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Loops - Python

In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on. There may be a situation when you need to execute a block of code several number of times.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times. The following diagram illustrates a loop statement −

Loops are required when there is a need to execute a piece of code one or more times. Just like any other language, python too supports loops. Various types of loops can be defined as follows:

-While loop :

Repeats a statement or group of statements while a given condition is TRUE. It tests the condition before executing the loop body. It helps in repeating a block of code till a particular condition holds true.

Format:

while condition:

code_to_execute

EXAMPLE:

a=0
while a<10:
print a
a=a+1

Output: 0;1;2;3;4;5;6;7;8;9

-For loop:

Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable. It uses a loop variable to repeat a block of code. Here are some points that you must keep in mind while working with for loop:

It must reference a list or range
For loop closes with a colon
The code that needs to be executed must be indented by one tab space

Format:

For condition:

Code_to_execute

EXAMPLE

a=[‘Sunday’,’Monday’,’Tuesday’,’Wednesday’,’Thursday’,’Friday’,’Saturday’] for item in a:
print item

Output: Sunday;Monday;Tuesday;Wednesday;Thursday;Friday;Saturday

-Nested loops

You can use one or more loop inside any other “while, for or do..while loop.” They are used in a scenario where there is a need to use one loop inside the other.

Format

for condition1:
for condition2:
code_to_execute2
code_to_execute1
or
while condition1:
while condition2:
code_to_execute2
code_to_execute1

EXAMPLE

Nested for loop
i=[‘a’,’b’,’c’]
j=[‘d’,’e’,’f’]
k=[‘g’,’h’,’i’]
l=[i,j,k]
m=1
for item in l:
print m
for element in item:
print element
m=m+1

Output:

1;a;b;c;2;d;e;f;3;g;h;i

-Nested While loop

x=1
y=10
z=20
while x print ‘x= ‘,x
print ‘x is less than y’
x=x+5
while y print ‘y= ‘,y
print ‘y is greater than x but less than z’
y=y+4

Output:

x= 1; x is less than y; y= 10; y is greater than x but less than z; y= 14; y is greater than x but less than z; y= 18; y is greater than x but less than z; x= 6; x is less than y; x= 11; x is less than y; x= 16; x is less than y; x= 21; x is less than y.

Normal execution of loops can be controlled with the help of control statements. The different types of control statements are described below:

-Break statement:

Terminates the loop execution and jumps to statement just after the loop

Format:

break

EXAMPLE:

x=1
while x<11:
if(x%2==0):
print ‘x is even ‘,x
else:
print ‘x is odd’,x
x=x+1
if(x==5):break
print ‘I will not go further’

Output:

x is odd 1;x is even 2;x is odd 3;x is even 4;I will not go further

-Continue Statement :

Skips the execution of remaining statements of the loop and jumps to the loop’s header line.

Format:

continue

EXAMPLE:

x = 10
while x:
x=x-1
if x%2!=0: continue
print(x,’is even and less than 10′)

Output:

(8, ‘is even and less than 10’); (6, ‘is even and less than 10’); (4, ‘is even and less than 10’);(2, ‘is even and less than 10’);(0, ‘is even and less than 10’)

–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.

Format :

pass.

EXAMPLE :

#!/usr/bin/python
for letter in ‘Python’:
if letter == ‘h’:
pass
print ‘This is pass block’
print ‘Current Letter :’, letter
print “Good bye!”

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