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Instructions

The example script uses the say and pull instructions to write information to the screen, and to read user input, respectively. As in any power language, these instructions have many variable-manipulation and formatting options, and they manage input/output for any of many different devices. The example script illustrates simple output to the screen and input from the keyboard.

Rexx has a couple dozen instructions. They include all those necessary for controlling program logic, as shown in Table 3.

Table 3 Rexx Instructions

Instruction

Use

call

Invokes a routine or enables an error condition

do

All forms of the do statement (do..while, do..until, do N times, etc.)

exit

Ends a program

if

All forms of the if statement (if..then, if..then..else, etc.)

iterate

Alters the flow of control by passing control directly back to a do instruction

leave

Alters the flow of control by passing control out of a do loop

return

Returns control to the caller

select

Selects a branch from a set of alternatives (implements a "case" construct)

signal

Transfers control to a label (similar to a "go to"), or enables or disables an error condition

Rexx offers the full set of programming constructs required for structured programming. But it also offers common unstructured constructs, should you want them.

Other Rexx instructions manage the environment and perform other tasks, as shown in Table 4.

Table 4 Miscellaneous Instructions

Instruction

Use

address

Dictates the external environment to which the script sends commands

arg

Reads input arguments, or parses arguments in a template

drop

Unassigns one or more variables

interpret

Dynamically evaluates instructions provided in an expression

nop

No operation

numeric

Controls precision and other aspects of numeric calculation

options

Passes commands to the Rexx interpreter (controls the interpreter)

parse

Assigns values to variables as a result of controlled parsing

procedure

Controls the scoping of variables across routines

pull

Reads input and applies parsing rules

push

Adds a line to the external data queue or stack in LIFO order

queue

Adds a line to the external data queue or stack in FIFO order

say

Evaluates an expression and writes a line to the default output stream

trace

Manages the built-in debugging facility

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