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Chapter 6

Structured Data

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Structured Data

• A data structure, or record, in COBOL is a method of combining several variables into one larger variable.– Example:

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Structured Data

• THE-WHOLE-MESSAGE is a structure variable or simply a structure.

• It is occasionally referred to as a compound variable or compound data.

• In a structure variable, the highest-level variable (the one that includes all the individual variables) has the level number 01.

• The 01 level must appear in Area A (columns 8 through 12).• The structure variable name appears in Area B (columns 12

through 72), and it does not have a PICTURE. • The variables that fall within the structure begin with numbers

higher than 01, and start in Area B (columns 12 through 72).

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Structured Data

• You can use the individual variables within a structure in the program as though they still were level 01 variables.

• In addition, you can use the structure variable as a variable. – For example, if all the variables within a structure

variable can be displayed, the structure variable itself can be displayed as a variable

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Structured Data

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Filler and How to use it?

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Filler and How to use it?

• They are used to format part of the display and are assigned values in the definition, but nothing is ever moved to these values in the program.

• They exist only to fill out THE-WHOLE-MESSAGE.

• this type of value in a structure variable can be defined as a filler by using the COBOL reserved word FILLER.

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Filler and How to use it?

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Filler and How to use it?

• A FILLER cannot be treated as a variable. • It is used to reserve space in a structure

variable. • You can assign a PICTURE and a VALUE to a

FILLER when it is defined, but you cannot use MOVE with FILLER.

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Calculating length of Data Structure

• A data structure is actually a series of individual variables, laid end to end in memory.

• The length of a simple data structure, such as this one (see example at next slide) used to create a displayable message, is the sum of all the lengths of the individual parts.

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Calculating length of Data Structure

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Calculating length of Data Structure

• A structure variable is treated as an alphanumeric variable. It has an implied PICTURE of X(nn), where nn is equal to the length of the structure variable.

• THE-WHOLE-MESSAGE has an implicit PICTURE of X(61).

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Calculating length of Data Structure

• You can move a value to a structure variable, but the move will affect the entire length of the variable.

• A structure variable and the variables that are the elements of a structure occupy the same memory area.

• When a variable is created by the compiler, it sets aside a number of bytes in memory that can be used to hold data.

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Calculating length of Data Structure

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Calculating length of Data Structure

• Compare figure 8.2 and figure 8.3• The two fillers, as well as EMP-NUMBER and EMP-

HOURLY, occupy some bytes that are in the same space in memory as the structure variable EMPLOYEE-DATA.

• When you use a command in COBOL to modify a variable in memory, the command looks at variables in memory as individual units.

• If you move a value to EMP-NUMBER, or use ADD 1 TO– EMP-NUMBER, COBOL acts on EMP-NUMBER as if it were a

single variable and ignores the fact that EMP-NUMBER is part of the structure EMPLOYEE-DATA.

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Calculating length of Data Structure

• If you move a message to EMPLOYEE-DATA, the command treats EMPLOYEE-DATA as if it were a PIC X(20) (the implied picture) and ignores the fact that EMPLOYEE-DATA has smaller variables within it.– Example

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Calculating length of Data Structure

• The variables in EMPLOYEE-DATA do not disappear, but the MOVE affects all 20 bytes of memory.

• The individual variables might no longer contain data that is correct for that variable type.

• From the figure:– EMP-NUMBER now contains ore, which certainly is not

valid numeric data. – This isn't a problem as long as you don't use a

command on EMP-NUMBER, such as ADD 1 TO EMP-NUMBER. I'll return to this issue in a moment.

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Calculating length of Data Structure

• This use of a structure variable is fairly common in display and print programs that might use a structure to format and display information line by line, and then at the end of the program might move a message to the entire structure and display it.

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Nested Structure Variables

• Any structure can contain another structure.

• The indention makes it clear that these variables are subordinate to THE-MESSAGE.

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Nested Structured Variables

• Refer to coding 1.• To calculate the length of a structure variable

containing one or more other structure variables.

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Misusing Structures

• It is common to find programs that define data structures in WORKING-STORAGE that never are used as structures.

• Grouping variables together under a structure variable because they are similar or to keep things tidy isn't a good practice.

• It is better to use comments in WORKING-STORAGE to separate groups of variables used for different purposes.

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Level 77

• When you see data in a structure in a program, you assume that the structure is used somewhere in the program as a structure, and you can be confused if it is not.

• A variable that is not a structure can be given a level number of 77 instead of 01:– For examples , see next slide

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Level 77

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Level 88

• Level 88 is a special level number used to improve the readability of COBOL programs and to improve IF tests.

• A level 88 looks like a level under another variable, – It does not have a PICTURE, – but it does have a value.

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Level 88

• A level 88 is always associated with another variable and is a condition name for that variable.

• Both of the following conditions test whether YES-NO is equal to "Y":

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Level 88

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Level 88

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Level 88

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Level 88

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Level 88

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Level 88

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Q&A

• Q What happens if I move to a FILLER?– A You can't. The compiler does not recognize FILLER as

a variable name; it is used only to reserve space in a structure variable.

– If you include a command such as MOVE 1 TO FILLER in a program, it will not compile and produces an error.

– You can move values to a structure variable that contains a FILLER.

– The MOVE will affect the structure variable and all of the variables within the structure, but you cannot directly move values to a FILLER.

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Q&A

• Q Why do I need to know the lengths of structures?– A You don't yet, but you will need to know how to

calculate this when you start creating printed reports (in Day 10, "Printing").

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Quiz

1. What is the length of THE-WHOLE-MESSAGE in the following example?

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Quiz

2. What is the implied PICTURE of THE-WHOLE-MESSAGE in question 1?

3. What is a data structure?4. If you move a value to a structure variable,

what happens to the values in the individual variables within the structure?

5. In the following code, what is another way of performing the test at line 004600?

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Quiz

6. In the following code, what is another way of performing the test at line 004600?

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Quiz

7. In the following code, what is another way of performing the test at lines 004600 and 004700?

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Quiz

8. Devise a level 88 condition name for YES-NO that would simplify the tests at lines 004600 through 004900 in the following code: