Inserting into a Linked List in COBOL

Inserting into a Linked List: After Current Node

(The following discussion uses data names and procedure names defined elsewhere.)

The following COBOL code inserts a new node after the node to which CURR-P points (we should have already verified that CURR-P is not NULL):

     PERFORM 9000-ALLOCATE-TN.
     SET ADDRESS OF TN-NODE    TO CURR-P.  
     SET ALT-TN-NEXT-P         TO TN-NEXT-P.
*
     IF TN-NEXT-P = NULL
        SET LAST-P             TO NEW-P.
*
     SET TN-NEXT-P             TO NEW-P.
     SET NEW-P                 TO NULL.

Remember, 9000-ALLOCATE-TN allocates a new node and initializes it in ALT-TN-NODE, leaving ALT-TN-NEXT-P set to NULL. The situation then looks like this:

[before insertion]

First we point the new node to its successor:

[point to successor]

In the example illustrated, the current node is in the middle of the list. If it is the last node, then we must point LAST-P at the new node, which is now becoming the last node. If we're not maintaining LAST-P then we don't need to give the last node any special treatment.

Next we point the current node to the new node, which becomes its successor:

[point current to new node]

Finally we nullify NEW-P:

[point NEW-P to NULL]

This last step is not really necessary, but it's good hygiene. If you keep an extra copy of a pointer longer than you need to, it may lead you into a pitfall later.

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