3GL Program Design - Dealing with errors and more work with decisions and loops

Dealing with errors and more work with decisions and loops

Exercise 2: Dial-in shopping

CALL connect_to_grocer
IF connect_OK
THEN CALL send_credit_card_number
             INPUT shopping_item
             DOWHILE shopping_item <> 'END'
              CALL transmit(shopping_item)
               INPUT shopping_item
            END DOWHILE
            CALL disconnect_link
ELSE DISPLAY 'Connection not available'
ENDIF

The dial-in shopping list algorithm has room for improvement. What problems does it have? Rewrite the algorithm to overcome the problems.

The major problem is that we dial-in to the grocer's computer and send a credit card number before testing the shopping list to make sure that it isn't empty. The loop priming, apart from not sending an empty shopping list, can also guard against an unecessary phone call.

INPUT shopping_item
IF shopping <> 'END'
THEN
CALL connect_to_grocer
IF connect_OK
    THEN CALL send_credit_card_number
               DOWHILE shopping_item <> 'END'
                CALL transmit(shopping_item)
                 INPUT shopping_item
              END DOWHILE
              CALL disconnect_link
    ELSE DISPLAY 'Connection not available'
ENDIF
ELSE DISPLAY 'Empty shopping list, call to grocer cancelled'
ENDIF

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