Written: August 14, 2026
Digit problems teach modulo arithmetic and loops that peel a number apart safely.
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
The last digit is n % 10. The first digit is what remains after repeatedly dividing by 10 until fewer than 10 remain.
- Read a positive integer n
- last = n % 10
- While n >= 10, n = n / 10
- Sum first (n) and last, then print
Working C example
#include <stdio.h>
int main(void) {
int n, first, last, sum;
printf("Enter a positive integer: ");
if (scanf("%d", &n) != 1 || n < 0) return 1;
last = n % 10;
first = n;
while (first >= 10) {
first /= 10;
}
sum = first + last;
printf("Sum of first and last digit = %d\n", sum);
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.
For a single-digit input, first and last are the same number—confirm your loop handles that.
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.
