Relational and Logical Operators in C

By: Tamil Selvan in C Tutorials on 2007-09-20

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:

   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=getchar()) != '\n' && c != EOF

need no extra parentheses. But since the precedence of != is higher than assignment, parentheses are needed in

   (c=getchar()) != '\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 in 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.

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