C Operators

C offers us a wide variety of operators that we can use to operate on data.

In particular, we can identify various groups of operators:

• arithmetic operators
• comparison operators
• logical operators
• compound assignment operators
• bitwise operators
• pointer operators
• structure operators
• miscellaneous operators

In this blog post I’m going to detail all of them, using 2 imaginary variables a and b as examples.

I am keeping bitwise operators, structure operators and pointer operators out of this list, as I will dedicate them a specific blog post.

Arithmetic operators

In this macro group I am going to separate binary operators and unary operators.

Binary operators work using two operands:

Unary operators only take one operand:

The difference between a++ and ++a is that a++ increments the a variable after using it. ++a increments the a variable before using it.

For example:

int a = 2;
int b;
b = a++; /* b is 2, a is 3 */
b = ++a; /* b is 4, a is 4 */

The same applies to the decrement operator.

Comparison operators

Logical operators

• ! NOT (example: !a)
• && AND (example: a && b)
• || OR (example: a || b)

Those operators are great when working with boolean values.

Compound assignment operators

The plain assignment operator is =. These compound operators assign a value and run an arithmetic operation at the same time:

Miscellaneous operators

The ternary operator

The ternary operator is the only operator in C that works with 3 operands, and it’s a short way to express conditionals.

This is how it looks:

<condition> ? <expression> : <expression>

Example:

a ? b : c

If a is non-zero, then b is evaluated, otherwise c is. Those are expressions, not statements.

The ternary operator does the same job as an if/else conditional, except it is shorter and it can sit inside an expression.

sizeof

The sizeof operator returns the size of the operand you pass. You can pass a variable, or even a type.

sizeof gives a size_t. Print it with %zu (the conversion specifier added in C99). %ld is the wrong type, and it breaks on platforms where long is 32-bit.

Example usage:

#include <stdio.h>

int main(void) {
  int age = 37;
  printf("%zu\n", sizeof(age));
  printf("%zu", sizeof(int));
}

Operator precedence

With all those operators (and more, which I haven’t covered in this post, including bitwise, structure operators and pointer operators), we must pay attention when using them together in a single expression.

Suppose we have this operation:

int a = 2;
int b = 4;
int c = b + a * a / b - a;

What’s the value of c? Do we get the addition being executed before the multiplication and the division?

There is a set of rules that help us solving this puzzle.

In order from less precedence to more precedence, we have:

• the = assignment operator
• the + and - binary operators
• the * and / operators
• the + and - unary operators

Operators also have an associativity rule, which is always left to right except for the unary operators, the assignment operators, and the ternary operator ?: (those are right to left).

In:

int c = b + a * a / b - a;

We first execute a * a / b, which due to being left-to-right we can separate into a * a and the result / b: 2 * 2 = 4, 4 / 4 = 1.

Then we can perform the sum and the subtraction: 4 + 1 - 2. The value of c is 3.

In all cases, however, I want to make sure you realize you can use parentheses to make any similar expression easier to read and comprehend.

Parentheses have higher priority over anything else.

The above example expression can be rewritten as:

int c = b + ((a * a) / b) - a;

and we don’t have to think about it that much.

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