Python Program to Reverse a Number

Example 1: Reverse a Number using a while loop

num = 1234
reversed_num = 0

while num != 0:
    digit = num % 10
    reversed_num = reversed_num * 10 + digit
    num //= 10

print("Reversed Number: " + str(reversed_num))

Output

4321

In this program, while loop is used to reverse a number as given in the following steps:

1. First, the remainder of the
   num
   divided by 10 is stored in the variable
   digit
   . Now, the
   digit
   contains the last digit of
   num
   , i.e. 4.
   
   digit
   is then added to the variable reversed after multiplying it by 10. Multiplication by 10 adds a new place in the reversed number. One-th place multiplied by 10 gives you tenth place, tenth gives you hundredth, and so on. In this case,
   reversed_num
   contains
   0 * 10 + 4 = 4
   .
   
   num
   is then divided by 10 so that now it only contains the first three digits: 123.
2. After second iteration,
   digit
   equals 3, reversed equals
   4 * 10 + 3 = 43
   and
   num = 12
   .
3. After third iteration,
   digit
   equals 2, reversed equals
   43 * 10 + 2 = 432
   and
   num = 1
   .
4. After fourth iteration,
   digit
   equals 1, reversed equals
   432 * 10 + 1 = 4321
   and
   num = 0
   .
5. Now
   num = 0
   , so the test expression
   num != 0
   fails and while loop exits. reversed already contains the reversed number 4321.

Example 2: Using String slicing

num = 123456
print(str(num)[::-1])

Output

654321

Using the string slicing concept, you can get reverse the string. ::-1 corresponds to start:stop:step. When you pass -1 as step, the start point goes to the end and stop at the front.