Relational and Logical Operators
The relational operators are
> >= < <=
They all have the same precedence. Just below them in precedence are the equality operators:
== !=
Relational operators have lower precedence than arithmetic operators, so an expression like i < lim-1 is taken as i < ( lim- 1 ), as would be expected.
More interesting are the logical operators && and ||. Expressions connected by && or || are evaluated left to right, and evaluation stops as soon as the truth or falsehood of the result is known. Most C programs rely on these properties. For example, here is a loop from the input function getline that we wrote in Chapter 1:
for (i=0; i<lim-1 && (c=getchar()) != ’\n' && c != EOF; ++i)
s[i] = c;
Before reading a new character it is necessary to check that there is room to store it in the array s, so the test i < lim-1 must be made first. Moreover, if this test fails, we must not go on and read another character.
Similarly, it would be unfortunate if c were tested against EOF before getchar is called; therefore the call and assignment must occur before the character in c is tested.
The precedence of && is higher than that of ||, and both are lower than relational and equality operators, so expressions like
i<lim-1 && ( c = qetchar ( ) ) != ’ \n’ && c != EOF
need no extra parentheses. But since the precedence of != is higher than assignment, parentheses are needed in
( c = qetchar ( ) ) != ’\n'
to achieve the desired result of assignment to c and then comparison with ’ \n ’.
By definition, the numeric value of a relational or logical expression is 1 if the relation is true, and 0 if the relation is false.
The unary negation operator ! converts a non-zero operand into 0, and a zero operand into 1. A common use of ! is in constructions like
if (!valid)
rather than
if (valid == 0)
It’s hard to generalize about which form is better. Constructions like !valid read nicely (“if not valid”), but more complicated ones can be hard to understand.
[The C Programming Language p.41-42]
