Loops In Python

4 minute read

Introduction

Sometimes the programs are going to be repetitive in nature. For example, suppose there is a problem statement to print 1 to 10(1,2,3,…..,9,10) . By looking into the problem in a deeper way we can see, to print 10 numbers we need 10 print() functions, which is not a scalable and perfect way to write code. The repetition being printing the number, but there is a small change, the number to be printed increases by 1 with each print() statement.

This becomes a necessity to use something which allows repetition in python as well as takes care of the changes.

In python(and every other programming language), such technicality is called as looping and normally python has two primitive loop commands, for and while loop.

A loop statement allows us to execute a statement or group of statements multiple times.

Let’s check the syntax of both the while and for loop and use them to print from 1 to 10.

# syntax - while loop

while expression:
    statement(s)
    

# syntax - for loop

for iterator_var in sequence:
    statements(s)

iterator_var is the variable used for iteration/traversing. Normally the for loop is used in traversing.

Let’s print 1 to 10 using these loops,

# declaring an iteration variable
iter_var = 1

# using the loop method
while(iter_var <= 10):
    
    # the statement which repeats
    print(iter_var,end=" ")
    
    # the change
    iter_var += 1
# using the for loop method
for iter_var in range(1,11):
    print(iter_var,end=" ")
OUTPUT:

1 2 3 4 5 6 7 8 9 10 
1 2 3 4 5 6 7 8 9 10 

Breaking apart the loops,

  • iter_var = 1 this denotes the declaration of an iteration variable and initially it’s set to be 1.
  • while() is the loop and iter_var <= 10 is the condition, here the condition is to print from 1 to 10 which ,means we have to traverse from 1 to 10 and print the value of each iter_var.
  • print(iter_var,end=" ") is the statement which repeats itself with each iteration.
  • iter_var+=1 is the change that we have explained above, with each iteration the number is increased by 1.

Similarly in for loop, we have used something known as range() in python, which is also an in-built function and takes two parameters (a,b), where a is inclusive and b is exclusive, that simply means the traverse will take place from a to b-1.

Nested Loops

Can we place one loop inside another ?

Let’s check that with a piece of code,

for iter_var in range(0,2):
    for iter_ma in range(0,2):
        print(iter_var + iter_ma,end = " ")
    print()
OUTPUT:

0 1 
1 2

Understanding what happened

To understand nested loops always break the loops in the following way,

for iter_var in range(0,2):
    <statement>

This is the outer-loop, when the outer loop executes, the <statement> part of the loop also executes.

The <statement> execution can take place only when the inner loop executes, this means the execution of the entire outer-loop is directly dependent on the results obtained from the inner loops. One of the ways to solve a nested loop is to start executing from the inner loop.

Here due to the inner loop the value of iter_ma changes from 0 to 1, with each execution of the outer loop. It means for every iter_var in the outer loop there exists two iter_ma, which is a better visualization of the problem here.

Loop control statements

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

Python supports the following control statements.

  • break statement

    It terminates the current loop and resumes execution at the next statement. The break statement can be used in both while and for loops. If you are using nested loops, the break statement stops the execution of the innermost loop and start executing the next line of code after the block.

    For example,

    for i in range(0,10):
        if(i==7):
            break
        else:
            print(i,end=" ")
    
    0 1 2 3 4 5 6 
    
  • continue statement

    The continue statement rejects all the remaining statements in the current iteration of the loop and moves the control back to the top of the loop.

    The continue statement can be used in both while and for loops.

    for i in range(0,10):
        if(i==7):
            continue
        else:
            print(i,end=" ")
    
    0 1 2 3 4 5 6 8 9 
    
  • pass statement

    It 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. The pass is also useful in places where your code will eventually go, but has not been written yet.

    for i in range(0,10):
        if(i==7):
            pass
        else:
            print(i,end=" ")
    
    0 1 2 3 4 5 6 8 9 
    

Exercise your way out

Q - For each \(i <= n\), print \(i^2\), where \(n\) is the taken from user.

Q - Write a program to print the following matrix to the terminal,

0 1 2
1 2 3
2 3 4

Q - Write a program to print A to Z, five characters per line.

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