3.15 where statements

Blitz++ provides the where function as an array expression version of the ( ? : ) operator. The syntax is:

where(array-expr1, array-expr2, array-expr3)

Wherever array-expr1 is true, array-expr2 is returned. Where array-expr1 is false, array-expr3 is returned. For example, suppose we wanted to sum the squares of only the positive elements of an array. This can be implemented using a where function:

double posSquareSum = sum(where(A > 0, pow2(A), 0));