Programming in C

442 Appendix A C Language Summary

Finally, because the assignment operators group from right to left, the statement

a = b = 0;

is evaluated as

a = (b = 0);

which has the net result of setting the values of aand bto 0. In the case of the

expression

x[i] + ++i

it is not defined whether the compiler evaluates the left side of the plus operator or the

right side first. Here, the way that it’s done affects the result because the value of imight

be incremented before x[i]is evaluated.

Another case in which the order of evaluation is not defined is in the following

expression:

x[i] = ++i

In this situation, it is not defined whether the value of iis incremented before or after

its value is used to index into x.

The order of evaluation of function arguments is also undefined.Therefore, in the

function call

f (i, ++i);

imight be incremented first, thereby causing the same value to be sent as the two argu-

ments to the function.

The C language guarantees that the &&and ||operators are evaluated from left to

right. Furthermore, in the case of &&, it is guaranteed that the second operand is not

evaluated if the first is 0 ; and in the case of ||, it is guaranteed that the second operand is

not evaluated if the first is nonzero.This fact is worth bearing in mind when forming

expressions such as

if ( dataFlag || checkData (myData) )

because, in this case,checkDatais called only if the value of dataFlagis 0 .To take

another example, if the array ais defined to contain nelements, the statement that

begins

if (index >= 0 && index < n && a[index] == 0))

references the element contained in the array only if indexis a valid subscript in the

array.

5.2 Constant Expressions

A constant expression is an expression in which each of the terms is a constant value.

Constant expressions are requiredin the following situations:

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