Transcript COBOL - Softsmith – Offshore Testing Services

COBOL
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Cobol is one of the most robust language in the software field, so far
Cobol turned 50, in 2009
Cobol has stood the test of time
Common Business Oriented Language
It is very simple, no confusion
English like language – people think better in natural language than in
programming language
Number of lines coded in COBOL as on date, exceeds the cumulative total
number of lines coded in all known programming languages
Data maintained by COBOL program will take 10 to 15 years by a super
computer, to be migrated to another platform or technology
Hence COBOL is here to stay further
Demand for COBOL still continues with same or more strength
COBOL has adapted well to stay fit in new millennium as well
Unfortunately people have the myth in mind, that COBOL is out dated
All big time software service companies, invariably work in COBOL and train
people in COBOL, year after year
Program Structure
• Each program has 4 divisions
– Identification division
– Environment division
– Data division
– Procedure division
• Each division has sections within them
• The section names are different for each
division
• Each section has definition or logic
statements
Identification Division
• This is the primary section that gives an identity to the program
• COBOL is not case sensitive – we can write code in uppercase or
lowercase letters
• PROGRAM-ID is the main phrase in this section that gives a unique
name to programs.
• DATE-WRITTEN and DATE-COMPILED are used to denote
documenting part of the program
• AUTHOR is the clause to specify the name of the programmer.
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Identification division.
Program-id. Myprog.
Author. Naga.
Date-written. 12-10-2008.
Date-compiled. 13-10-2008.
Margins
• Since cobol evolved from punched cards time, the
program lines are restricted to 80 column width
• In this also, there are column boundaries that play major
role
• Columns 1 to 6 are meant for line numbers
• Column 7 is meant for comment. A star mark in this
column, denotes that line is a comment
• Column 8 is called A margin. Columns 8 to 11 is called A
area
• Column 12 is called B margin. Columns 12 to 80 is
called B area
• Some statements must start from A area, and some
other must start from B area.
• If margin rules are not followed, the compiler will issue a
syntax error
Environment Division
• This is used to specify the computer types, special
symbols and files related data.
• This can have CONFIGURATION SECTION and INPUTOUTPUT SECTION.
• Configuration section can have the following.
– SOURCE-COMPUTER. TDM180.
– OBJECT-COMPUTER. TDM180.
– SEPCIAL-NAMES.
- DECIMAL POINT IS COMMA, CURRENCY SIGN IS “$”.
• In some countries the decimal points is not a dot and the
currency signs keep varying.
• Input-output section is mainly for file specifications. This
is given at the end of the presentation.
Data Division
• This is used to declare variables, screens,
parameters to other programs and file record
descriptions.
• This can have file section, working-storage
section, linkage section, screen section.
• If sections are not used, they can be omitted as
well. But usually every program will have at least
one variable declaration.
• Unlike other languages, ALL VARIABLES
DECLARED IN A PROGRAM ARE GLOBAL
VARIABLES.
Variables
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Variables are to be declared in working-storage section
Variables can be scalar, arrays, records, file descriptor records
The name of variable can have alphanumeric, with first character as alphabet, and can have only dash (hyphen) in
it. Hypen must be embedded
The type of variable is determined by the picture clause
77 level variables are scalar variables; arrays and records are explained later in this presentation
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77 var-1 picture 999.
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9 stands for a single digit number. This is a 3 digit number
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77 var-2 picture xxx.
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77 var-3 picture S99.99
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X stands for alphanumeric. This is a 3 character alphanumeric
S sign for numeric variable stands for sign + or -. A dot in picture clause with trailing 9s will indicate the number of decimals. Dot
stands for explicit decimal point
77 var-4 picture 9(5) value 100.
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Value specifies what must be the initial value when variable is created
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77 var-4 picture a(20) value all “a”.
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9(4) in picture clause is same as 9999, and x(5) is same as xxxxx. The repetition count can be given in brackets.
77 var-5 picture 99V99
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Picture A stands for alphabetic. All clause in value, will fill the entire variable with that value.
V in picture clause with trailing 9s will indicate the number of decimals. V stands for explicit decimal point. In storage, the decimal
point will not be stored as a character. Hence when storing in disk, it will store 99.78 as 9978, without decimal point. But it will
know last 2 digits are for decimal.
77 var-6 picture zzz9.99.
If this variable has a value of 59.28, and picture is set as 9999.99, it will display as 0059.28. If we do not want to
display leading zeroes, we must use z to suppress zeroes.
Procedure Division
• This is the place where the program logic is coded.
• This can have user defined sections. Usually, this is
segmented as multiple paragraphs.
• Each paragraph is a set of statements that can act as a
function or procedure as used in other languages
• Since all variables are global, every variable is
accessible from every paragraph within that program.
• Statement delimiter is dot.
• Procedure division para names must start in A area
• Procedure division statements must start in B area
• The program starts from the very first line of the
procedure division till the statement STOP RUN is
reached.
Initializing Variables
• Move is used to set the value of variables
• Assume v1 is numeric and s1 is alphanumeric
text
– Move zeroes to n1.
– Move spaces to s1
• There is a verb Initialize that can automatically
determine the type of variable and move either
zeroes or spaces
– Initialize n1.
– Initialize s1.
Simple User Input
• Accept is the verb to get values from user
• Accep1 n1. – This will get the values from user,
from the cursor position
• Accept (10, 15) n1. – This will get the value of
the variable at line 10 column 15 on the screen
• Accept (10, 15) n1 with prompt. – This will do
same as above, except it will display dashes so
that you the length of the variable
• Accept n1 from date. – This will give you the
current system date into variable
• Accept n1 from time. – This will give you the
current system time into variable
Simple Output
• Display is the verb to display values of
variables on screen
• Display n1. – This will display the values
on screen, from the cursor position
• Display (10, 15) n1. – This will display the
value of the variable at line 10 column 15
on the screen
• Display Blank screen. – This will erase the
content of screen. This must be defined in
screen section.
Arithmetic Verbs
• Unlike other languages, cobol prefers to
explicitly spell out the operations.
– Add v1 to v2 giving v3.
– Subtract v1 from v2 giving v3.
– Multiply v1 by v2 giving v3.
– Divide v1 by v2 giving v3 remainder v4.
• Formula can be expressed thru compute
verb
– Compute v1 = (v2 + v3) * (v2 – v3).
Conditions
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If and Evaluate are used for conditional branching
The relational operators as well as spelt out phrases can be used
If a < b – this can be written as if a is less than b
>, <, =, >=, <= are the available relational operators
NOT, AND, OR are the logical operators
Less than, greater than, equal to are the spelt out phrases
• If a is less than b then
Do some thing
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Do the other way
• End-if.
• To check an alphanumeric variable for what type of data it holds, we
can use if a is numeric, if a is alphabetic, conditions. Based on the
value this will be true or false. We can then take decisions.
Multiple Conditions
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We can use if-then-else if for multi level conditions
To make it very simple, we can use evaluate verb
Evaluate a
When 1 perform para1
When 2 perform para2
When 3 perform para3
When other perform para4
End-evaluate
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Each when statement will be treated as a separate if condition. When 3
here means, if a is equal to 3.
If no condition is matched, the other clause is executed. This is similar to
the switch statements in other languages.
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Without checking anything, we can transfer the program flow using go to.
GO TO mypara. This will suddenly branch to that para. Go To is not
supposed to be used, as it is not a good programming practice.
Perform Loop
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Perform is the loop in cobol
There are many variations of this verb
Perform para1. This will execute only that para only once
Perform para1 5 times. This will repeat the execution
Perform para1 thru para5. This will execute any para
from para1 to para5, that appear one after the other on
the source code
• Perform para1 until a < b. This will execute the para until
the condition becomes true.
• Perform para1 varying v1 from 1 by 1 until v1 > 10. This
will use v1 as the loop counter and increment it by 1, till it
reaches 10.
Records
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Records are nothing but collection of related variables
These are equivalent to structures in other languages
The record variables do have level numbers
Level numbers 1 to 76 are used
01 employee-record.
05 employee-code picture 9(5).
05 employee-name picture x(30).
05 dob.
10 yyyy picture 9999.
10 mm picture 99.
10 dd picture 99.
05 designation picture x(3).
05 doj.
10 yyyy picture 9999.
10 mm picture 99.
10 dd picture 99.
05 salary picture 9(6).99.
The record can have any levels. They are grouped by level numbers.
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Yyyy is the sub field of dob. Dob itself is a sub field of employee-record.
To denote a field in a record, we need to qualify with OF.
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Yyyy of doj of employee-record
Designation of employee-record
Move Corresponding
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When we have 2 similar records, and we want to move data from one to the other, we
can use this
We need not write separate move statements for each field of the record
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01 rec1.
05 field1 pic 999.
05 field2 pic xx.
05 filed3 pic x.
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01 rec2.
05 field2 pic xx.
05 filed3 pic x.
05 field1 pic 999.
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To move data in a single statements, to respective fields, we can say
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Move corresponding rec1 to rec2.
This will move field1 of rec1 to field1 of rec2, field2 of rec1 to field2 of rec2, field3 of
rec1 to field3 of rec2.
Though the fields are in different positions in the 2 records, cobol will match the field
names and then move values.
Condition Names
• Variables hold values, and based on these values we may have to
take decisions in code
• Usual way is to have an if condition to see whether the variable
satisfies a condition
• This can be easily done in a more readable way using condition
names
• 77 mm pic 99.
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88 jan value 01.
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88 feb value 02.
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88 march value 03.
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88 april value 04.
• We can write if condition to check the month as if mm = 2 then…
• The same can be written in a better way as if jan then. When mm is
set to 1, this name jan is set to true and if condition becomes true.
• People prefer to write English like conditions, such as, if file-isclosed, if account-is-active, if ticket-is-confirmed etc. This makes
programs simple
Arrays
• Arrays are nothing but a set of variables with same
name, differentiated by an index number
• 01 rec1.
– 05 student-name picture x(30) occurs 100 times.
• Occurs clause determines the number of elements in an
array
• Move spaces to student-name (10) will access the 10th
element of the array
• Arrays start with index as 1
• Any element that is accessed beyond the limit will throw
a runtime error and hence we need to be careful
• We can have multi level arrays also, but they are very
rarely used in business applications
File Handling
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Cobol supports sequential files, relative files and indexed files
To handle files, we must specify file type and mode in input-output section
The following is a standard one when we need to access files - SELECT myfile ASSIGN TO DISK
Myfile is just a logical name that we want to use later in program. It can be EmployeeFile, PayFile
etc
In data division, we must have a FD (file descriptor) part and record definition of the same
There are RD and SD definitions (reporting and sorting descriptions). These 2 are not discussed
in this presentation
We can access disk, tape and printer files. In that case, the assign to clause will change.
In procedure division, OPEN, READ, WRITE, START, REWRITE, DELETE, CLOSE are the
common verbs that we will use to deal with files.
Every file operation must be checked for the status of the operation. COBOL gives the status of
last operation in a variable called file-status.
In working-storage, we must define a variable ws-fs.
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01 ws-fs.
05 s1 pic 9.
05 s2 pic 9.
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It contains 2 parts, first digit and second digit. A zero in both digits indicates, success; all other
codes must be handled specially.
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Standard practice is to load record with values before WRITE and initialize record before READ
operations
Defining Sequential Files
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In sequential files, we can add records one after the other and read them one after the other.
We cannot directly jump to any record we want; we need to go from the beginning
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SELECT myfile assign to disk
Organization is sequential
Access mode is sequential
file status is ws-fs.
In the FD part of file section, we must use the standard phrases to attach a physical file to the logical name myfile.
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FD FD myfile
RECORD CONTAINS 62 CHARACTERS
LABEL RECORD IS STANDARD
DATA RECORD IS employee-record
VALUE OF FILE-ID "myfile.DAT".
01 employee-record.
05 emp-code pic 9(5).
05 emp-name pic x(30).
05 dob pic 9(8).
05 doj pic 9(8).
05 designation pic x(3).
05 ctc pic 9(6)V99.
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The record character count given in the 01 record, must match with FD description.
Label record is to tell the file system of the machine that they must be standard. Other clause can be OMITTED
FILE-ID is the one that actually binds a physical file myfile.dat to the logical file.
Handling Sequential Files
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Open statement must have the mode specified.
The open modes are input, output, extend
We cannot delete a record in a sequential file; however we can even rewrite records, only in certain cobol versions
In many to most cobol versions, opening sequential file in I-O mode is not allowed
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For input mode, the file must exist; in output mode, the previous file data will be erased; in extend mode, the
previous file data can be appended with new data
Open Input myfile.
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To write data to the file, we must first load valid data to the file record, as given in the FD part
Then we need to issue a WRITE command
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Open extend myfile.
WRITE emp-record
END-WRITE
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To read a record, we must use READ command. The AT END clause, will tell whether we reached end of file or
not.
READ myfile
AT END move 1 to ws-end-of-file
END-READ
The read will get the record that is fetched from the disk to the FD record.
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To close the file, use, CLOSE myfile.
When program ends, all open files will be closed.
Relative Files
• Relative files are used, to directly jump to any
record based on the position of the record
• E.g. If we want to go to the 57th record in a file,
we can use relative files
• However, relative files are nowadays very rarely
used and they are replaced by indexed files
• For relative file, we must define organization is
relative, access mode is random.
• Since it is not used practically in current day
scenarios, we do not address it in this
presentation
Defining Indexed Files
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Indexed files are one of the most used file types as they establish a good DBMS
Records in indexed files can be accessed sequentially or randomly as we like
Using relative files, based on some key values, we can search for a record and the search is binary search and not sequential search;
hence it is faster
COBOL can handle billions of records in indexed file itself
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SELECT myfile assign to disk
Organization is indexed
Access mode is dynamic
Record key is emp-code of employee-record
file status is ws-fs.
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In the FD part of file section, we must use the standard phrases to attach a physical file to the logical name myfile.
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FD FD myfile
RECORD CONTAINS 62 CHARACTERS
LABEL RECORD IS STANDARD
DATA RECORD IS employee-record
VALUE OF FILE-ID "myfile.DAT".
01 employee-record.
05 emp-code pic 9(5).
05 emp-name pic x(30).
05 dob pic 9(8).
05 doj pic 9(8).
05 designation pic x(3).
05 ctc pic 9(6)V99.
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In the above file, we can search for any emp-code much faster.
Accessing Indexed Files
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The open mode can be input, i-o, or output.
Close command is same for all types of files
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If the file is opened with sequential access mode, we can use, this type of command to keep on reading the next records
READ SupplierFile
NEXT RECORD
AT END SET EndOfFile TO TRUE
END-READ
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If the access mode is set to direct access, we can use this type of read command, to go to the specific record that has the key.
READ SupplierFile
INVALID KEY DISPLAY "SUPP STATUS :-", SupplierStatus
END-READ
If a record with the key value populated before read, is not found, invalid key statement is executed
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If the access mode is dynamic, we can use start verb to position the reading point based on some key value.
START Oil-Details-File
KEY IS GREATER THAN 150
INVALID KEY DISPLAY "Start Error”
END-START.
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We can use equal to, less than and other operators in the Key clause of start. If a key is found satisfying the condition, the start is
successful; else invalid key statement is executed
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WRITE command is also used with INVALID KEY clause
REWRITE can be used to update a record, provided we have executed a START command before rewriting, so that the correct record is
pointed.
REWRITE is also used along with INVALID KEY option
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Program Branching, Termination
• We can use CALL command to call another compiled
cobol program
• CALL “prog2”.
• The “prog2” must be the one appearing in program-id of
the called program
• If we define, linkage section, we can use call “prog2”
using linkage-entries.
• Linkage section is not discussed in this presentation.
• To stop a program from execution, use STOP RUN.
• To exit from a called program, use EXIT PROGRAM.
• To exit from a paragraph, use EXIT.
Example Programs
• Refer to this site.
• You will get a lot of examples.
• Syntax may slightly vary between IBM, HP
and other COBOL versions.
• But, get the concept, right.
• http://www.csis.ul.ie/COBOL/examples/def
ault.htm