C Arithmetic Operators

Programs calculate things all the time: a cart total, the time left on a timer, how many buses a school trip needs. In C, you do this math with C arithmetic operators. In this lesson, you'll learn all five of them, why 7 / 2 gives 3 in C, and how the remainder operator % solves everyday problems in a single line.

What are arithmetic operators in C?

An operator is a symbol that tells C to do something with one or more values. The values it works on are called operands. In price * 3, the operator is * and the operands are price and 3.

C has five arithmetic operators:

Here they are in a complete program:

#include <stdio.h>

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

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

    return 0;
}

a + b = 22
a - b = 12
a * b = 85
a / b = 3
a % b = 2

Notice the %% inside the last printf(). Because % starts a format specifier, you write %% to print a single percent sign.

How does division work in C?

This is the part that surprises most beginners. When both operands are integers, C does integer division: it divides and then throws away everything after the decimal point. It does not round. 7 / 2 is 3, and even 9 / 10 is 0.

If at least one operand is a floating-point number (like 7.0 or a double variable), C does normal decimal division instead:

#include <stdio.h>

int main(void) {
    int slices = 7;
    int friends = 2;

    printf("Each friend gets %d slices\n", slices / friends);
    printf("Exact share: %.2f slices\n", 7.0 / 2);
    printf("Exact share: %.2f slices\n", (double) slices / friends);

    return 0;
}

Each friend gets 3 slices
Exact share: 3.50 slices
Exact share: 3.50 slices

The last line uses a cast, (double), to turn slices into a double before dividing. You met casts in the type conversion lesson.

What does the % (modulus) operator do?

The % operator, often called modulus or mod, gives the remainder after integer division. It answers "what's left over?"

There is a neat rule that always holds for integers: (a / b) * b + a % b equals a. For 17 and 5: 3 * 5 + 2 = 17.

Two important limits:

• % works only with integers. 7.5 % 2 is a compile error. For decimals, use fmod() from <math.h> (covered in C math functions).
• Like division, % 0 is not allowed. Never use zero as the right operand.

Division and remainder with negative numbers

In C, integer division always truncates toward zero: it simply chops off the decimal part. So -7 / 3 is -2 (not -3). The remainder then follows from the rule above, which means the result of % takes the sign of the left operand.

#include <stdio.h>

int main(void) {
    printf("-7 / 3 = %d\n", -7 / 3);
    printf("-7 %% 3 = %d\n", -7 % 3);
    printf("7 %% -3 = %d\n", 7 % -3);
    printf("-7 %% -3 = %d\n", -7 % -3);

    return 0;
}

-7 / 3 = -2
-7 % 3 = -1
7 % -3 = 1
-7 % -3 = -1

Check the second line with the rule: (-2) * 3 + (-1) = -7. If you've used Python, note that Python gives 2 for -7 % 3, so code ported between the two languages can behave differently.

Practical uses of the % operator

Remainders show up everywhere once you start looking:

• Even or odd: n % 2 is 0 for even numbers.
• Converting units: 135 minutes is 135 / 60 hours and 135 % 60 minutes.
• Taking turns: with 4 players, turn % 4 cycles 0, 1, 2, 3, 0, 1, ...
• Last digit: n % 10 gives the last digit of a positive number.

#include <stdio.h>

int main(void) {
    int minutes = 135;
    printf("%d minutes = %d h %d min\n", minutes, minutes / 60, minutes % 60);

    int first = 14;
    int second = 9;
    printf("%d %% 2 = %d (0 means even)\n", first, first % 2);
    printf("%d %% 2 = %d (1 means odd)\n", second, second % 2);

    int order_number = 50473;
    printf("Last digit: %d\n", order_number % 10);

    return 0;
}

135 minutes = 2 h 15 min
14 % 2 = 0 (0 means even)
9 % 2 = 1 (1 means odd)
Last digit: 3

Mixing integers and decimals

When an operator gets an int and a double, C converts the int to double first, and the result is a double. This happens for +, -, * and / alike.

#include <stdio.h>

int main(void) {
    int quantity = 3;
    double price = 2.49;
    double total = quantity * price;

    printf("Total: %.2f\n", total);
    printf("Negated: %.2f\n", -total);

    return 0;
}

Total: 7.47
Negated: -7.47

The minus in -total is the unary minus operator: it works on a single operand and flips its sign. There is also a unary plus (+x), which changes nothing and is rarely used.

Real-world example: splitting a bill and packing boxes

Money is safest stored as whole cents in an int, which also makes / and % very handy. Here, three friends split a $100.00 restaurant bill, and a shop packs 50 eggs into cartons of 12.

#include <stdio.h>

int main(void) {
    int bill_cents = 10000;     /* $100.00 */
    int people = 3;

    int share = bill_cents / people;
    int leftover = bill_cents % people;

    printf("Each person pays $%d.%02d\n", share / 100, share % 100);
    printf("Leftover: %d cent\n", leftover);

    int eggs = 50;
    int per_carton = 12;

    printf("Full cartons: %d\n", eggs / per_carton);
    printf("Loose eggs: %d\n", eggs % per_carton);
    printf("Cartons needed: %d\n", (eggs + per_carton - 1) / per_carton);

    return 0;
}

Each person pays $33.33
Leftover: 1 cent
Full cartons: 4
Loose eggs: 2
Cartons needed: 5

The last line is a classic trick for rounding up with integers: adding per_carton - 1 before dividing pushes any partial carton up to a full one. 50 eggs need 5 cartons, even though only 4 are full.

Common mistakes with arithmetic operators

1. Expecting a decimal from integer division

#include <stdio.h>

int main(void) {
    double average = 7 / 2;
    printf("%f\n", average);

    return 0;
}

3.000000

Storing the result in a double is too late: 7 / 2 was already computed as the integer 3. Write 7.0 / 2 or cast one operand.

2. Dividing by zero

Integer division or remainder by zero (x / 0, x % 0) is undefined behavior: C makes no promise about what happens. On many systems the program simply crashes (on Linux you'll often see "Floating point exception"). Make sure the right operand can never be zero.

3. Using % with decimal numbers

double hours = 7.5;
double rest = hours % 2;   /* error */

GCC refuses with error: invalid operands to binary %. Use fmod(hours, 2) from <math.h> instead.

4. Writing a single % in printf()

printf("50% off\n"); confuses printf(), because % o looks like the start of a format specifier. Write printf("50%% off\n");.

Practice exercise

#include <stdio.h>

int main(void) {
    int total_seconds = 3725;

    int hours = total_seconds / 3600;
    int remaining = total_seconds % 3600;
    int minutes = remaining / 60;
    int seconds = remaining % 60;

    printf("%d h %d min %d s\n", hours, minutes, seconds);

    return 0;
}

1 h 2 min 5 s

What's next?

You can now calculate with C. Next, learn shortcuts like += and *= for updating a variable in place in C assignment operators.