The PERFORM verb is one of the most important in COBOL
(alongside MOVE).
A D V E R T I S E M E N T
PERFORM has already been encountered in the Four Divisions
section, where it was used to call paragraphs from within a control paragraph.
Of course, it doesn't have to be a control (or main) paragraph.
In the above code, the paragraph FIRST-PROCESS is executed. When the full stop
at the end of this paragraph is encountered the logic will return to
XYZ-PARAGRAPH at the next line, i.e. line 320. This is called an Out-of-Line
PERFORM.
The PERFORM verb can form the bases of a repetitive loop
(or sub-routine) until a certin condition has been met.
For Example:
/p>
In the above code, COUNT-PROCESS is executed until the value of W-COUNT has
reached 11.
The format for an Out-of-Line PERFORM is:
PERFORM [paragraph-name] UNTIL [condition]
An In-Line PERFORM, rather than execute a paragraph (aka procedure), allows for
the repeated execution of a series of commands. The format for an In-Line
PERFORM is:
PERFORM UNTIL
{action}...
END-PERFORM
Example:
:
000290 PROCEDURE DIVISION.
000300 XYZ-PARAGRAPH.
000305 MOVE ZERO TO W-COUNTER
000310 PERFORM UNTIL W-COUNTER > 10
000320 COMPUTE W-COUNTER = W-COUNTER + 1
000330 DISPLAY 'This is loop number: ' W-COUNTER
000340 END-PERFORM
000350 DISPLAY 'Counter is now equal to: ' W-COUNTER
000360 STOP RUN.
END-PERFORM defines the scope of the PERFORM loop, and is a Scope
terminator. Other such scope terminators exist for other commands that will
be described further on. The code above will loop 11 times (showning numbers 1
to 11). This is because when W-COUNTER is equal to 10, the condition (W-COUNTER)
is still false. 1 is then added, and W-COUNTER is displayed as 11, and now
when W-COUNTER is tested the condition will be true and the logic will then jump
to the statement that immediately follows END-PERFORM.
This type of PEFORM tests the condition before the
following statements are allowed to proceed. Using WITH TEST can be used to
define when the test is done:
:
000290 PROCEDURE DIVISION.
000300 XYZ-PARAGRAPH.
000305 MOVE ZERO TO W-COUNTER
000310 PERFORM WITH TEST AFTER UNTIL W-COUNTER > 10
000320 COMPUTE W-COUNTER = W-COUNTER + 1
000330 DISPLAY 'This is loop number: ' W-COUNTER
000340 END-PERFORM
000350 DISPLAY 'Counter is now equal to: ' W-COUNTER
000360 STOP RUN.
Now the condition is tested after the commands within the PERFORM..END-PERFORM
loop has be executed once. (WITH TEST BEFORE has same effect as initial
example).
If you wanted to loop a desired number of times you could
use TIMES
PERFORM 5 TIMES
COMPUTE W-NUMBER = XYZ * 3
END-PERFORM
The format is:
PERFORM {identifier or literal} TIMES
{action}...
END-PERFORM
To have a loop using an increment (such as a 'for..do'
loop in Pascal or FOR in BASIC), the PERFORM VARYING statement is used.
The Format is:
PERFORM {paragraph-name if out-of-line} VARYING {identifier-1}
FROM {identifier-2 or literal} BY {identifier-3 or literal}
UNTIL {condition}
END-PERFORM
What does all that mean? Well look at the example:
:
000290 PROCEDURE DIVISION.
000300 XYZ-PARAGRAPH.
000310 PERFORM VARYING W-COUNTER FROM 1 BY 2
000320 UNTIL W-COUNTER > 10
000330 DISPLAY 'This is loop number: ' W-COUNTER
000340 END-PERFORM
000350 DISPLAY 'Counter is now equal to: ' W-COUNTER
000360 STOP RUN.
This code will display:
This is loop number: 1
This is loop number: 3
This is loop number: 5
This is loop number: 7
This is loop number: 9
Counter is now equal to: 11
This because with each loop, W-COUNTER has increased from 1
by increments of 2. When W-COUNT was equal to 11 then the condition W-COUNTER >
10 is now true and so the loop is exited. If you wanted to count downwards you
could code:
PERFORM VARYING W-COUNTER FROM 20 BY -1
UNTIL W-COUNTER < ZERO.
The last thing to mention is PERFORM..THRU. If a program had a series of
paragraphs, just for the sake of argument, called PROCESS-1, PROCESS-2,
PROCESS-3 and PROCESS-4, then if you wished to execute these paragraphs in the
order that they are written you could code: PERFORM PROCESS-1 THRU PROCESS-4
with any out-of-line loops and conditions you might want. Seemingly, this is not
good programming practise so is generally avoided.
