Operator Precedence in C: Understanding Expression Evaluation
Operator precedence determines the sequence in which operators in an expression are executed. This order of precedence can affect the result of an expression substantially. For example, suppose you are to process job applications and you want to only accept applicants who are 25 or older and have graduated from Harvard or Yale. Here’s the age condition you can represent by this conditional expression:
Age >= 25
Suppose that you represent graduation by the variables Yale and Harvard, which may be true or false. Now you can write the condition as follows:
Age >= 25 && Harvard || Yale
Unfortunately, this will result in protest because you will now accept Yale graduates who are under 25. In fact, this statement will accept Yale graduates of any age. But if you’re from Harvard, you must be 25 or over to be accepted. Because of operator precedence, this expression is effectively the following:
(Age >= 25 && Harvard) || Yale
So now you take anybody at all from Yale. I’m sure those wearing a Y-front sweatshirt will claim that this is as it should be, but what you really meant was this:
Age >= 25 && (Harvard || Yale)
Because of operator precedence, you must put the parentheses in the expression to force the order of operations to be what you want.
In general, the precedence of the operators in an expression determines whether it is necessary for you to put parentheses in to get the result you want, but if you are unsure of the precedence of the operators you are using, it does no harm to put the parentheses in. Below table shows the order of precedence for all the operators in C, from highest at the top to lowest at the bottom.
Operator Precedence Table
| Description | Operator | Associativity |
| Function Expression | ( ) | Left to Right |
| Array Expression | [ ] | Left to Right |
| Structure operator | -> | Left to Right |
| Structure operator | . | Left to Right |
| Unary minus | - | Right to left |
| Increment/Decrement | ++ -- | Right to left |
| One’s compliment | ~ | Right to left |
| Negation | ! | Right to left |
| Address of | & | Right to left |
| Value of address | * | Right to left |
| Type cast | ( type ) | Right to left |
| Size in bytes | sizeof | Right to left |
| Multiplication | * | Left to Right |
| Division | / | Left to Right |
| Modulus | % | Left to Right |
| Addition | + | Left to Right |
| Subtraction | - | Left to Right |
| Left shift | << | Left to Right |
| Right shift | >> | Left to Right |
| Less than | < | Left to Right |
| Less than or equal to | <= | Left to Right |
| Greater than | > | Left to Right |
| Greater than or equal to | >= | Left to Right |
| Equal to | == | Left to Right |
| Not equal to | != | Left to Right |
| Bitwise AND | & | Left to Right |
| Bitwise exclusive OR | ^ | Left to Right |
| Bitwise inclusive OR | | | Left to Right |
| Logical AND | && | Left to Right |
| Logical OR | || | Left to Right |
| Conditional | ? : | Right to left |
| Assignment |
= *= /= %= += -= &= ^= |= <<= >>= |
Right to left Right to left Right to left Right to left Right to left |
| Comma | , | Right to left |
For better understanding
| Common operators | ||||||
|---|---|---|---|---|---|---|
| assignment | increment decrement |
arithmetic | logical | comparison | member access |
other |
|
a = b |
++a |
+a |
!a |
a == b |
a[b] |
a(...) |