Written: August 15, 2026
Reversing digits trains the modulo pattern: peel the last digit, append it to a growing result, shrink the original.
This walkthrough keeps the logic small and readable so you can type it, run it, and then change one input at a time to see what happens.
What you will build
While n > 0: rev = rev*10 + n%10; n /= 10.
- Read a non-negative integer n
- Initialize rev = 0
- Loop: take last digit, shift rev left by one decimal place, add digit
- Print rev
Working C example
#include <stdio.h>
int main(void) {
int n, rev = 0;
printf("Enter a non-negative integer: ");
if (scanf("%d", &n) != 1 || n < 0) return 1;
while (n > 0) {
rev = rev * 10 + n % 10;
n /= 10;
}
printf("Reversed = %d\n", rev);
return 0;
}How the logic works
Read the program top to bottom: includes and main first, then the statements that change variables, then the print that proves the result.
If your output looks wrong, print the variables before and after the critical lines. That single habit catches most beginner bugs faster than rewriting the whole file.
Trailing zeros in the input disappear in the integer reverse (120 → 21). Mention that if your lab expects a string reverse instead.
Common mistakes
Forgetting a semicolon, using the wrong format specifier in printf/scanf, and mixing up assignment (=) with comparison (==) are the usual culprits.
Compile with warnings enabled (`gcc -Wall`) so the compiler points at risky casts and unused variables before you chase them by hand.
Try this next
Change the sample inputs, add a second test case, and briefly note what stayed the same. Teaching yourself with tiny experiments sticks better than copying a longer program you never run.
When you can explain every line without looking, you are ready for the next exercise in the series.
