What Is a Variable?
Computers are capable of dealing with a large amount of data, and the data should be able to change while the computer is running. So, how do you change data? You use something called a variable.
New Term: A variable is a value that can be changed while the program is running.
When a variable is created in a program, an area of memory is set aside to hold values. In most programming languages, including COBOL, a variable is given a name. The name can be used in the program to refer to the value.
The value stored in memory can be modified while the program is running by using the variable name. You'll see some examples of using variables later in today's lesson; but you first must understand how to define a variable.
Defining Numeric Variables in COBOL
The following is an example of a COBOL numeric variable. Variable names use the same characters as paragraph names: A through Z, 0 through 9, and the hyphen (-). This is described more fully in the section "Naming Variables in COBOL," later in today's lesson.
001400 01 THE-NUMBER PICTURE IS 9999.
A COBOL variable definition contains at least three parts:
· The level number
· The name
· The PICTURE
In the syntax, the level number is 01. For now, every variable you will be using will have a level number of 01. The level number 01 must be in Area A, columns 8 through 11. In the previous code fragment, the 01 starts in column 8.
The second part of the variable definition is the name of the variable and, in this case, is THE-NUMBER. This is the data name used to identify the variable. The data name is assigned by the programmer. The variable will be referred to by its data name, THE-NUMBER, anywhere in the program that the variable must be set or modified. The name of the variable must start in Area B, columns 12 through 72. In this example, THE-NUMBER starts in column 12.
The PICTURE defines two things about a variable: the size of the variable (the number of bytes used in memory for the value) and the type of data that can be stored in the variable. In this example, the picture 9999 indicates that four numeric characters can be stored in the variable named THE-NUMBER. Similarly, a variable with a PICTURE IS 99 could hold two numeric characters. The PICTURE IS clause and the actual picture 9999 must start somewhere in Area B, columns 12 through 72.
The PICTURE IS clause in the definition of a variable is the COBOL syntax for introducing the size of a variable and the type of data that a variable holds.
The 9999 in the picture does not indicate that the variable contains the value 9999. It indicates that the variable can be used for numeric values in the range 0 through 9,999. The 9999 picture indicates that four numeric digits can be stored in this variable. The picture 9999 will hold any of the values from 0 through 9,999. The values 17 and 6,489 will both fit in THE-NUMBER, but the value 65,413 is too large.
Look at Listing 2.4, add01.cbl, for the general format of the program now that it contains variables. Three variables are created in this program: FIRST-NUMBER, SECOND-NUMBER, and THE-RESULT. Each variable has the level number 01. The first two have pictures of 99 and will hold values ranging from 0 through 99. The third variable, THE-RESULT, has a picture of 999 and will hold a value of 0 through 999. Once again, the PICTURE IS clause does not set the value of the variable; it sets only the largest and smallest values that a variable can hold and the fact that the variable will hold numeric data.
TYPE: Listing 2.4. Using variables.
000100 IDENTIFICATION DIVISION.
000200 PROGRAM-ID. ADD01.
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
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:".
002700 DISPLAY THE-RESULT.
002800
002900 PROGRAM-DONE.
003000 STOP RUN.
003100
Load and compile or type in with your editor and compile Listing 2.4, add01.CBL. When you run the program, you will be asked to enter the first number, as shown in the following output. Note that the final blank line 003100 in the listing has no effect on the program. You can leave it out if you wish.
OUTPUT:
C>pcobrun add01
Enter the first number.
First, type 97 and then press Enter. You will be asked for a second number, in a screen looking something like this:
C>pcobrun add01
Enter the first number.
97
Enter the second number.
Now type 33 and press Enter. The two numbers are added together and displayed:
C>pcobrun add01
Enter the first number.
97
Enter the second number.
33
The result is:
130
C>
ANALYSIS: Take a look at the DATA DIVISION. Here is your first example of a section, the WORKING-STORAGE SECTION. A SECTION in COBOL is created by typing a name, similar to a paragraph name, followed by one or more spaces, followed by the word SECTION and a period. SECTIONs in COBOL can be required or optional, depending on which DIVISION they are in. WORKING-STORAGE SECTION is a reserved name and a required section in the DATA DIVISION if your program uses any variables--and most programs do.
DO/DON'T:
DO precede the word SECTION with at least one space (WORKING-STORAGE SECTION).
DON'T precede SECTION with an extra hyphen (WORKING-STORAGE-SECTION).
Each of the variables is defined with a 01, a variable name, and a PICTURE. The PICTURE IS clauses are lined up on the right. There is no reason for this other than tidiness. As long as the PICTURE clause starts and ends in Area B, there are no other restrictions on alignment or position.Now look at Listing 2.4 again, but this time from the perspective of a running program. In the DATA DIVISION, space is created for two variables with pictures of 99 and one variable with a picture of 999.
In the PROCEDURE DIVISION, a message is displayed for the user at line 001600, asking the user to enter the first variable. At line 001800, this value is accepted from the keyboard, using the ACCEPT verb. ACCEPT is a verb that causes the program to wait for input from the keyboard. Digits can be typed until the user presses Enter. When the user presses Enter, the value of the digits entered is moved to the variable named immediately after the ACCEPT verb.
When the program is running and encounters the sentence
ACCEPT FIRST-NUMBER
the computer stops and waits for the user to type. Whatever value is typed is stored in the two bytes of FIRST-NUMBER.
At line 002000, the user is asked for another number. This is accepted at line 002200 using the ACCEPT verb and stored in SECOND-NUMBER.
At line 002400, the COBOL COMPUTE verb is used to add the two values together. In the following statement, the values that have been stored at FIRST-NUMBER and SECOND-NUMBER are retrieved and added together, using + to perform the addition:
COMPUTE THE-RESULT = FIRST-NUMBER + SECOND-NUMBER.
The result of this addition is stored at THE-RESULT.
Finally, in lines 002600 and 002700, the result of the addition is displayed.
In this example, two of the variables--FIRST-NUMBER and SECOND-NUMBER--have been modified or "varied" by the user entering values at the keyboard. The third variable, THE-RESULT, was modified by the program. The program uses the COMPUTE statement to calculate a new value and store it in THE-RESULT. This is what variables are all about: the ability to vary their values while the program is running.
Some versions of COBOL (ACUCOBOL, for example) are picky about accepting data from the keyboard. If a field is defined with a PICTURE IS 99, you must enter two digits in response to an ACCEPT. To enter the number 3, you must enter 03. To enter 7 into a PICTURE 99999 field, you must enter 00007. ACUCOBOL includes an option to change this behavior, and I am informed that, as of their version 3.0 compiler, the ACCEPT verb behaves in a more relaxed manner, allowing numeric entry without the preceding zeroes. Other versions of COBOL, such as Micro Focus Personal COBOL, use this more relaxed approach to ACCEPT. If you want to enter a 3 into a PICTURE 999, just enter a 3 and press Enter. The COBOL language will correctly store 003 in the PICTURE 999.
For the time being, enter all the digits to avoid any problems. If a program complains of non-numeric data in a numeric field, you probably have not entered enough leading zeroes.
Type the program from Listing 2.4 into your computer and compile it. Run it several times, entering different values each time. You will see that the three variables truly are variable, because their values are determined while the program is running. You do not have to edit and recompile the program each time that you want to get a new result.
Take one more look at Listing 2.4, add01.cbl, for a quick review. Line numbers appear in columns 1 through 6, the sequence area. Comments start in the indicator area, and they start with an asterisk in that column. DIVISIONs, SECTIONs, paragraphs, and the 01 level number of a variable start in columns 8, Area A. Everything else starts and ends in Area B, usually at column 12.
