Operator Precedence

C contains many operators, and because of the way in which operator precedence works, the interactions between multiple operators can become confusing.

x=5+3*6;

X receives the value 23, not 48, because in C multiplication and division have higher precedence than addition and subtraction.

char *a[10];

Is a a single pointer to an array of 10 characters, or is it an array of 10 pointers to character? Unless you know the precedence conventions in C, there is no way to find out. Similarly, in E.11 we saw that because of precedence statements such as *p.i = 10; do not work. Instead, the form (*p).i = 10; must be used to force correct precedence.

The following table from C Programming Language, by Kernighan and Ritchie, shows the precedence hierarchy in C. The top line has the highest precedence.

Operators                                  Associativity
( [ - .                                    Left to right
! - ++  -{-  + *  &  (type-cast)  sizeof   Right to left
(in the above line, +, - and * are the unary forms)
*  / %                                     Left to right
+  -                                       Left to right
<<  >>                                     Left to right
<  <=  >  >=                               Left to right
==  !=                                     Left to right
&                                          Left to right
^                                          Left to right
|                                          Left to right
&&                                         Left to right
||                                         Left to right
?:                                         Left to right
=  +=  -=  *=  /=  %=  &=  ^=  |=  <<= >>= Right to left
,                                          Left to right

Using this table, you can see that char *a[10]; is an array of 10 pointers to character. You can also see why the parentheses are required if (*p).i is to be handled correctly. After some practice, you will memorize most of this table, but every now and again something will not work because you have been caught by a subtle precedence problem.