Finding the Previous Node

(The following discussion uses data names defined elsewhere.)

The following Cobol code searches a linked list to find the node just prior to the node to which CURR-P points:

     SET PREV-P                        TO FIRST-P.
*
     IF PREV-P NOT = NULL
        SET ADDRESS OF TN-NODE         TO PREV-P
        PERFORM UNTIL PREV-P = NULL
        OR TN-NEXT-P = CURR-P 
           SET PREV-P                  TO TN-NEXT-P
           IF PREV-P NOT = NULL
              SET ADDRESS OF TN-NODE   TO PREV-P
           END-IF 
        END-PERFORM.

This procedure is almost identical to that described elsewhere for searching the list. The only significant differences are:

We have a different search criterion;

We use PREV-P instead of CURR-P to traverse the list, since we are using CURR-P for something else.

The above code fragment finishes with TN-NODE representing the node prior to the current node. In the context of the entire program, we might want to add the line:

     SET ADDRESS OF TN-NODE TO CURR-P.

...because the rest of the program might expect CURR-P to be pointing to TN-NODE.

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