Arithmetic Operators — Math in C

C in 100 Seconds, Episode 4

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C gives you five arithmetic operators. Four of them work exactly like you'd expect. The fifth one has a catch.

The Five Operators

int a = 17, b = 5;

a + b   // 22  — addition
a - b   // 12  — subtraction
a * b   // 85  — multiplication
a / b   // 3   — division (integer!)
a % b   // 2   — modulus (remainder)

Addition, subtraction, and multiplication do what you'd expect. Modulus gives you the remainder after division. But division? That's where it gets interesting.

The Integer Division Gotcha

When you divide two integers, C throws away the decimal part:

17 / 5    // 3, not 3.4

The result is truncated, not rounded. C doesn't promote the result to a float — it stays as an integer because both operands are integers.

To get the decimal result, make at least one operand a float:

17.0 / 5  // 3.4

That .0 changes everything. Now C performs floating-point division instead.

Why Does This Matter?

Integer division is one of the most common sources of bugs in C. Calculate an average with sum / count where both are int, and you'll get a truncated result with no warning. C trusts you to know the difference.

Full Code

#include <stdio.h>

int main() {
  int a = 17, b = 5;

  printf("%d + %d = %d\n", a, b, a + b);
  printf("%d - %d = %d\n", a, b, a - b);
  printf("%d * %d = %d\n", a, b, a * b);
  printf("%d / %d = %d\n", a, b, a / b);
  printf("%d %% %d = %d\n", a, b, a % b);

  printf("\n17 / 5 = %d (integer division truncates)\n", 17 / 5);
  printf("17.0 / 5 = %.1f (use float for decimals)\n", 17.0 / 5);

  return 0;
}

Compile and Run

gcc arithmetic.c -o arithmetic
./arithmetic

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Next episode: If/Else — making decisions in your code.

Student code: github.com/GoCelesteAI/c-in-100-seconds/episode04