PERFORM Statement













PERFORM Statement

       Simple form of PERFORM acts very similar to GO TO statement, except that after execution 
       of paragraph control will return back to the next statement of PERFORM statement.

   
       Format - 1
            
             PERFORM  procedure-name-1  



       Example 1 -

       Below picture represents the difference between GO TO and PERFORM statements.

        
               
    
       In above example, when we use PERFORM in the place of GO TO statement, after execution 
       of PARA-1, control return back to the next statement of PERFORM and continued the 
       execution. 



     
       Example 2 -

             PERFORM  PARA-1  THRU  PARA-2 

     
       PARA-1 & PARA-2 are paragraph names. Execution of PERFORM statement causes control transfers 
       to the first statement of PARA-1. control executes all the statements from the first statement 
       of PARA-1 to last statement of PARA-2 and the control return to the next executable statement 
       of PERFORM statement.  if there are any other paragraphs between PARA-1 and PARA-2, all those 
       statements are included for execution.








    PERFORM with TIMES phrase 

    
     Format - 2
            
             PERFORM  procedure-name-1     data-item-1/literal-1  TIMES


     
      This is very similar to what we discussed in 1st format of PERFORM, except that, here we can 
      specify , how many times we want to execute same set of commands.


  
      Example -    PERFORM  PARA-1  THRU PARA-2  10 TIMES.

      All the statements in the scope of PARA-1 thru PARA-2 are get executed 10 times.  Control then 
      passes to the next executable statement following PERFORM statement






    
    PERFORM with UNTIL phrase 

   
     Format - 3
            
            PERFORM  procedure-name-1      
                                       UNTIL condition-1
   
     
     In this format all the statments come under scope of procedure-name-1 & procedure-name-2 are get 
     executed till condition-1 becomes true. It is very similar to WHILE statement in other programming
     languages.
    
 
     - TEST BEFORE is the default one.  condition-1 will be tested before execution of statements.
       equalent to DO WHILE statement in other programming langauges.

     - If TEST AFTER is used, condition-1 will be tested after one execution of statements.
       This is equalent to DO UNTIL statement in other programming languages.

     In either case, if condition-1 is true, control is transfered to next executable statement after 
     PERFORM statement.
  





   PERFORM with VARYING phrase

         
     Format - 4
            
          PERFORM  procedure-name-1  [THRU/THROUGH procedure-name-2]   [TESTBEFORE/TESTAFTER]
           
                   VARYING    FROM   BY  UNTIL condition-1     

                   [ AFTER  FROM  BY  AFTER codition-2 ] 

                   [ AFTER  FROM  BY  AFTER codition-3 ] ... 
            
             
           d- -> data-item
           i- -> index-name
           l- -> literal 




    This format is little complex to understand for new learners.  Let us try to understand what exactly
    means of this syntax.   It is very similar to Format-3 ( PERFORM... UNTIL ), except  in this format
    it allows us to increase one or more data-item values automatically.

    Example 1 -    PERFORM PARA-1 VARIYING  WS-I FROM 1 BY  2  UNTIL WS-I > 10

    In above example , all statements in PARA-1 are executed till the codition associated with UNTIL 
    becomes true. For first iteration WS-I contains the value of 1 ( as it is specified after FROM ), 
    for second iteration WS-I value increase by 2 ( as it is specified after BY keyword) and condition 
    will be tested, if it false statements in PARA-1 will get executed and control come back to PERFORM, 
    Now WS-I value increased again by 2 and condition will be tested, if it is false, statements in 
    PARA-1 will get executed again. This loop continues till the condition becomes true.





    PERFORM with VARYING , AFTER phrases



    Example 2 -   PERFORM PARA-1 VARIYING  WS-I FROM 1 BY  2  UNTIL WS-I > 10
                                       AFTER WS-J FROM 1 BY 3 UNTIL WS-J > 20


    In this example, in addition to WS-I, WS-J value also get changed.
    For every valid value in WS-I, WS-J value start from 1 till WS-J > 20 becomes true.
    PERFORM statement execution ends only when WS-I > 10 becomes true.