C Programming #16: Operator precedence, associativity and order of evalation

In this article there related and important topic will be covered

1. Operator precedence
2. Associativity
3. Order of Evaluation.

Operator precedence and Associativity

Say in mathematics we are given with an expression.

(2 * 3 + 2) - 6 / 2

We know that in maths we use the formula BODMAS - Brackets Of Division or Multiplication and Addition or Subtraction. First thing inside brackets which is 2 * 3 + 2 => 6 + 2 => 8. Expression is now simplified to 8 - 6 / 2 => 8 - 3 => 5. This is called Operator precedence.

Take another example
8 - 2 + 1
Here subtraction and addition have same priority in such case evaluated from left to right. 8 - 2 + 1 => 6 + 1 => 7. This is called Associativity.

More details can be found in this link.

C adds one detail called Order of Evaluation. which is discussed in next part of this article.

Here is table that has Operator precedence and Associativity defined.

Operator	Description	Associativity
++ --	post-increment, post-decrement	left to right
++ -- + - !~ & sizeof	pre-increment, pre-decrement Unary + and Unary - Logical not and bit-wise not Address operator Size of variable/expression	right to left
* / %	Multiplication, division and remainder	left to right
+ -	Addition subtraction	left to right
<< >>	bit-wise left shift, bit-wise right shift	left to right
< <= > >=	Less than or less than or equal to greater than or greater than or equal to	left to right
== !=	Equal, Not Equal	left to right
&	Bit-wise AND	left to right
^	Bit-wise XOR	left to right
|	Bit-wise OR	left to right
&&	Logical AND	left to right
||	Logical OR	left to right
?:	Conditional operator	right to left
=	Assignment, Other Assignment short cut	right to left
,	Comma	left to right

Order of Evaluation

for example we have

2 + 3 + 8

both (1 + 3) + 8 and 1 + (3 + 8) both will same result 12. Even though there is Associativity that tells left to right, the result would be same in both cases. In such cases C standard allows any order of evaluation. Hence order is not guaranteed as the result is same. You might not appreciate now, it becomes glaringly clear if number were some function (covered later).

Operator where the order of evaluation is not guaranteed (when one or more of same operator is used in same parenthesis level)

1. Addition
2. Multiplication
3. Binary AND
4. Binary OR
5. Binary XOR

In all other operators the order of evaluation is guaranteed.

But when it comes to 2 - 3 + 8, it is always (2 - 3) + 8 which is 7. Not 2 - (3 + 8) !

Examples

#include <stdio.h>

int main() 
{
   int a, b, c;

   a = 2 + 3 * 2;
   b = 4 / 2 - 1;
   c = 10 / 2 % 3;
 
   printf("Value of a, b, c is %d, %d, %d\n", a, b, c);
 
   a = b = c = 1;

   printf("Value of a, b, c is %d, %d, %d\n", a, b, c);

   a = b = c == 2;

   printf("Value of a, b, c is %d, %d, %d\n", a, b, c);
 
   return 0;
}

output of the above program is

Value of a, b, c is 8, 1, 2
Value of a, b, c is 1, 1, 1
Value of a, b, c is 0, 0, 1

Links

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