Python Programming Essentials - M16 - Control Flow Statements and Loops

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Control flow statements
Pavan Verma
@YinYangPavan
Founder, P3 InfoTech Solutions Pvt. Ltd.
Python Programming Essentials
1

© SkillBrew http://skillbrew.com
Truth value testing
Any object can be tested for truth value, for use in
an if or while condition or as operand of the Boolean operations
below. The following values are considered False:
1. None
2. False
3. zero of any numeric type, for example 0, 0L, 0.0, 0j
4. any empty sequence, for example: '', ( ), [ ]
5. any empty mapping, for example: { }
2

Truth value testing (2)
3
mylist = []
if mylist:
print mylist
else:
print "list is empty“
Output:
list is empty
var1 = 0
if var1:
print “inside if"
else:
print "inside else“
Output:
inside else

Truth value testing (3)
4
var1 = None
if var1:
print "inside if"
else:
print "inside else“
Output:
"inside else"

Built in Constants
5
Constant Description
True The true value of the bool type
False The false value of the bool type
None None is frequently used to
represent the absence of a value,
as when default arguments are
not passed to a function

Built in Constants (2)
6
>>> 3 > 2
True
>>> 3 < 2
False
>>> True + 4
5
Internally True is represented as 1 and
False is represented as 0

if statement
7
if statement lets you make decisions
If the expression evaluates to True the if
block is executed
if expression:
# statements

if statement
from random import randint
dice = randint(1, 6)
guess = int(raw_input('Guess the number:'))
if guess == dice:
print "Wohoo you got lucky“
else:
print "Better luck next time. The number
was %d" % dice
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True

if-elif-else
guess = int(raw_input('Guess the number: '))
if guess == dice:
elif guess < dice:
print "You guessed lower“
else:
print "You guessed higher"
9

for loop
10
for loop lets you iterate over
a sequence
Iterable is a stream of values
like lists, strings, tuples,
dictionary
for item in iterable:
# do something with item

for loop (2)
11
>>> colors = ['red', 'green', 'yellow', 'brown']
>>> for color in colors:
print color
Output:
red
green
yellow
brown

range function
12
>>> range(5)
[0, 1, 2, 3, 4]
>>> for i in range(5):
print i
Output:
0
1
2
3
4
range creates lists containing arithmetic
progressions

range function (2)
13
>>> range(4, 10)
[4, 5, 6, 7, 8, 9]
>>> range(4, 10, 2)
[4, 6, 8]
range(start, end) can take
start and end as inputs. It will
generate list from start to end
(excluding)
range(start, end, step)
can also take an optional third
parameter step

Access index in for loop
14
>>> for index, color in enumerate(colors):
print "color {0} at pos {1}".format(color, index)
Output:
color red at pos 0
color green at pos 1
color yellow at pos 2
color brown at pos 3

zip function
15
>>> countries = ['India', 'America', 'Australia']
>>> capitals = ['Delhi', 'Washington DC', 'Canberra']
>>> zip(countries, capitals)
Output:
[('India', 'Delhi'), ('America', 'Washington DC'),
('Australia', ‘Canberra')]
zip: Takes in a pair of streams and gives
you a stream of pairs

zip function (2)
16
>>> countries = ['India', 'America', 'Australia']
>>> capitals = ['Delhi', 'Washington DC', ‘Canberra']
>>> for country, capital in zip(countries, capitals):
print “{0} is capital of {1}“.format(capital,
country)
Output:
Delhi is capital of India
Washington DC is capital of America
Canberra is capital of Australia

while loop
keep_guessing = True
while keep_guessing:
if guess == dice:
print "Wohoo you got lucky"
keep_guessing = False
elif guess < dice:
else:
print "You guessed higher“
print "while loop is over"
17
while condition:
# loops till the condition is true

Infinite loops and break statement
if guess == dice:
elif guess < dice:
else:
18
variable keep_guessing always
stays True, hence the while
loop becomes an infinite loop

Infinite loops and break statement (2)
if guess == dice:
break
elif guess < dice:
else:
19
The break statement is used to
break out of a loop statement

continue statement
while True:
if guess > 6:
print "Hey enter a number between 1-6"
continue
if guess == dice:
break
else:
print "Guess Again"
20
The continue statement will
skip the rest of the statements
in the current loop block and
start from beginning of the
block

pass statement
 The pass statement in Python is used when a
statement is required syntactically but you do not
want any command or code to execute
 The pass statement is a null operation; nothing
happens when it executes.
 pass is also useful in places where your code will
eventually go, but has not been written yet
21

pass statement (2)
22
for i in range(10):
pass
def donothing():
pass
class DoNothing():
pass

Summary
 Truth value testing
 Built in constants
 If statement
 for loop
 while loop
 break, continue, pass
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Resources
 Constants True, False, None
http://docs.python.org/2/library/constants.html
 break statement
http://www.tutorialspoint.com/python/python_break_statement.htm
 range function http://docs.python.org/2/library/functions.html#range
 continue statement
http://www.tutorialspoint.com/python/python_continue_statement.htm
 pass statement
http://www.tutorialspoint.com/python/python_pass_statement.htm
 Loops tutorials http://www.pythonforbeginners.com/loops/
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