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.