5f}".format(2.35))x=+002.35000

>>> print("{:08b}".format(15))00001111

format_spec ::= [[fill]align][sign][#][0][width][,][.precision][type]fill ::= <any character>align ::= "<" | ">" | "=" | "^"sign ::= "+" | "-" | " "width ::= integerprecision ::= integertype ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"

.format spec: (from https://docs.python.org/3/library/string.html#formatspec):

• Please see online documentation for more explanations and examples of format specification.

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Writing to a File• In general when you do an experiment under computer control you will want to

save data to a file.

• Here we are only concerned with writing data to the file as we will use numpy to read it back in (i.e. we will skip the general Python file reading routines).

• Procedure: >>> fh = open("example.txt", "w")>>> fh.write(str(x)+”\n”) >>> fh.close()

• The open() command takes the filename and the mode: (read: “r” (default), write: “w”, append: “a”) as arguments.

• The write() command takes a single string argument and writes the string to a file. ”\n” indicates a new line.

• It is likely that you will write() in a loop to save multiple values to the file.

• It is essential to close() the file afterwards. 35

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Writing to a file

• If an error occurs then the file may not get closed properly.

• Alternative way:

with open(“results.txt”,”w") as f:s=string(some_data) f.write(s+’\n’)

• The file will be closed automatically once the “with” code block exits

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Python 3 Language Basics - veritas.ucd.ie

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