C Program Reverse digits in an integer number

The program segment given below obtains a number by reversing the digits in the given number.

scanf(“%d”, &num);

rev_num = 0;

do {

digit = num % 10;

rev_num = rev_num * 10 + digit;

num /= 10;

} while(num > 0);

printf(“reverse number= %d”, rev_num);

This program segment first accepts a number num from the keyboard and initializes variable rev_ num to zero. Then it uses a do…while loop to separate the digits in num and construct number rev_num in reverse order of digits, one digit at a time. Observe carefully the statement used to obtain the reversed number:

rev_num = rev_num * 10 + digit;

In this statement, the current value of rev_num is first multiplied by I0 and the value of digit is added to it. The steps in the formation of the reverse number are illustrated below for num = 1234.

The code to determine the reverse number can also be written using the while loop. The code given below avoids use of variable digit. The I/O statements are omitted to save space.

rev_num = 0;

while (num > 0) {

rev_num = rev_num * 10 + num % 10;

num /= 10;

This code can be made more concise using the for loop as shown below.

for (rev_num = 0; num > 0; num /= 10)

rev num = rev num * 10 + num % 10;