Python Program to Reverse a Number

This article is created to cover some programs in Python, that find and prints reverse of a number entered by user. Here are the list of approaches used:

  • Find reverse of a number using while loop
  • Using for loop
  • Using list
  • Using function
  • Using recursion

Reverse a Number using while Loop

To find reverse of a given number in Python, you have to ask from user to enter a number, then reverse and print like shown in the program given below. The question is, write a Python program to reverse a number. Here is its answer:

print("Enter a Number: ")
num = int(input())

rev = 0
while num!=0:
  rem = num%10
  rev = rem + (rev*10)
  num = int(num/10)

print("\nReverse =", rev)

Here is its sample run:

find reverse of number python

Now supply the input say 236 as number, press ENTER key to find and print its reverse like shown in the snapshot given below:

reverse of number python

The dry run of above program with 236 as user input, goes like:

  • When user enters the number say 236, it gets stored in num. So num=236
  • The condition (of while loop) num!=0 or 236!=0 evaluates to be true, therefore program flow goes inside the loop and executes all the three statements present inside the block of this loop
  • So num%10 or 236%10 or 6 gets initialized to rem
  • Then rem + (rev*10) or 6 + (0*10) (rev=0 initially) or 6 gets initialized to rev
  • Finally int(num/10) or int(236/10) or int(23.6) or 23 gets initialized to num. Now the condition of while loop again gets evaluated with new value of num
  • That is, the condition num!=0 or 23!=0 again evaluates to be true, therefore program flow again goes inside the loop
  • This process continues until the condition evaluates to be false
  • In this way, after exiting from the loop, the variable rev holds the reverse of given number
  • So print the value of rev on output, as reverse of a number, just after exiting from the loop

Modified Version of Previous Program

I've included try-except block in this program, to handle with invalid inputs. The end used here to skip insertion of an automatic newline. And the str() method converts into a string type value.

print("Enter a Number: ", end="")
try:
  num = int(input())
  rev = 0
  temp = num
  while num!=0:
    rev = (num%10) + (rev*10)
    num = int(num/10)
  print("\nReverse of " +str(temp)+ " is " +str(rev))
except ValueError:
  print("\nInvalid Input!")

Here is its sample run with user input 549:

python find reverse of a number

Reverse a Number using for Loop

This program uses for loop to do the same job, that is of reversing a number. Let's have a look at the program first:

print("Enter a Number: ", end="")
num = int(input())

rev = 0
numlen = len(str(num))
temp = num

for i in range(numlen):
  rev = (num%10) + (rev*10)
  num = int(num/10)

print("\nReverse of " +str(temp)+ " is " +str(rev))

Produces same output as of previous program. The following code (from above program):

for i in range(numlen):

is used to execute following statements (present inside the block of above for loop):

rev = (num%10) + (rev*10)
num = int(num/10)

numlen number of times. For example, if given number is 549, then using the following statement:

numlen = len(str(num))

the length of str(549) or "549" gets initialized to numlen, that is 3. So numlen=3. Therefore both statements present inside for loop's block, executes 3 times.

Note - The statements inside for loop gets executed x number of times. The value of x indicates the total digit of given number.

Reverse a Number using List

Now this program uses list to reverse a given number. The append() is used to add or append new element (number here) at the end of list.

print("Enter a Number: ", end="")
num = int(input())

temp = num
nums = []
while num!=0:
  rem = num%10
  nums.append(rem)
  num = int(num/10)

print(end="\nReverse of " +str(temp)+ " is ")
for i in range(len(nums)):
  print(nums[i], end="")
print()

Reverse a Number using Function

This program uses a user-defined function named rev(), that returns reverse of a number passed as its argument.

def rev(n):
  r = 0
  while n!=0:
    r = (n%10) + (r*10)
    n = int(n/10)
  return r

print("Enter a Number: ", end="")
num = int(input())

print("\nReverse of " +str(num)+ " is " +str(rev(num)))

Reverse a Number using Recursion

This is the last program of this article, created using a recursive function. That is, a recursive function named rev() is implemented here to return reverse of a number. The second argument passed to this function is the argument corresponds to rev (as defined in previous programs), with its initial value as 0

def rev(n, r):
  if n==0:
    return r
  else:
    return rev(int(n/10), (n%10)+(r*10))

print("Enter a Number: ", end="")
num = int(input())

print("\nReverse of " +str(num)+ " is " +str(rev(num, 0)))

Same Program in Other Languages

Python Online Test


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