COBOL is a structured language. You already have seen some of the rules for layout, but structure goes deeper than just physical layout. Structuring applies to the DATA and the PROCEDURE DIVISIONs of a COBOL program. Today, you learn the structure of the PROCEDURE DIVISION and explore the following topics:
From now on, all programs will have some sort of data in them, so it is a good idea to modify cobshl01.cbl, created in Day 1, "Your First COBOL Program." Listing 3.1, cobshl02.cbl, now includes WORKING-STORAGE.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. COBSHL02. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 WORKING-STORAGE SECTION. 000600 000700 PROCEDURE DIVISION. 000800 PROGRAM-BEGIN. 000900 001000 PROGRAM-DONE. 001100 STOP RUN.
The normal course of execution of a COBOL program is from the first statement in the PROCEDURE DIVISION to the last. Let's look at Listing 3.2, add01.cbl, line by line.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. ADD03. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 FIRST-NUMBER PICTURE IS 99. 000900 01 SECOND-NUMBER PICTURE IS 99. 001000 01 THE-RESULT PICTURE IS 999. 001100 001200 PROCEDURE DIVISION. 001300 001400 PROGRAM-BEGIN. 001500 DISPLAY "This program will add 2 numbers.". 001600 DISPLAY "Enter the first number.". 001700 001800 ACCEPT FIRST-NUMBER. 001900 002000 DISPLAY "Enter the second number.". 002100 002200 ACCEPT SECOND-NUMBER. 002300 002400 COMPUTE THE-RESULT = FIRST-NUMBER + SECOND-NUMBER. 002500 002600 DISPLAY "The result is " THE-RESULT. 002700 002800 002900 PROGRAM-DONE. 003000 STOP RUN. 003100
ANALYSIS: The program starts executing at line 001300 in the PROCEDURE DIVISION. Blank lines are skipped, so nothing happens on that line. Line 001400 is a paragraph name. Paragraphs in COBOL are used as bookmarks. The program doesn't do anything for line 001400 except to note internally that it has started a paragraph named PROGRAM-BEGIN.
At line 001500, the program displays a message on-screen. At line 001600, another is displayed. Line 001700 is blank, so it is skipped. At line 001800, the program stops, waits for keyboard input, and places the results in the variable FIRST-NUMBER.
This type of step-by-step action occurs until line 002900, when the program notes that it has begun executing a paragraph named PROGRAM-DONE. At line 003000, the statement STOP RUN is executed, and this halts the execution of the program.
Because paragraph names are used only as bookmarks, it is possible to insert more paragraph names into this program. Remember that you can assign your own paragraph names. The rules for naming paragraphs are similar to the rules for naming variables:
DO/DON'T:
DO use uppercase paragraph names if you want your code to be portable.DON'T use lowercase paragraph names. Even the simplest COBOL programs have a tendency to survive and grow. One day, you might find yourself porting the program to a new computer, and you will curse yourself for having used lowercase. Listing 3.3 is sprinkled with some extra paragraph names. If you compare Listing 3.2 and Listing 3.3, you will see that the sentences are identical. Because a paragraph name does not cause any command to be executed, these two programs behave identically. Enter them both into the computer and compile them. Run them one after the other and you will see no difference.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. ADD04. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 FIRST-NUMBER PIC 99. 000900 01 SECOND-NUMBER PIC 99. 001000 01 THE-RESULT PIC 999. 001100 001200 PROCEDURE DIVISION. 001300 001400 PROGRAM-BEGIN. 001500 001600 DISPLAY "This program will add 2 numbers.". 001700 001800 GET-FIRST-NUMBER. 001900 002000 DISPLAY "Enter the first number.". 002100 002200 ACCEPT FIRST-NUMBER. 002300 002400 GET-SECOND-NUMBER. 002500 002600 DISPLAY "Enter the second number.". 002700 002800 ACCEPT SECOND-NUMBER. 002900 003000 COMPUTE-AND-DISPLAY. 003100 COMPUTE THE-RESULT = FIRST-NUMBER + SECOND-NUMBER. 003200 003300 DISPLAY "The result is " THE-RESULT. 003400 003500 003600 PROGRAM-DONE. 003700 STOP RUN. 003800
ANALYSIS: Listing 3.4 includes an empty paragraph at line 001400. At lines 001500, 001800, 002400, and 003000, the paragraph names have been changed to STEP-01, STEP-02, and so on. If you inspect the code, you will notice that, because of the placement of the paragraph names, the paragraph PROGRAM-BEGIN contains no statements. Some compilers allow this, and others complain of an empty paragraph with either a warning or an error. Personal COBOL and ACUCOBOL both allow it. If you want to test your compiler, you can type this and compile it.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. ADD05. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 FIRST-NUMBER PIC 99. 000900 01 SECOND-NUMBER PIC 99. 001000 01 THE-RESULT PIC 999. 001100 001200 PROCEDURE DIVISION. 001300 001400 PROGRAM-BEGIN. 001500 STEP-01. 001600 DISPLAY "This program will add 2 numbers.". 001700 001800 STEP-02. 001900 002000 DISPLAY "Enter the first number.". 002100 002200 ACCEPT FIRST-NUMBER. 002300 002400 STEP-03. 002500 002600 DISPLAY "Enter the second number.". 002700 002800 ACCEPT SECOND-NUMBER. 002900 003000 STEP-04. 003100 COMPUTE THE-RESULT = FIRST-NUMBER + SECOND-NUMBER. 003200 003300 DISPLAY "The result is " THE-RESULT. 003400 003500 003600 PROGRAM-DONE. 003700 STOP RUN. 003800
In any programming language, certain words have special meanings in the language. In COBOL, DISPLAY "Hello" causes the word Hello to be displayed on-screen. DISPLAY has a special meaning in COBOL. It means "put the next thing on the screen."
New Term: Reserved words are reserved in the language to have a special meaning, and the programmer cannot use these words for some other purpose.
The words DATA and DIVISION appearing together mean that the section of the program where data is defined is beginning. DATA, DIVISION, and DISPLAY are reserved words. Therefore, if you create a program that displays something, it would be incorrect to name the program DISPLAY as in the following:
PROGRAM-ID. DISPLAY.
The compiler probably would complain of an invalid program name, because DISPLAY is reserved for a special meaning in COBOL. You already have learned several reserved words: COMPUTE, ACCEPT, ADD, PROCEDURE, DIVISION, and others. You learn about most of them as you move through this book.
DO/DON'T:
DO name programs, variables, and paragraphs with descriptive names that make their use obvious.DON'T name programs, variables, or paragraphs with reserved words.
You also have seen that WORKING-STORAGE is a reserved word for the name of the SECTION in the DATA DIVISION that contains the data. The compiler will complain if you try to name a variable WORKING-STORAGE. That combination of words is reserved for use by COBOL.
STOP RUN has appeared in every program so far. STOP RUN is a sentence in COBOL, just as DISPLAY "Hello" is. STOP and RUN are both reserved words, and the sentence STOP RUN does exactly what it says; it stops the execution of the program.
Some COBOL implementations do not require a STOP RUN; for example, Personal COBOL does not. Most compilers will compile a program that does not include a STOP RUN, and the problem, if there is one, occurs while the program is running. A program will come to the end and appear to start over again, or it will come to the end and crash with some sort of ABORT message. If you want to check out your COBOL compiler, take any of the examples already covered and remove the PROGRAM-DONE paragraph and the STOP RUN sentence. Compile the program and then try to run it. See what happens when the program reaches the end.
STOP RUN can occur anywhere in the program, and it will stop execution. In all the examples so far, the STOP RUN is placed in its own separate paragraph to make it stand out as the end of the program, but Listing 3.2 could have been written without the PROGRAM-DONE paragraph name, as long as it included STOP RUN. It would have worked as well. In Listing 3.5, the STOP RUN at line 002400 causes the program to terminate at that spot.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. ADD06. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 FIRST-NUMBER PIC 99. 000900 01 SECOND-NUMBER PIC 99. 001000 01 THE-RESULT PIC 999. 001100 001200 PROCEDURE DIVISION. 001300 001400 PROGRAM-BEGIN. 001500 DISPLAY "This program will add 2 numbers.". 001600 001700 001800 DISPLAY "Enter the first number.". 001900 002000 ACCEPT FIRST-NUMBER. 002100 002200 DISPLAY "Fooled you.". 002300 002400 STOP RUN. 002500 002600 DISPLAY "Enter the second number.". 002700 002800 ACCEPT SECOND-NUMBER. 002900 003000 COMPUTE THE-RESULT = FIRST-NUMBER + SECOND-NUMBER. 003100 003200 DISPLAY "The result is " THE-RESULT. 003300 003400 003500 PROGRAM-DONE. 003600 STOP RUN. 003700
Obviously, the PROGRAM-DONE paragraph at line 003500 is misleading in this example because the program stops before this point.
A program that executes from beginning to end and then stops might be useful for something, but it wouldn't do much more than the examples already covered. Suppose you had one action that you performed several times in a program. In top-to-bottom execution, you would have to code that same logic over and over.
The PERFORM verb avoids this problem of coding repetitive actions. In order to illustrate the effect of a PERFORM, Listing 3.6 uses another version of the "Hello world" program. A PERFORM is a kind of "jump" with a "bounce back."
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. HELLO04. 000300 000400* This program illustrates the use of a PERFORM 000500 000600 ENVIRONMENT DIVISION. 000700 DATA DIVISION. 000800 PROCEDURE DIVISION. 000900 001000 PROGRAM-BEGIN. 001100 DISPLAY "Today's message is:". 001200 PERFORM SAY-HELLO. 001300 001400 PROGRAM-DONE. 001500 STOP RUN. 001600 001700 SAY-HELLO. 001800 DISPLAY "Hello world". 001900
OUTPUT:
Today's message is: Hello world C> C>
ANALYSIS: At line 001200, PERFORM SAY-HELLO indicates the following:
A paragraph ends in two ways. Either another paragraph begins or the end of the source code file is reached. In Listing 3.6, the paragraph PROGRAM-BEGIN ends at line 001400 when PROGRAM-DONE begins. The paragraph PROGRAM-DONE ends at line 001700 when SAY-HELLO begins, and SAY-HELLO ends just after line 001900 at the end of the source code file.
Ignoring blank lines (because they will not execute), the sequence of execution in Listing 3.6 is the following: Line 001000. Internally note that the paragraph PROGRAM-BEGIN has started.
Line 001100. DISPLAY "Today's message is:" on-screen.
Line 001200. Locate the paragraph SAY-HELLO at line 001700. Jump to line 001700, the beginning of SAY-HELLO.
Line 001700. Internally note that the paragraph SAY-HELLO has started.
Line 001800. DISPLAY "Hello world" on-screen.
End of file. COBOL recognizes that it has hit the end of the SAY-HELLO paragraph, but it also knows that it is in the middle of a PERFORM requested at line 001200. Whenever a paragraph ends because of an end-of-file or because a new paragraph starts, COBOL checks whether it is in the middle of a PERFORM. If it is, it returns to the line that requested the PERFORM. In this example, the SAY-HELLO paragraph ends, and execution resumes at the end of line 001200. There are no further instructions on that line, so execution continues at line 001400.
Line 001400. Internally note that the paragraph PROGRAM-DONE has started. Line 001500. Stop execution of the program. The top-to-bottom course of a COBOL program continues unless it is interrupted by a PERFORM. The paragraph being performed is also executed top to bottom. When the PERFORM is complete, the program returns to the point just after the PERFORM was requested, and it continues from there to the bottom.
It is important to recognize this "keep-on-trucking" flow of a COBOL program, because it is critical to place your PERFORMed paragraphs below the STOP RUN statement. Listing 3.7 has the SAY-HELLO paragraph placed differently.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. HELLO05. 000300 000400* This program illustrates the incorrect placement of a 000500* Paragraph that is the target of a perform 000600 000700 ENVIRONMENT DIVISION. 000800 DATA DIVISION. 000900 PROCEDURE DIVISION. 001000 001100 PROGRAM-BEGIN. 001200 DISPLAY "Today's message is:". 001300 PERFORM SAY-HELLO. 001400 001500 SAY-HELLO. 001600 DISPLAY "Hello world". 001700 001800 PROGRAM-DONE. 001900 STOP RUN. 002000
If you followed the steps describing the flow of hello05.cbl, it should come as no surprise that the program displays Hello world twice:
OUTPUT:
Today's message is: Hello world Hello world C> C>
ANALYSIS: Ignoring blank lines, the sequence of execution in Listing 3.7 is as follows: Line 001100. Internally note that the paragraph PROGRAM-BEGIN has started.
Line 001200. DISPLAY "Today's message is:" on-screen.
Line 001300. Jump to line 001500, the beginning of SAY-HELLO.
Line 001500. Internally note that the paragraph SAY-HELLO has started.
Line 001600. DISPLAY "Hello world" on-screen.
Line 001800. COBOL recognizes that it has hit the end of the SAY-HELLO paragraph, but it also knows that it is in the middle of a PERFORM requested at line 001300. The SAY-HELLO paragraph ends, and execution resumes at the end of line 001300. There are no further instructions on that line, so execution continues at line 001500.
Line 001500. Internally note that the paragraph SAY-HELLO has started.
Line 001600. DISPLAY "Hello world" on-screen.
Line 001800. COBOL recognizes that it has hit the end of the SAY-HELLO paragraph, but in this case it is not in the middle of a PERFORM.
Line 001800. Internally note that the paragraph PROGRAM-DONE has started. Line 001900. Stop execution of the program.
A PERFORM has several uses in COBOL. First, it is used to section off a repetitive action that is performed in several places or several times in a program to prevent writing the same code over and over.(I hinted at this use when I introduced PERFORM earlier in today's lesson.) This creates two advantages: It not only cuts down on the amount of typing, but it cuts down on the number of potential errors. Less typing means fewer opportunities to copy it incorrectly.
Imagine typing the following retail profit formula five or six times in different places in a program, and you begin to see the sense of putting this in a paragraph called COMPUTE-RETAIL-PROFIT and using the PERFORM verb every time you want it done:
COMPUTE MARGIN-PERCENT =
( (GOODS-PRICE - GOODS-COST) /
GOODS-PRICE ) * 100 .
The second use of PERFORM might not be so obvious, because the programs used so far in this book have been relatively small. A PERFORM serves to break up a program into smaller, more manageable pieces. If you're changing the sales commission from 10 percent to 11 percent, it's much easier to search through a long program looking for a paragraph named CALCULATE-COMMISSION than to plow through a long list of code not broken into paragraphs. It is very common for programs to perform more than one major task. A payroll system might have a program in it used both to calculate pay totals and to print checks after the hours for each employee are entered into the computer.
A program that carries out these actions might have one area of the program to deal with the calculations, while further down in the program another area of code deals with the printing activity. Above these two areas is the main logic of the program that performs both pieces of the program:
MAIN-LOGIC.
PERFORM CALC-PAYROLL-TOTALS.
PERFORM PRINT-PAYROLL-CHECKS.
The third reason to use a PERFORM is that the program is easier to read and understand if the paragraphs are named sensibly. In the following code fragment, it is fairly easy to figure out what the paragraph PAY-THE-SALESPERSON is doing, even though the whole paragraph is made up of PERFORMs:
PAY-THE-SALESPERSON
PERFORM GET-SALES-TOTAL.
PERFORM CALCULATE-COMMISSION.
PERFORM PRINT-THE-CHECK.
A fourth reason to use PERFORM would be to conserve memory. Look back at the retail profit formula and imagine that piece of code repeated five or six times in a single program. Each time the formula appears in the program it takes up several bytes of memory. Coding it once in a single paragraph and performing that paragraph five or six times uses far less memory.
Let's take a closer look at the first reason for a PERFORM, which is to handle repetitive code. Listing 3.8, "The Lady from Eiger," is similar to jack03.cbl, used in Day 2, "Using Variables and Constants." At line 001800, THE-NUMBER is initialized to 0, and then (instead of moving 1 to THE-NUMBER at line 002300) 1 is added to THE-NUMBER.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. EIGER03. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 THE-MESSAGE PIC X(50). 000900 01 THE-NUMBER PIC 9(2). 001000 01 A-SPACE PIC X. 001100 001200 PROCEDURE DIVISION. 001300 PROGRAM-BEGIN. 001400 001500* Initialize the space variable 001600 MOVE " " TO A-SPACE. 001700* Start THE-NUMBER at 0 001800 MOVE 0 TO THE-NUMBER. 001900 002000* Set up and display line 1 002100 MOVE "There once was a lady from Eiger," 002200 TO THE-MESSAGE. 002300 ADD 1 TO THE-NUMBER. 002400 DISPLAY 002500 THE-NUMBER 002600 A-SPACE 002700 THE-MESSAGE. 002800 002900* Set up and Display line 2 003000 MOVE "Who smiled and rode forth on a tiger." 003100 TO THE-MESSAGE. 003200 ADD 1 TO THE-NUMBER. 003300 DISPLAY 003400 THE-NUMBER 003500 A-SPACE 003600 THE-MESSAGE. 003700 003800* Set up and display line 3 003900 MOVE "They returned from the ride" TO THE-MESSAGE. 004000 ADD 1 TO THE-NUMBER. 004100 DISPLAY 004200 THE-NUMBER 004300 A-SPACE 004400 THE-MESSAGE. 004500 004600* Set up and display line 4 004700 MOVE "With the lady inside," TO THE-MESSAGE. 004800 ADD 1 TO THE-NUMBER. 004900 DISPLAY 005000 THE-NUMBER 005100 A-SPACE 005200 THE-MESSAGE. 005300 005400* Set up and display line 5 005500 MOVE "And the smile on the face of the tiger." 005600 TO THE-MESSAGE. 005700 ADD 1 TO THE-NUMBER. 005800 DISPLAY 005900 THE-NUMBER 006000 A-SPACE 006100 THE-MESSAGE. 006200 006300 006400 PROGRAM-DONE. 006500 STOP RUN. 006600
OUTPUT:
01 There once was a lady from Eiger, 02 Who smiled and rode forth on a tiger. 03 They returned from the ride 04 With the lady inside, 05 And the smile on the face of the tiger. C> C>
ANALYSIS: If you inspect lines 002300 through 002700 and lines 003200 through 003600, you'll find that the actions are identical. These appear again in lines 004000 through 004400. In fact, these two sentences are repeated five times in the program, appearing again in lines 004800 through 005200 and 005700 through 006100.
The DISPLAY command (lines 002400 through 002700, 003300 through 003600, and so on) is rather long because it is a DISPLAY of three variables. The odds of typing incorrectly are reasonably high. A typographical error wouldn't be so bad (because the compiler would complain if something were spelled incorrectly), but suppose that in one of the five instances you left out A-SPACE in the list of variables to display. It wouldn't be a disaster. It would show up the first time you ran the program; one of the lines would look odd, and you would be able to track it down quickly. Unfortunately, not all errors of this sort are so easy to spot. If this were a series of calculations, you might be able to spot an error in the final result, but you wouldn't know where it had originated.
There are two repetitive actions in Listing 3.8:
ADD 1 TO THE-NUMBER
and
DISPLAY THE-NUMBER A-SPACE THE-MESSAGE.
It is simple to extract these two lines and create a paragraph that performs both of these actions:
ADD-NUMBER-AND-DISPLAY. ADD 1 TO THE-NUMBER DISPLAY THE-NUMBER A-SPACE THE-MESSAGE.
Listing 3.9 is an example of using this paragraph, with PERFORMs inserted at the appropriate points.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. EIGER04. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 THE-MESSAGE PIC X(50). 000900 01 THE-NUMBER PIC 9(2). 001000 01 A-SPACE PIC X. 001100 001200 PROCEDURE DIVISION. 001300 PROGRAM-BEGIN. 001400 001500* Initialize the space variable 001600 MOVE " " TO A-SPACE. 001700* Start THE-NUMBER at 0 001800 MOVE 0 TO THE-NUMBER. 001900 002000* Set up and display line 1 002100 MOVE "There once was a lady from Eiger," 002200 TO THE-MESSAGE. 002300 PERFORM ADD-NUMBER-AND-DISPLAY. 002400 002500* Set up and Display line 2 002600 MOVE "Who smiled and rode forth on a tiger." 002700 TO THE-MESSAGE. 002800 PERFORM ADD-NUMBER-AND-DISPLAY. 002900 003000* Set up and display line 3 003100 MOVE "They returned from the ride" TO THE-MESSAGE. 003200 PERFORM ADD-NUMBER-AND-DISPLAY. 003300 003400* Set up and display line 4 003500 MOVE "With the lady inside," TO THE-MESSAGE. 003600 PERFORM ADD-NUMBER-AND-DISPLAY. 003700 003800* Set up and display line 5 003900 MOVE "And the smile on the face of the tiger." 004000 TO THE-MESSAGE. 004100 PERFORM ADD-NUMBER-AND-DISPLAY. 004200 004300 PROGRAM-DONE. 004400 STOP RUN. 004500 004600 ADD-NUMBER-AND-DISPLAY. 004700 ADD 1 TO THE-NUMBER. 004800 DISPLAY 004900 THE-NUMBER 005000 A-SPACE 005100 THE-MESSAGE. 005200
Code eiger04.cbl, compile it, and run it. Work out the flow of each PERFORM and how the program returns to the main stream of the logic. Here is the output of eiger04.cbl:
OUTPUT:
01 There once was a lady from Eiger, 02 Who smiled and rode forth on a tiger. 03 They returned from the ride 04 With the lady inside, 05 And the smile on the face of the tiger. C> C>
ANALYSIS: At line 002300, the computer locates ADD-NUMBER-AND-DISPLAY at line 004600 and jumps to that line. This paragraph is PERFORMed, and when complete, execution resumes at the end of line 002300. Execution continues on line 002600. Then at line 002800, another jump is made to line 004600, and the program returns to the end of line 002800. This process continues until all five PERFORM requests have been executed and ADD-NUMBER-AND-DISPLAY has been performed five times.
It is possible to request a PERFORM when you already are within a PERFORM, by nesting them together. If a paragraph that is being PERFORMed itself requests a PERFORM of another paragraph, COBOL keeps track of the layers of PERFORMed paragraphs and returns to the correct level.
In general, when a PERFORMed paragraph ends, the program returns to the line that requested the PERFORM, at a position in the line just after the PERFORM was requested. In Listing 3.10, the ADD-NUMBER-AND-DISPLAY paragraph at line 005100 has been broken down into two sentences that each PERFORM smaller paragraphs. These PERFORM sentences refer to paragraphs at lines 005800 and 006100--ADD-THE-NUMBER and DISPLAY-THE-MESSAGE. Each of these paragraphs does only one thing. In practice, you rarely would create a paragraph to execute a single statement, but this example serves to illustrate nested PERFORMs.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. EIGER05. 000300 000400* This program illustrates nested PERFORMS 000500 ENVIRONMENT DIVISION. 000600 DATA DIVISION. 000700 000800 WORKING-STORAGE SECTION. 000900 001000 01 THE-MESSAGE PIC X(50). 001100 01 THE-NUMBER PIC 9(2). 001200 01 A-SPACE PIC X. 001300 001400 PROCEDURE DIVISION. 001500 PROGRAM-BEGIN. 001600 001700* Initialize the space variable 001800 MOVE " " TO A-SPACE. 001900* Start THE-NUMBER at 0 002000 MOVE 0 TO THE-NUMBER. 002100 002200* Set up and display line 1 002300 MOVE "There once was a lady from Eiger," 002400 TO THE-MESSAGE. 002500 PERFORM ADD-NUMBER-AND-DISPLAY. 002600 002700* Set up and Display line 2 002800 MOVE "Who smiled and rode forth on a tiger." 002900 TO THE-MESSAGE. 003000 PERFORM ADD-NUMBER-AND-DISPLAY. 003100 003200* Set up and display line 3 003300 MOVE "They returned from the ride" TO THE-MESSAGE. 003400 PERFORM ADD-NUMBER-AND-DISPLAY. 003500 003600* Set up and display line 4 003700 MOVE "With the lady inside," TO THE-MESSAGE. 003800 PERFORM ADD-NUMBER-AND-DISPLAY. 003900 004000* Set up and display line 5 004100 MOVE "And the smile on the face of the tiger." 004200 TO THE-MESSAGE. 004300 PERFORM ADD-NUMBER-AND-DISPLAY. 004400 004500 PROGRAM-DONE. 004600 STOP RUN. 004700 004800* This paragraph is PERFORMED 5 times from within 004900* PROGRAM-BEGIN. This paragraph in turn PERFORMS 005000* Two other paragraphs 005100 ADD-NUMBER-AND-DISPLAY. 005200 PERFORM ADD-THE-NUMBER. 005300 PERFORM DISPLAY-THE-MESSAGE. 005400 005500* These two paragraphs will each be performed 5 times as 005600* they are each performed every time ADD-NUMBER-AND-DISPLAY 005700* is performed. 005800 ADD-THE-NUMBER. 005900 ADD 1 TO THE-NUMBER. 006000 006100 DISPLAY-THE-MESSAGE. 006200 DISPLAY 006300 THE-NUMBER 006400 A-SPACE 006500 THE-MESSAGE. 006600
ANALYSIS: The flow of the program at line 005100 is executed every time PERFORM ADD-NUMBER-AND-DISPLAY is requested: Line 005100. Internally note that the paragraph ADD-NUMBER-AND-DISPLAY has started.
Line 005200. Locate the paragraph called ADD-THE-NUMBER and jump to it at line 005800.
Line 005800. Internally note that the paragraph ADD-THE-NUMBER has started.
Line 005900. ADD 1 TO THE-NUMBER.
Line 006100. COBOL notes that ADD-THE-NUMBER has ended and returns to line 005200, where there are no further instructions.
Line 005300. Locate the paragraph named DISPLAY-THE-MESSAGE and jump to it at line 006100.
Line 006100. Internally note that the paragraph DISPLAY-THE-MESSAGE has started.
Lines 006200 through 006500. These lines are executed as one long sentence, displaying all the variables.
End of file (the last line of the source code). COBOL notes the end of DISPLAY-THE-MESSAGE and knows it is in the middle of a PERFORM. It returns to line 005300, where there are no further instructions. Line 005800. (Remember that blank lines and comment lines have no effect on the program, so the next active line is 005800.) COBOL notes that ADD-THE-NUMBER starts here; therefore, the current paragraph, ADD-NUMBER-AND-DISPLAY, must have ended. Execution returns to whatever line originally requested the PERFORM of ADD-NUMBER-AND-DISPLAY.
DO/DON'T:
DO locate repetitive actions in your programs, and create separate paragraphs containing those actions. Then PERFORM the paragraph wherever those actions are needed.DON'T keep typing the same code over and over in one program.
It is common for PERFORMs to be nested in COBOL programs. In fact, Listing 3.11 shows how an experienced programmer actually might organize "The Lady from Eiger." The callouts down the right side show you the sequence of execution.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. EIGER06. 000300 000400* This program illustrates nested PERFORMS in a 000500* structured program. 000600 ENVIRONMENT DIVISION. 000700 DATA DIVISION. 000800 000900 WORKING-STORAGE SECTION. 001000 001100 01 THE-MESSAGE PIC X(50). 001200 01 THE-NUMBER PIC 9(2). 001300 01 A-SPACE PIC X. 001400 001500 PROCEDURE DIVISION. 001600 001700* LEVEL 1 ROUTINES 001800 PROGRAM-BEGIN. 001900 002000 PERFORM PROGRAM-INITIALIZATION. 002100 PERFORM MAIN-LOGIC. 002200 002300 PROGRAM-DONE. 002400 STOP RUN. 002500 002600*LEVEL 2 ROUTINES 002700 PROGRAM-INITIALIZATION. 002800* Initialize the space variable 002900 MOVE " " TO A-SPACE. 003000* Start THE-NUMBER at 0 003100 MOVE 0 TO THE-NUMBER. 003200 003300 MAIN-LOGIC. 003400* Set up and display line 1 003500 MOVE "There once was a lady from Eiger," 003600 TO THE-MESSAGE. 003700 PERFORM ADD-NUMBER-AND-DISPLAY. 003800 003900* Set up and Display line 2 004000 MOVE "Who smiled and rode forth on a tiger." 004100 TO THE-MESSAGE. 004200 PERFORM ADD-NUMBER-AND-DISPLAY. 004300 004400* Set up and display line 3 004500 MOVE "They returned from the ride" TO THE-MESSAGE. 004600 PERFORM ADD-NUMBER-AND-DISPLAY. 004700 004800* Set up and display line 4 004900 MOVE "With the lady inside," TO THE-MESSAGE. 005000 PERFORM ADD-NUMBER-AND-DISPLAY. 005100 005200* Set up and display line 5 005300 MOVE "And the smile on the face of the tiger." 005400 TO THE-MESSAGE. 005500 PERFORM ADD-NUMBER-AND-DISPLAY. 005600 005700* LEVEL 3 ROUTINES 005800* This paragraph is PERFORMED 5 times from within 005900* MAIN-LOGIC. 006000 006100 ADD-NUMBER-AND-DISPLAY. 006200 ADD 1 TO THE-NUMBER. 006300 DISPLAY 006400 THE-NUMBER 006500 A-SPACE 006600 THE-MESSAGE. 006700
ANALYSIS: The "main" stream of the program runs from line 001800 to line 002400 and is quite short. All the work of the program is accomplished by requesting PERFORMs of other paragraphs. Some of these in turn request other PERFORMs. You should look over this listing and compare it carefully to Listing 3.9. You will see that the effect of the two programs is identical, and both of them use paragraphs to isolate repetitive logic. The version in eiger06.cbl additionally uses paragraphs to break out and document which parts of the code are for initializing (or setting up variables) before the main logic and which parts are the main logic of the code.
Today, you learned about the structure of the PROCEDURE DIVISION, including the following basics:
DO-SOMETHING.
PERFORM DO-SOMETHING.
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. MSG01. 000300 000400 ENVIRONMENT DIVISION. 000500 DATA DIVISION. 000600 000700 WORKING-STORAGE SECTION. 000800 000900 PROCEDURE DIVISION. 001000 001100 PROGRAM-BEGIN. 001200 001300 PERFORM MAIN-LOGIC. 001400 001500 PROGRAM-DONE. 001600 STOP RUN. 001700 001800 MAIN-LOGIC. 001900 PERFORM DISPLAY-MSG-1. 002000 PERFORM DISPLAY-MSG-2. 002100 002200 DISPLAY-MSG-1. 002300 DISPLAY "This is message 1.". 002400 002500 DISPLAY-MSG-2. 002600 DISPLAY "This is message 2.". 002700
000100 IDENTIFICATION DIVISION. 000200 PROGRAM-ID. ADD07. 000300 ENVIRONMENT DIVISION. 000400 DATA DIVISION. 000500 000600 WORKING-STORAGE SECTION. 000700 000800 01 FIRST-NUMBER PIC 99. 000900 01 SECOND-NUMBER PIC 99. 001000 01 THE-RESULT PIC 999. 001100 001200 PROCEDURE DIVISION. 001300 001400 PROGRAM-BEGIN. 001500 001600 PERFORM ADVISE-THE-USER. 001700 PERFORM GET-FIRST-NUMBER. 001800 PERFORM GET-SECOND-NUMBER. 001900 PERFORM COMPUTE-AND-DISPLAY. 002000 002100 PROGRAM-DONE. 002200 STOP RUN. 002300 002400 ADVISE-THE-USER. 002500 DISPLAY "This program will add 2 numbers.". 002600 002700 GET-FIRST-NUMBER. 002800 002900 DISPLAY "Enter the first number.". 003000 ACCEPT FIRST-NUMBER. 003100 003200 GET-SECOND-NUMBER. 003300 003400 DISPLAY "Enter the second number.". 003500 ACCEPT SECOND-NUMBER. 003600 003700 COMPUTE-AND-DISPLAY. 003800 003900 COMPUTE THE-RESULT = FIRST-NUMBER + SECOND-NUMBER. 004000 DISPLAY "The result is " THE-RESULT. 004100
004600 COMPUTE THE-RESULT = FIRST-NUMBER + 004700 SECOND-NUMBER + 004800 THIRD-NUMBER.
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