Nested if and if-else statements

The if-else statement allows a choice to be made between two possible alternatives. Sometimes a choice must be made between more than two possibilities. For example the sign function in mathematics returns -1 if the argument is less than zero, returns +1 if the argument is greater than zero and returns zero if the argument is zero. The following C++ statement implements this function:

if (x < 0)
   sign = -1;
else
   if (x == 0)
      sign = 0;
   else
      sign = 1;

This is an if-else statement in which the statement following the else is itself an if-else statement. If x is less than zero then sign is set to -1, however if it is not less than zero the statement following the else is executed. In that case if x is equal to zero then sign is set to zero and otherwise it is set to 1.

Novice programmers often use a sequence of if statements rather than use a nested if-else statement. That is they write the above in the logically equivalent form:

if (x < 0)
   sign = -1;
if (x == 0)
   sign = 0;
if (x > 0)
   sign = 1;

This version is not recommended since it does not make it clear that only one of the assignment statements will be executed for a given value of x. Also it is inefficient since all three conditions are always tested.

http://www.macs.hw.ac.uk/~pjbk/pathways/cpp1/node99.html