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Operators in C: Arithmetic, Relational, Logical, Assignment and More

Learn C operators with examples: arithmetic operators and the modulus operator, increment and decrement (prefix vs postfix), relational and logical operators with short-circuiting, assignment operators, the ternary operator, sizeof, the comma operator, and precedence.

Beginner6 min readUpdated
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Operators combine values into new values. C's operators are also used, with small differences, by C++, Java, JavaScript and many other languages, so learning them here pays off everywhere.

Arithmetic#

C
int a = 17, b = 5;
printf("%d %d %d %d %d\n", a + b, a - b, a * b, a / b, a % b);
printf("%d\n", -17 % 5);
Output
22 12 85 3 2
-2
  • / with two integers is integer division (Type Conversion).
  • % gives the remainder and only works on integers. With a negative left operand the result is negative (since C99).
  • n % 2 == 0 tests for even numbers; % 10 gives the last digit.

Increment and decrement#

C
int count = 5;
count++;
printf("%d\n", count);

int x = 5;
int y = x++;      /* y gets the old value, then x becomes 6 */
int z = ++x;      /* x becomes 7 first, then z gets 7 */
printf("%d %d %d\n", x, y, z);
Output
6
7 5 7

x++ (postfix) gives the old value; ++x (prefix) gives the new value. Never use the same variable twice in one expression with ++, as in i = i++ + ++i;: that is undefined behaviour.

Assignment operators#

C
int balance = 1000;
balance += 500;
balance -= 200;
balance *= 2;
balance /= 4;
balance %= 300;
printf("%d\n", balance);
Output
50

balance += 500 means balance = balance + 500.

Relational operators#

Comparisons produce 1 (true) or 0 (false), of type int:

C
int age = 20;
printf("%d %d %d\n", age > 18, age == 21, age != 21);
Output
1 0 1
OperatorMeaning
==, !=Equal, not equal
<, >, <=, >=Less, greater, and so on

Logical operators#

OperatorTrue when
&&Both sides are true (non-zero)
||At least one side is true
!The operand is false (zero)
C
int age = 25, has_ticket = 1;
printf("%d\n", age >= 18 && has_ticket);
printf("%d\n", age < 12 || age > 60);
printf("%d\n", !has_ticket);
Output
1
0
0

&& and || short-circuit: the right side is not evaluated if the left side already decides the result. This makes checks like if (p != NULL && p->value > 0) safe.

The ternary operator#

C
int stock = 0;
printf("%s\n", stock > 0 ? "In stock" : "Sold out");
int bigger = 7 > 3 ? 7 : 3;
printf("%d\n", bigger);
Output
Sold out
7

sizeof and the comma operator#

sizeof gives a size in bytes (Data Types). The comma operator evaluates left then right and gives the right value; you will mostly see it in for loop headers: for (i = 0, j = n - 1; i < j; i++, j--).

Precedence#

When an expression has several operators, precedence decides the order. Roughly from highest to lowest:

  1. () [] -> . and postfix ++ --
  2. Unary ! - ++ -- * (dereference) & (address-of) sizeof and casts
  3. * / %
  4. + -
  5. << >>
  6. < <= > >=
  7. == !=
  8. &, then ^, then |
  9. &&, then ||
  10. ?:
  11. Assignment: = += -= ...
C
printf("%d\n", 2 + 3 * 4);
printf("%d\n", (2 + 3) * 4);
printf("%d\n", 10 - 4 - 3);
Output
14
20
3

Use parentheses whenever the order is not obvious. Bitwise operators are covered in Bitwise Operators.

Practice#

Practice exercise

Given a number of seconds, 3725, print it as hours, minutes and seconds (1 h 2 min 5 s) using / and %, and print whether the number of seconds is even.

Show solution
C
#include <stdio.h>

int main(void) {
    int total = 3725;
    int hours = total / 3600;
    int minutes = total % 3600 / 60;
    int seconds = total % 60;
    printf("%d h %d min %d s\n", hours, minutes, seconds);
    printf("%s\n", total % 2 == 0 ? "even" : "odd");
    return 0;
}
Output
1 h 2 min 5 s
odd

Key takeaway#

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