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33 lines
1.1 KiB
33 lines
1.1 KiB
2 years ago
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Exercise 3.21: Ben Bitdiddle decides to test the
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queue implementation described above. He types in the procedures to the Lisp
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interpreter and proceeds to try them out:
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(define q1 (make-queue))
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(insert-queue! q1 'a)
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((a) a)
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(insert-queue! q1 'b)
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((a b) b)
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(delete-queue! q1)
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((b) b)
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(delete-queue! q1)
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(() b)
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“It’s all wrong!” he complains. “The interpreter’s response shows that the
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last item is inserted into the queue twice. And when I delete both items, the
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second b is still there, so the queue isn’t empty, even though it’s
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supposed to be.” Eva Lu Ator suggests that Ben has misunderstood what is
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happening. “It’s not that the items are going into the queue twice,” she
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explains. “It’s just that the standard Lisp printer doesn’t know how to make
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sense of the queue representation. If you want to see the queue printed
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correctly, you’ll have to define your own print procedure for queues.” Explain
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what Eva Lu is talking about. In particular, show why Ben’s examples produce
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the printed results that they do. Define a procedure print-queue that
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takes a queue as input and prints the sequence of items in the queue.
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