Loops in Python

 Loops in Python

 

Sometimes we may require to repeat a set of statements multiple times continuously, in these situation the Language provides Loop Statement for the same.

 

Python programming language provides three ways for executing the loops. 

While Loop:

 

In Python , while loop is used to execute a block of statements repeatedly until the given condition is satisfied. 

 

Syntax:

while expression:

    statement(s)

 

Example:

 

#Program

 

count=0

while(count<3):

    count=count+1

    print("Hi Student")

Output

Hi Student

Hi Student

Hi Student

 

Here, the variable count is incremented every time we enter the loop and so if the count reaches the value 3, the loop terminates and proceeds to the next.

 

Using else statement with while loops:

 

As discussed above, while loop executes the block until a condition is satisfied . When the condition becomes false, the statement immediately after the loop is executed.

 

The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won’t be executed.

 

It is similar to that of if … else statement except that both blocks will be executed under normal conditions.

 

Syntax:

while condition:

     #executes these statements

else:

     #executes these statements

 

Example

#Python program to illustrate combining else with value

 

count=0

while(count<3):

    count=count+1

    print("Hi Student")

else:

    print("Bye")

Output

Hi Student

Hi Student

Hi Student

Bye

 

While the loop is repeated 3 times the else block is executed only once at the last.

 

Single statement while block:

Just like the short hand if statement, if the while block has only a single statement then we can declare the entire loop in a single line as shown below:

 

#Python program to illustrate Single statement while block

 

Infinite loop

 

count=0

while(count==0):print("Hi Student")

 

Note: It is suggested not to use this type of loops as it is a never ending infinite loop where the condition is always true and you have to forcefully terminate the program execution.

 

for in loop:

For loops are used for sequential traversal. For example : traversing a list or string or array etc. In Python , there is no C language style for loop, i.e., for(i=0;i<n;i++). There is ‘for in’ loop which is similar to for each loop in other languages. Let us learn how to use for in loop for sequential traversals.

 

syntax:

 

for iterator_val in sequence:

    statements(s)

 

It can be used to iterate over a range and also iterators.

Example1:

#Python program to illustrate iterating over range 0 to n-1

 

n=4

for i in range(0,n):

    print(i)

 

Output

0

1

2

3

 

Example 2:

 

#Python program to illustrate Iterating over a list,tuple,string,dictionary and set

 

#List Iteration

print("List Iteration")

l=["hi","hello","welcome"]

for i in l:

    print(i)

 

#Iterating over a tuple(immutable)

print("\nTuple Iteration")

t=("welcome","to","the","world")

for i in t:

    print(i)

#Iterating over a String

print("\n String Iteration")

 

s="Hello World"

for i in s:

    print(i)

#Iterating over dictionary

print("\nDictionary Iteration")

d=dict()

d['xyz']=123

d['abc']=345

for i in d:

    print("%s %d"%(i,d[i]))

#Iterating over a set

print("\n Set Iteration")

set1={1,2,3,4}

for i in set1:

    print(i)

 

Output

 

List Iteration

hi

hello

welcome

 

Tuple Iteration

welcome

to

the

world

 

 String Iteration

H

e

l

l

o

 

W

o

r

l

d

 

Dictionary Iteration

xyz 123

abc 345

 

 Set Iteration

1

2

3

4

 

Iterating by the index of sequences:

 

We can also use the index of elements in the sequence to iterate. The key idea is to first calculate the length of the list and then iterate over the sequence within the range of this length.

 

Example

 

list=["hi","hello","apple"]

for index in range(len(list)):

    print(list[index])

Output

hi

hello

apple

 

Using else statement with for loops:

 

We can also combine else statement with for loop like in while loop. But as there is no condition in for loop based on which the execution will terminate so the else block will be executed immediately after for block finishes execution.

Below example explains how to do this

 

#Python  program to illustrate combining else with for

 

list=["hi","hello","apple"]

for index in range(len(list)):

    print(list[index])

else:

    print("Now inside else Block")

Output

hi

hello

apple

Now inside else Block

 

Nested Loops:

Python programming language allows to use one loop inside another loop. That means there can be a for statement inside a while statement or vice versa or For within For and While within While. Following section shows few examples to illustrate the concept.

 

Syntax:

 

for iterator_var in sequence:

     for iterator_var in sequence:

          statements(s)

 

The syntax for a nested while loop statement in the python programming language is as follows:

 

while expression:

     while expression:

          statement(s)

 

#Python program to illustrate nested for loops in Python

 

for i in range(1,5):

    for j in range(i):

        print(i,end=' ')

    print()

 

Output

 

1

2 2

3 3 3

4 4 4 4

for i in range(1,5):

    for j in range(i):

        print(j+1,end=' ')

    print()

 

Output

1

1 2

1 2 3

1 2 3 4

 

Loop Control Statements:

 

Loop Control statements change execution from their normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements.

 

Continue Statement:

 

It returns the control to the beginning of the loop, from the point from where it is executed thus skipping the statements below it in the Loop.

 

Example

 

#Prints all letters except ‘C’ and ‘ ‘

 

for letter in 'Hello World':

    if letter=='o' or letter==' ':

        continue

    print('Current Letter:',letter)

    var=10

 

Output

Current Letter: H

Current Letter: e

Current Letter: l

Current Letter: l

Current Letter: W

Current Letter: r

Current Letter: l

Current Letter: d

 

Break Statement:

 

Unlike Continue , this statement will shift the control to outside the Loop and terminates it.

Example

 

for letter in 'Hello World':

    #break the loop as soon it sees 'o' or 'e'

    if letter=='o' or letter==' ':

        break

    print('Current Letter:',letter)

   

Output

Current Letter: H

Current Letter: e

Current Letter: l

Current Letter: l

 

Pass Statement:

 This is useful when we want to test run a set of statements before finishing it completely.

 

Example

#An empty loop

for letter in 'Hello World':

    pass

print('Last Letter:',letter)

   

Output

Last Letter: d