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 a and b to 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 i might
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 i is 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);
i might be incremented first, thereby causing the same value to be sent as the two arguments
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, checkData is called only if the value of dataFlag is 0.To take
another example, if the array a is defined to contain n elements, the statement that
begins
if (index >= 0 && index < n && a[index] == 0))
...
references the element contained in the array only if index is 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 required in the following situations: