Nested IF…THEN…ELSE instructions

Code examples help you learn how to specify IF…THEN…ELSE instructions.

Sometimes it is necessary to have one or more IF…THEN…ELSE instructions within other IF…THEN…ELSE instructions. Having one type of instruction within another is called nesting. With nested IF instructions, it is important to match each IF with an ELSE and each DO with an END.

IF weather = fine THEN
   DO
      SAY 'What a lovely day!'
      IF tenniscourt = free THEN
         SAY 'Let''s play tennis!'
      ELSE NOP
   END
ELSE
   SAY 'We should take our raincoats!'

Not matching nested IFs to ELSEs and DOs to ENDs can have some surprising results. If you eliminate the DOs and ENDs and the ELSE NOP, as in the following example, what is the outcome?

Figure 1. Example of Missing Instructions


/******************************** REXX *******************************/
/* This program demonstrates what can happen when you do not include */
/* DOs, ENDs, and ELSEs in nested IF...THEN...ELSE instructions.     */
/*********************************************************************/
weather = 'fine'
tenniscourt = 'occupied'

IF weather = 'fine' THEN
   SAY 'What a lovely day!'
   IF tenniscourt = 'free' THEN
      SAY 'Let''s play tennis!'
ELSE
   SAY 'We should take our raincoats!'

Looking at the program, you might assume the ELSE belongs to the first IF. However, the language processor associates an ELSE with the nearest unpaired IF. The outcome is as follows:

What a lovely day!
We should take our raincoats!

Exercise: using the IF…THEN…ELSE instruction

Write the REXX instructions for the following flowchart:

ANSWER


IF a = 0 THEN
   IF c = 2 THEN
      z = 1
   ELSE NOP
ELSE
   IF z = 2 THEN
      IF c = 3 THEN
         a = 1
      ELSE
         a = 3
   ELSE NOP