1 Python Chapter 4 Branching statements and loops © Samuel Marateck 2010
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1
PythonChapter 4
Branching statements and loops
© Samuel Marateck 2010
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PythonChapter 4
Branching statements and loops
© Samuel Marateck 2010
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The type of a variable is determined by whatis defined by the type of the literal assigned to it. Example: value = 12So value is an integer identifier. If you then assign ‘asd’ to value,value = ‘asd’What type a variable is value now?
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value becomes a string variable.
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We now introduce a new type of literal, the
boolean literal. It can have two values,
True and False. Note that these two literals
must be capitalized.
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In computer science, an expression consist
of a variable, literal or both. So examples of
expressions are: x, 3.3, x+2, and ‘asd’.
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A boolean expression must have a true or
false value. Can you give an example of
a boolean expression ?
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Can you give an example of a boolean
expression ?
True and False are the simplest boolean
expressions.
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Normally, boolean expressions have a
comparison operator. The following slide is a
table of some of these operators.
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Operator Explanation
== Equal to
!= Not equal to
> Greater than
>= Greater than or equal to
< Less than
<= Less than or equal to
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Note that the operators consisting of two
characters cannot have an embedded blank.
So > = would cause an error. It should be
written >=.
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Next we show how to use a boolean
expression in an if statement.
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if x == 3:
print(x)
print(‘nyu’)
print(‘end’)
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The boolean expression in if x == 3: is
x==3. If it’s true, then the block of
statements following the if is executed. Here
the block is:
print(x)
print(‘nyu’)
Note that the block must be indented.
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The Python environment automatically
indents statements in the block. To end the
block you must manually unindent.
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if x == 3: print(x) print(‘nyu’)print(‘end’)After the block is executed, the statements after the block, here print(‘end’) executed. If the boolean expression is false, the block is skipped and the statements after the block, here print(‘end’), are executed.
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Now we explore the if-else construct:
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if x == 3:
print(x)
print(‘nyu’)
else:
print(x**2)
print(‘end’)
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If x==3 is true the block
print(x)
print(‘nyu’)
is executed, then print(‘end’) is executed.
If x==3 is false, the block after the else:,
print(x**2) is executed. Then print(‘end’) is
executed.
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In
if True :
print(‘doggy’)
the literal True is the boolean expression
and it is true, so print(‘doggy’) is executed.
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The for loop
In for j in range(10): print(j)the range of j is 0 ≤ j ≤ 9 so print(j) is executed ten times, once for each value of j in this range. Note that j is an integer and is called the loop index.
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To add the integers from 1 to 10, set a variable sum to zero and add to it in the loopsum = 0for j in range(11): sum = sum + j print(sum)Since the print is indented, it is executed with sum = sum + jFor each value of j where now 0 ≤ j ≤ 10. The last two statements is the block comprising the scope of the loop.
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A table showing how sum is incremented.
j sum sum + j
0 0 0
1 0 1
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A table showing how sum is incremented.
j sum sum + j
0 0 0
1 0 1
2 1 3
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A table showing how sum is incremented.
j sum sum + j
0 0 0
1 0 1
2 1 3
3 3 6
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A table showing how sum is incremented.
j sum sum + j
0 0 0
1 0 1
2 1 3
3 3 6
4 6 10
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A table showing how sum is incremented.
j sum sum + j
0 0 0
1 0 1
2 1 3
3 3 6
4 6 10
5 10 15
etc
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A table showing how sum is incremented.
j sum sum + j
0 0 0
1 0 1
2 1 3
3 3 6
4 6 10
5 10 15
6 15 21
etc
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Since when j = 0, sum is not incremented,
we can rewrite the loop as:
sum = 0
for j in range(1, 11):
sum = sum + j
print(sum)
Now the range of j is 1 ≤ j ≤ 10 and the increment
by default is 1.
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If you want the increment to be 2, write:
for j in range(1, 11, 2):
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If you want j to decrease, starting at 10,
and going to 1, write:
for j in range(10, 0, -1):
The -1 indicates a negative increment of 1.
the range of j is 10>= j >= 1.
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In general for a positive for loop increment,
the second number in range must be greater
than the first for the loop to be executed at
least once. So in for j in range(2,2) the loop
will be executed zero times. The second 2
means that the upper limit of the loop index
is 1.
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In general for a negative for loop increment,
the second number in range must be less
than the first for the loop to be executed at
least once. So in for j in range(2,2, -1) the
loop will be executed zero times. The
second 2 means that the limit of the loop
index is 3.
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If you want the sum to be printed only at the
end of the loop’s execution, unindent the
print,
for j in range(1, 11):
sum = sum + j
print(sum)
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In general, in for j in range(n), the range of j
is 0 ≤ j ≤ n-1.
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If you want to build a string consisting of increasing numbers, e.g.,012345678910111213141516171819, trys = ‘’for j in range(0,20): s = s + jprint(s)Because s is a string, the ‘+’ here means concatenation not addition.
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While wont the following program compile?
How do you correct it?
s = ‘’
for j in range(0,20):
s = s + j
print(s)
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Since s is a string, j must be converted to a
string by writing str(j)
s = ‘’
for j in range(0,20):
s = s + str(j)
print(s)
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
1 ‘0’ ’01’
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
1 ‘0’ ’01’
2 ’01’ ’012’
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
1 ‘0’ ’01’
2 ’01’ ’012’
3 ’012’ ’0123’
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
1 ‘0’ ’01’
2 ’01’ ’012’
3 ’012’ ’0123’
4 ’0123’ ’01234’
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
1 ‘0’ ’01’
2 ’01’ ’012’
3 ’012’ ’0123’
4 ’0123’ ’01234’
5 ’01234’ ’012345’
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A table showing how the string s is incremented.
j s s + str(j)
0 ‘’ ‘0’
1 ‘0’ ’01’
2 ’01’ ’012’
3 ’012’ ’0123’
4 ’0123’ ’01234’
5 ’01234’ ’012345’
6 ’012345’ ’0123456’
etc
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How do you get the computer to print only
digits, e.g.,
01234567890123456789
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Let’s pause here and ask, what is 4//10?
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Let’s pause here and ask, what is 4//10?
The answer is 0. So the remainder, 4%10, is
4.
What is 14//10?
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What is 14//10?
The answer is 1. So the remainder, 14%10,
is 4.
How should we rewrite the program to get
01234567890123456789?
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s = ‘’
for j in range(0,20):
n = j%10
s = s + str(n)
print(s)
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Using characters in a for
Given a string s = ‘asd1’, in
for c in s:
print(c)
during execution, the Python virtual machine
iterates though the string s and prints each
character in the string.
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You don’t have to use c and s. So given the string t = ‘asd1’, you could writefor b in t: print(b)and get the same results.
How do you write a program that counts the characters in a string?
inu
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s = ‘asd1’,
n = 0
for c in s:
n = n + 1
print(‘number of characters is ‘, n)
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What happens if instead you write:
s = ‘asd1’,
for c in s:
n = 0
n = n + 1
print(‘number of characters is ‘, n)
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Since n= 0 is in the loop, the counter n is set
to zero for each character.
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Nested loops
How do you produce the following pattern in
which the number of columns and rows is
dictated by the loop indices.
XXXXXX
XXXXXX
XXXXXX
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How do you write a program that produces:
XXXXXX
and then skip to a new line?
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for j in range(6):
print(x, end = ‘’)
print() # skips to a new line
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How do we repeat this pattern 3 times?
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We nest
for j in range(6):
print(x, end = ‘’)
print() # skips to a new line
in another loop
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for k in range(3):
for j in range(6):
print(x, end = ‘’)
print() # skips to a new line
Note that the print() is executed after the
for j loop, so it’s executed three times not six
times. The loop index of the outer loop k must be
different than the loop index of the inner loop, j.
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