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 A Tutorial Introduction

   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. 

Revison. Write a loop equivalent to the _for loop above without using _&& or _||.