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COBOL basics

Introduction

This section presents the fundamentals of constructing COBOL programs. It explains the notation used in COBOL syntax diagrams and enumerates the COBOL coding rules. It shows how user-defined names are constructed and examines the structure of COBOL programs.

COBOL idiosyncrasies

COBOL is one of the oldest programming languages in use. As a result it has some idiosyncrasies which programmers used to other languages may find irritating.

When COBOL was developed (around the end of the 1950′s) one of the design goals was to make it as English-like as possible. As a result, COBOL uses structural concepts normally associated with English prose such as section, paragraph and sentence. It also has an extensive reserved word list with over 300 entries and the reserved words themselves, tend to be long. COBOL programs tend to be verbose especially when compared to languages like C.

When COBOL was designed, programs were written on coding forms (see below) , punched on to punch cards, and loaded into the computer using a punch card reader. These media (coding forms and punch cards) required adherence to a number formatting restrictions that some COBOL implementations still enforce today, long after the need for them has gone.

Although modern COBOL (COBOL 85 and OO-COBOL) has introduced many of the constructs required to write well structured programs it also still retains elements which, if used, make it difficult, and in some cases impossible, to write good programs.

COBOL syntax

COBOL syntax is defined using particular notation sometimes called the COBOL MetaLanguage.

In this notation, words in uppercase are reserved words. When underlined they are mandatory. When not underlined they are “noise” words, used for readability only, and are optional. Because COBOL statements are supposed to read like English sentences there are a lot of these “noise” words.

Words in mixed case represent names that must be devised by the programmer (like data item names).

When material is enclosed in curly braces { }, a choice must be made from the options within the braces. If there is only one option then that item in mandatory.

Material enclosed in square brackets [ ], indicates that the material is optional, and may be included or omitted as required.

The ellipsis symbol … (three dots), indicates that the preceding syntax element may be repeated at the programmer’s discretion.

Some notes on syntax diagrams

To simplify the syntax diagrams and reduce the number of rules that must be explained, in some diagrams special operand endings have been used (note that this is my own extension – it is not standard COBOL).

These special operand endings have the following meanings:

$i	uses an alphanumeric data-item
$il	uses an alphanumeric data-item or a string literal
#i	uses a numeric data-item
#il	uses a numeric data-item or numeric literal
$#i	uses a numeric or an alphanumeric data-item

An example syntax diagram

Note that for clarity data items may be separated from one another by means of an optional comma.
This has been done in the COMPUTE statement opposite

In COBOL, evaluating an arithmetic expression and assigning the result to a data item is achieved by means of the COMPUTE statement. The syntax diagram for the COMPUTE is shown below.

This syntax diagram may be interpreted as follows;

We must start a COMPUTE statement with the keyword COMPUTE.

We must follow the keyword with the name(s) of the numeric data item (or items – note the ellipsis symbol (…)) to be used to receive the result of the expression. The #i suffix at the end of word Result tells us that a numeric identifier/data item must be used.

Since the ellipsis symbol is placed outside the curly brackets we can interpret this to mean that each result field can have its own ROUNDED phase. In other words we could have a COMPUTE statement like -

COMPUTE Result1 ROUNDED, Result2 = ((9*9)+8)/5

where Result1 would be assigned a value of 18 and Result2 would be assigned a value of 17.8.

The square brackets after the Arithmetic Expression indicate that the next items are optional but if used we must choose between the ON SIZE ERROR or NOT ON SIZE ERROR phrases.

Because the END-COMPUTE is contained within the square brackets it must only be used when a SIZE ERROR or NOT SIZE ERROR phrase is used.

COBOL coding rules

Traditionally, COBOL programs were written on coding forms and then punched on to punch cards. Although nowadays most programs are entered directly into a computer, some COBOL formatting conventions remain that derive from its ancient punch-card history.

On coding forms, the first six character positions are reserved for sequence numbers. The seventh character position is reserved for the continuation character, or for an asterisk that denotes a comment line.

The actual program text starts in column 8. The four positions from 8 to 11 are known as Area A, and positions from 12 to 72 are Area B.

Although many COBOL compilers ignore some of these formatting restrictions, most still retain the distinction between Area A and Area B.

When a COBOL compiler recognizes the two areas, all division names, section names, paragraph names, FD entries and 01 level numbers must start in Area A. All other sentences must start in Area B.

In our example programs we use the compiler directive (available with the NetExpress COBOL compiler) – $ SET SOURCEFORMAT”FREE” – to free us from these formatting restrictions.

Ancient COBOL coding form

Name construction

All user-defined names, such as data names, paragraph names, section names condition names and mnemonic names, must adhere to the following rules:

1. They must contain at least one character, but not more than 30 characters.
2. They must contain at least one alphabetic character.
3. They must not begin or end with a hyphen.
4. They must be constructed from the characters A to Z, the numbers 0 to 9, and the hyphen.
5. They must not contain spaces.
6. Names are not case-sensitive: TotalPay is the same as totalpay, Totalpay or TOTALPAY.

The structure of COBOL programs

COBOL programs are hierarchical in structure. Each element of the hierarchy consists of one or more subordinate elements.

The hierarchy consists of Divisions, Sections, Paragraphs, Sentences and Statements.

A Division may contain one or more Sections, a Section one or more Paragraphs, a Paragraph one or more Sentences and a Sentence one or more Statements.

We can represent the COBOL hierarchy using the COBOL metalanguage as follows;

Divisions
A division is a block of code, usually containing one or more sections, that starts where the division name is encountered and ends with the beginning of the next division or with the end of the program text.

Sections
A section is a block of code usually containing one or more paragraphs. A section begins with the section name and ends where the next section name is encountered or where the program text ends.

Section names are devised by the programmer, or defined by the language. A section name is followed by the word SECTION and a period.
See the two example names below -

SelectUnpaidBills SECTION.
FILE SECTION.

Paragraphs
A paragraph is a block of code made up of one or more sentences. A paragraph begins with the paragraph name and ends with the next paragraph or section name or the end of the program text.

A paragraph name is devised by the programmer or defined by the language, and is followed by a period.
See the two example names below -

PrintFinalTotals.
PROGRAM-ID.

Sentences and statements
A sentence consists of one or more statements and is terminated by a period.
For example:

MOVE .21 TO VatRate
MOVE 1235.76 TO ProductCost
COMPUTE VatAmount = ProductCost * VatRate.

A statement consists of a COBOL verb and an operand or operands.
For example:

SUBTRACT Tax FROM GrossPay GIVING NetPay