Arithmetic Operators

C provides five basic arithmetic operators: + (addition), - (subtraction), * (multiplication), / (division), and % (modulo, the remainder of integer division). These operators work on integer and floating-point types, and understanding exactly how they behave — especially with mixed types and negative numbers — avoids some of the most common bugs in beginner C code.

#include <stdio.h>

int main(void) {
    int a = 10, b = 3;

    printf("%d\n", a + b); // 13
    printf("%d\n", a - b); // 7
    printf("%d\n", a * b); // 30
    printf("%d\n", a / b); // 3  (integer division truncates)
    printf("%d\n", a % b); // 1  (remainder)
    return 0;
}

Integer Division Truncates

7 / 2 is 3, not 3.5

When both operands of / are integers, C performs integer division: the fractional part is discarded (truncated toward zero). This trips up many beginners who expect division to always yield a precise, fractional answer. To get a fractional result, at least one operand must be a floating-point type.

int a = 7, b = 2;
printf("%d\n", a / b);              // 3   (integer division)
printf("%f\n", (double) a / b);     // 3.5 (one operand cast to double)
printf("%f\n", 7.0 / 2);            // 3.5 (a literal 7.0 forces double math)

Modulo Only Works on Integers

The % operator is only defined for integer operand types in standard C — writing 10.5 % 3 is a compile error. To compute a remainder for floating-point values, use fmod() from <math.h> instead.

#include <math.h>
#include <stdio.h>

int main(void) {
    int wholeRemainder = 10 % 3;              // 1 — fine, both operands are int
    /* double bad = 10.5 % 3; */              // ERROR: % does not accept doubles
    double floatRemainder = fmod(10.5, 3.0);  // 1.5 — use fmod instead

    printf("%d %f\n", wholeRemainder, floatRemainder);
    return 0;
}

Mixing int and float in Arithmetic

When an expression mixes an int with a double (or float), C implicitly promotes the integer operand to the floating-point type before performing the operation — this is called the "usual arithmetic conversions". The result of the whole expression is then a floating-point value.

int count = 3;
double total = 10.0;

double average = total / count; // count is promoted to 3.0, average is 3.333...

Modulo with Negative Numbers

Truncation toward zero since C99

For negative operands, the sign of the result of % was implementation-defined before C99. Since C99, both / and % truncate toward zero, so the result of % has the same sign as the dividend (the left operand). For example, -7 % 2 is -1, and 7 % -2 is 1.

printf("%d\n", -7 % 2);  // -1 (C99+: truncates toward zero)
printf("%d\n", 7 % -2);  //  1
printf("%d\n", -7 % -2); // -1

+, -, *, /, % are the five arithmetic operators in C
Integer / truncates toward zero; cast an operand to double for a fractional result
% only accepts integer operands — use fmod() for floating-point remainders
Mixed int/float expressions promote the integer operand to floating-point
Since C99, % truncates toward zero and matches the sign of the dividend