Operator Precedence

Operator Precedence Table

C has a a lot of implied behavior, operator chaining, and undefined behavior traps. Hence the importance of operator precedence.

Empty cells copy the same contents as the one above it.

Precedence	Operator	Description	Associativity
1	()	Parentheses (function call)	Left-to-Right
	[]	Array access
	.	For structs (access fields)
	->	struct . and dereference (*)
	++, --	Postfix increment and decrement
2	++, --	Prefix increment, decrement	Right-to-Left
	+, -	Unary plus/minus, related to the signs of numbers (pos/neg)
	!, ~	Logical NOT, Bitwise complement
	(type)	Type casting
	*	Dereference operator
	&	Get memory address
	sizeof	Get the size of something in bytes
3	*, /, %	Multiplication, division, modulus	Left-to-Right
4	+, -	Addition, subtraction	Left-to-Right
5	<<, >>	Bitwise shift left, Bitwise shift right	Left-to-Right
6	<, <=	Less than, less than or equal to	Left-to-Right
	>, >=	Greater than, greater than or equal to
7	==, !=	Is equal to, is not equal to	Left-to-Right
8	&	Bitwise AND	Left-to-Right
9	^	Bitwise XOR	Left-to-Right
10	\|	Bitwise OR	Left-to-Right
11	&&	Logical AND	Left-to-Right
12	\|\|	Logical OR	Left-to-Right
13	? (expr) : (expr)	(CONDITION) ? (expr) : (expr) (Tenary conditional)	Right-to-Left
14	=	Assignment	Right-to-Left
	+=, -=	Addition, subtraction assignment
	*=, /=	Multiplication, division assignment
	%=, &=	Modulus, bitwise AND assignment
	^=, \|=	Bitwise exclusive, inclusive OR assignment
	<<=, >>=	Bitwise shift left, right assignment
15	,	Comma (expression separator)	Left-to-Right

Postfix vs Prefix

#include <stdio.h>

int main(void)
{
    int i, j = 0;
    printf("i: %d j: %d", i++ + 1, ++j + 1);
    return 0;
}

Postfix returns the value of the variable first, then does the increment (or decrement w/ i-- or j--), and the opposite goes for prefix.

So the value of the printf for i++ + 1 would be 1, but after that, i’s value would be 1 (due to i++). And the value for ++j + 1 would be 2, but since the + 1 isn’t a += ,++, or something similar, j’s value would be 1 (again, due to ++j).

When you’re declaring a for loop: (etc)

for (int i = 0; i < n; ++i)
    ;

It doesn’t really matter, purely user preference.

What about expressions like these?

int a[3] = {1, 2, 3};
int i = 3;

while (i--)
{
    ... // code
}

int a[3] = {1, 2, 3};

int * b = a;
for (int i = 0; i < 3; ++i)
    *b++ = 0; // set everything to 0

Remember that postfix ++ or -- doesn’t increment on the spot, AND it first returns the current value of the variable (the value before it pluses/minuses 1). It will increment once the expression is finished. It’s like a “I will increment soon” signal.

So the i-- on the first iteration will go through because the while loop condition is while (3), but once you go into the loop body, i is 2.

#C #C/conceptual