Python Loops

Sometimes we want to repeat a set of statements in our program. For instance: Print 1 to 1000.

Loops make it easy for a programmer to tell the computer, which set of instructions to repeat, and how!

What is Loops?

In programming, loops are a sequence of instructions that does a specific set of instructions or tasks based on some conditions and continue the tasks until it reaches certain conditions.

Types of loops in Python

Python provides three types of looping techniques:

LoopDescription
for LoopThis is traditionally used when programmers had a piece of code and wanted to repeat that ‘n’ number of times.
while LoopThe loop gets repeated until the specific Boolean condition is met.
Nested LoopsProgrammers can use one loop inside another; i.e., they can use for loop inside while or vice – versa or for loop inside for loop or while inside while.

Primarily there are two types of loops in Python

  1. While loop
  2. For loop

We will look into this one by one!

While loop

The syntax of a while loop looks like this:

''' while condition:
    #Body of the loop '''

  • The block keeps executing until the condition is true/false

In while loops, the condition is checked first. If it evaluates to true, the body of the loop is executed, otherwise not!

If the loop is entered, the process of condition check and execution is continued until the condition becomes false.

Quick Quiz: Write a program to print 1 to 50 using a while loop.

An Example:

i = 0
while i<5:
    print(“Sandeep”)
    i = i+1

(Above program will print Sandeep 5 times)

Note: if the condition never becomes false, the loop keeps getting executed.

Quick Quiz: Write a program to print the content of a list using while loops.

For loop

A for loop is used to iterate through a sequence like a list, tuple, or string (iterables)

The syntax of a for loop looks like this:

l = [1, 7, 8]
for item in l:
	print(item)

(Above program will print 1, 7, and 8)

Range function in Python

The range function in python is used to generate a sequence of numbers.

We can also specify the start, stop, and step-size as follows:

            range(start, stop, step_size)

  • step size is usually not used with range()

An example demonstrating range() function

for i in range(0, 7):		#range(7) can also be used
	print(i)		#prints 0 to 6

For loop with else

An optional else can be used with a for loop if the code is to be executed when the loop exhausts.

Example:

l = [1, 7, 8]
for item in l:
	print(item)
else:
	print(“Done”)	#This is printed when the loop exhausts!

Output:

1

7

8

Done

Python Loop Control Statements

Control StatementsDescription
Break statementIt is used to exit a while loop or a for a loop. It terminates the looping & transfers execution to the statement next to the loop.
Continue statementIt causes the looping to skip the rest part of its body & start re-testing its condition.
Pass statementIt is used in Python to when a statement is required syntactically, and the programmer does not want to execute any code block or command.

The break statement

‘break’ is used to come out of the loop when encountered. It instructs the program to – Exit the loop now.

Syntax:

break

Example:

for i in range(0, 80):
	print(i)	#This will print 0, 1, 2 and 3
	if i == 3:
		break

The continue statement

‘continue’ is used to stop the current iteration of the loop and continue with the next one. It instructs the program to “skip this iteration.”

Example:

for i in range(4):
	print(“printing”)
	if i == 2:	#if i is 2, the iteration is skipped
		continue
	print(i)

pass statement

pass is a null statement in python. It instructs to “Do nothing.”

Example:

l = [1, 7, 8]
for item in l:
	pass		#without pass, the program will throw an error

Exercise On Python Loops

  1. Write a program to print the multiplication table of a given number using for loop.
  2. Write a program to greet all the person names stored in a list l1 and which starts with S.
l1 = [“Sandeep”, “Sohan”, “Sachin”, “Rahul”]
  1. Attempt problem 1 using a while loop.
  2. Write a program to find whether a given number is prime or not.
  3. Write a program to find the sum of first n natural numbers using a while loop.
  4. Write a program to calculate the factorial of a given number using for loop.
  5. Write a program to print the following star pattern.
    *

  ***  

***** for n=3
  1. Write a program to print the following star pattern:
*

**

*** for n = 3
  1. Write a program to print the following star pattern:
* * *
*   *             #For n=3
* * * 

   10. Write a program to print the multiplication table of n using for loop in reversed order.

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