IBM SC34-5764-01 Manual page 89

Exercise - Writing an Internal and an External Subroutine
Write a program that plays a simulated coin toss game and produces the accumulated scores.
There should be four possible inputs:
v 'HEADS'
v 'TAILS'
v '' (Null—to quit the game)
v None of these three (incorrect response).
Write an internal subroutine without arguments to check for valid input. Send valid input to an external
subroutine that uses the RANDOM built-in function to generate random outcomes. Assume HEADS = 0
and TAILS = 1, and use RANDOM as follows:
RANDOM(0,1)
Compare the valid input with the value from RANDOM. If they are the same, the user wins one point; if
they are different, the computer wins one point. Return the result to the main program where results are
tallied.
ANSWER
/***************************** REXX ********************************/
/* This program plays a simulated coin toss game.
/* The input can be heads, tails, or null ("") to quit the game.
/* First an internal subroutine checks input for validity.
/* An external subroutine uses the RANDOM built-in function to
/* obtain a simulation of a throw of dice and compares the user
/* input to the random outcome. The main program receives
/* notification of who won the round. It maintains and produces
/* scores after each round.
/*******************************************************************/
PULL flip
computer = 0; user = 0
CALL check
DO FOREVER
CALL throw
IF RESULT = 'machine' THEN /* The computer won
computer = computer + 1
ELSE
user = user + 1
SAY 'Computer score = ' computer
PULL flip
CALL check
END
EXIT
Figure 41. Possible Solution (Main Program)
/* Gets "HEADS", "TAILS", or ""
/* from input stream.
/* Initializes scores to zero
/* Calls internal subroutine, check */
/* Calls external subroutine, throw */
/* Increase the computer score
/* The user won
/* Increase the user score
' Your score = ' user
/* Call internal subroutine, check */
Writing Subroutines and Functions
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Chapter 6. Writing Subroutines and Functions
67