conditional execution -- execute some code if a condition is true

conditional execution -- execute some code if a condition is true

The basic way to control the execution of code is with the if expression. Such an expression typically has the form

     if X then Y else Z

and is evaluated as follows. First X is evaluated. If the result is true, then the value of Y is provided, and if the result is false, then the value of Z is provided. An error is signalled if the value of X is anything but true or false.

i1 : (-4 .. 4) / 

          (i -> if i < 0 then "neg" 

               else if i == 0 then "zer" 

               else "pos")



o1 = (neg, neg, neg, neg, zer, pos, pos, pos, pos)



o1 : Sequence

The else clause may be omitted from an if expression. In that case, if value of the predicate X is false, then null is provided as the value of the if expression.

i2 : (-4 .. 4) / 

          (i -> if i < 0 then "neg" 

          else if i == 0 then "zer")



o2 = (neg, neg, neg, neg, zer, , , , )



o2 : Sequence

There are a variety of predicate functions (such as <, used above) that yield true or false and can be used as the predicate in an if expression. For a list, see Boolean. Boolean results may be combined with not, and, and or.