Selected Help on C

[Last modified : 2003.07.09]

Contents

• Operator Precedence and Associativity
• Value of Equality and Relational Expressions
• Preprocessing

See Also

• The C Standard Library

Operators Precedence and Associativity

From highest to lowest precedence [with either left-to-right ("L to R") or right-to-left ("R to L") associativity]:

1. [L to R] function() [] -> .
2. [R to L] ! ~ ++ -- + - * & (type) sizeof
3. [L to R] * / %
4. [L to R] + -
5. [L to R] << >>
6. [L to R] < <= > >=
7. [L to R] == !=
8. [L to R] &
9. [L to R] ^
10. [L to R] |
11. [L to R] &&
12. [L to R] ||
13. [R to L] ?:
14. [R to L] = += -= *= /= %= &= ^= |= <<= >>=
15. [L to R] ,

where:

• "function()" refers to function invocation
• "(type)" refers to type casting
• unary +, - and * have higher precedence than the corresponding binary operators

Note that because the precedence of bitwise operators &, ^ and | is lower than that of the equality operators (== and !=), bit-testing expressions must be parenthesised to give proper results:

if ((value & BITMASK) == REQUIREDBITS) {...

Value of Equality and Relational Expressions

The equality (== and !=) and relational (<, <=, >= and >) operators all yield 0 for false and 1 for true.

Preprocessing

#define DAYSPERWEEK 7

Simple textual substitution. 7 substituted for token DAYSPERWEEK. Note that in most cases, a better alternative is:
const int DaysPerWeek = 7;

#define MAX(a, b) ((a) > (b) ? (a) : (b))

Simple textual substitution with parameter replacement. Note that, unlike the corresponding function, the parameters and result may be any scalar type or even pointers. Also in contrast to a function, a parameter may be evaluated more than once, giving undesired effects:
MAX(++i, ++j) will increment either i twice and j once, or vice versa.

#define PATHFORMAT(d) #d "/%s"

PATHFORMAT(/etc) will be expanded to:
"/etc" "/%s"
which will yield:
"/etc/%s"

#define JOIN(a, b) a ## b

JOIN(var, 01) will be expanded to:
var01