Rexx Programming/How to Rexx/switch conditional

Rexx allows our program to examine numerous possible scenarios, one by one, until it finds a scenario which is actually the case. The keywords for doing this are SELECT, WHEN, THEN, and OTHERWISE. When we're done listing out scenarios and their possible responses, we close the construct with END.

/* Try to tell someone whether they have a fever or not. */ say "Enter your temperature in degrees Celsius:" pull degrees select when DataType(degrees) \= "NUM" then say "That's not a number. I can't help you." when degrees < 36.5 then say "Your body temperature is a bit low." when degrees > 37.5 then say "I think you have a fever." otherwise say "You're temperature seems normal." end /* NOT real medical advice -- just an example! */

After the example script receives its input, it starts running various tests. First, it checks whether the input is even a number. Then, it checks whether the temperature is too low, or too high. If neither of those conditions is met, it reports a normal temperature. After any of the WHEN conditions is met, the program stops checking, executes a line of code, and skips to the end.

Multiline responses
You can allow more complex functionality by using DO/END blocks.

say "Would you like to convert feet to meters or meters to feet?" say "Type M to enter a number of meters or F to enter a number of feet." pull choice select when choice = "M" then do  say "Enter a number of meters:" pull meters feet = meters / 0.3048 say "That's equal to" feet "ft." end when choice = "F" then do  say "Enter a number of feet:" pull feet meters = feet * 0.3048 say "That's equal to" meters "m." end otherwise say "I don't understand what you entered." end

Testing more than one variable
WHEN conditions can be as complex as we want them to be. Here's the famous FIZZBUZZ program, which counts normally except for multiples of 3, 5 and 15.

/* FizzBuzz */ do count = 1 to 30 divisible_by_3 = count // 3 = 0 divisible_by_5 = count // 5 = 0 select when divisible_by_3 & divisible_by_5 then say "FIZZBUZZ" when divisible_by_3 then say "FIZZ" when divisible_by_5 then say "BUZZ" otherwise say count end end

Isn't this just the same as IF/ELSE?
Yes. If you've already read up on IF statements, you can write the same kind of code using those. SELECT is just an convenience for grouping related conditional constructs, especially if you'd need a whole lot of ELSE IF lines.

/* Code with SELECT: */ select when number < 0 then type = 'negative' when number > 0 then type = 'positive' otherwise type = 'neutral' end /* Same code with IF */ if number < 0 then type = 'positive' else if number > 0 then type = 'positive' else type = 'neutral'

Why "switch"?
Control structures like this are sometimes called "switch conditionals" because some other languages use a "switch" keyword to analyze a single variable and branch to different execution pathways for different cases represented by different possible values. It often works with only a finite number of possible values and doesn't necessarily skip the other tests automatically. The Rexx version is more verbose but also more general. SELECT is often used in Rexx where other languages would have to use IF/ELSE.