Python Loops

Python For Loop

for loop in Python can be used to iterate over a sequence (list, tuple, string) or other iterable objects

It allows to execute one or more statements for each item of the sequence

Example

Print each fruit in a fruit list

fruits = ["lemon", "orange", "melon"]
for x in fruits:
    print(x) 

Output

lemon
orange
melon

Alternatively, indexes can be used to derive the same

fruits = ["lemon", "orange", "melon"]
for x in range(0, len(fruits)):
    print(fruits[x]) 

Above code uses len() function to get size of fruits list

and range() function to get integer values from 0 to (excluding) size of the list

These values are then used as indexes to retrieve items from fruits list

Syntax of Python for Loop

for item in sequence:
    Body of for 

Here, item is a variable that takes the value of the corresponding item of a sequence for each iteration

Loop continues until last item in the sequence, unless break statement is used (or statements like System.exit())

The body of for loop is supposed to be indented (with spaces or tabs), otherwise it throws error

Python uses indentation as its method of grouping statements

In Python, all the statements indented by the same number of space characters are considered to be part of a single block of code

Python while Loop

A while loop statement in Python programming language repeatedly executes a target statement(or block of statements) as long as a given condition is true

Syntax

The syntax of a while loop is -

while code:
   statement(s) 

Here, statement(s) may be a single statement or a block of statements

condition may be any expression, which is evaluated and the loop iterates if it is True

Any non-zero value is also considered as true

When the condition is tested and the result is false, the loop body is skipped and the first statement after the while loop is executed

While loop might not ever run, in case the condition is initially False

Example

#!/usr/bin/python

count = 0
while(count <9):
    print('The count is:', count)
    count = count +1 

When above code is executed, it produces following result

The count is: 0
The count is: 1
The count is: 2
The count is: 3
The count is: 4
The count is: 5
The count is: 6
The count is: 7
The count is: 8 

In above example an indexing variable count is defined which is set to 0

The block consisting of the print and increment statements, is executed repeatedly until count is no longer less than 9

With each iteration, the value of the index count is displayed and then increased by 1

Python break Statement

With a break statement, a loop can be stopped during its iteration

Example

Following example uses break to exit a while loop when value of i is 3:

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1 

Example

With a break statement, a loop can be stopped before it has looped through all the items

Exit the loop when x is "orange":

fruits = ["lemon", "orange", "melon"]
for x in fruits:
    print(x)
    if x == "orange":
        break 

Looping Through a String in Python

In Python, Strings are iterable objects, which contain a sequence of characters

Example

Looping through the letters in the word "bazinga":

for x in "bazinga":
    print(x)

Output

b
a
z
i
n
g
a 

Python range() function

range() function can be used to retrieve a sequence of numbers or to loop through a set of code for a specified number of times

range(10) generates numbers from 0 to 9 (10 numbers)

The start, stop and step size can also be defined as range(start, stop, step size)

If a single argument is provided, it is taken as the value of stop parameter, which decides the last value of the sequence

start is 0 by default and step size defaults to 1 if not provided

Example

print from 1 to 5 using range() function:

>>> for x in range(6):
...    print(x)
...
0
1
2
3
4
5

The starting value can be specified by adding a parameter like range(2, 6) which returns values from 2 to 6 (excluding 6)

>>> for x in range(2, 6):
...     print(x)
...
2
3
4
5 

Example using step size as 3

>>> for x in range(2, 20, 3):
...     print(x)
...
2
5
8
11
14
17

range function does not store all the values in memory but generates the next number on the go

To force this function to output all the items, the list() function can be used

>>> range(3, 8)
range(3, 8)
>>> list(range(3, 8))
[3, 4, 5, 6, 7]

Python Infinite Loop

A loop becomes infinite loop if a condition never becomes False

This results in a loop that never ends

It can be as a result of human error

An infinite loop might be useful in client/server programming where the server needs to run continuously so that client programs can communicate with it as and when required

#!/usr/bin/python
var=1
while var==1: # This constructs an infinite loop, assuming value of var is not changed
    num = raw_input("Enter a number :")
    print(num, "is entered")
print("Unreachable") 

Above code can be executed as :

Enter a number  :20
20 is entered
Enter a number  :29
29 is entered
Enter a number  :3
3 is entered
Enter a number :Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
KeyboardInterrupt 

Since above example goes in an infinite loop, the program has to be exited explicitly (using CTRL+C etc)

Python continue Statement

continue statement is used to proceed to next iteration of a loop, without executing any further statement(s) of current iteration

Example

Continue to the next iteration if i is 3:

i = 0
while i < 7:
    i += 1 
    if i in [2, 5]:
        continue
    print(i) 

This produces following output

1
3
4
6
7 

Example

Do not print orange :

fruits = ["lemon", "orange", "melon"]
for x in fruits:
    if x is "orange":
        continue
    print(x) 

Output

lemon
melon

It is used when one or more statements are to be omitted for specified conditions while iterating

Python pass Statement

The pass statement is a null operation; nothing happens when it executes

pass is of use in places where code is unimplemented (skeleton code) since, after a conditional statement at least one statement is required in an indented block

Syntax

pass 

Example

#!/usr/bin/python
for letter in 'Python':
    if letter =='h':
        pass
        print('This is pass block')
    print('Current Letter :',letter) 

Output

Current Letter : P
Current Letter : y
Current Letter : t
This is pass block
Current Letter : h
Current Letter : o
Current Letter : n 

Using else Statement with Loops

Python supports an else statement associated with a loop statement

• If else statement is used with a for loop, the else statement is executed when the loop has exhausted iterating the list

• If else statement is used with a while loop, the else statement is executed when the condition becomes false

else with while loop

Following example uses an else statement with a while statement that prints a number as long as it is less than 5, otherwise else statement gets executed

#!/usr/bin/python

count = 0
while count<5:
    print(count, "is less than 5")
    count = count + 1
else:
    print(count," is not less than 5") 

When above code is executed, it produces following result -

0 is less than 5
1 is less than 5
2 is less than 5
3 is less than 5
4 is less than 5
5 is not less than 5 

else with for loop

A for loop can also have an optional else block

The else keyword in a for loop specifies a block of code to be executed when the loop is finished:

In case a break statement is used to stop a for loop, the else part is ignored

Example

Print all numbers from 0 to 5, and print a message when the loop has ended:

for x in range(6):
    print(x)
else:
    print("Out of range") 

Output

0
1
2
3
4
5
Out of range