19 lines
685 B
19 lines
685 B
2 years ago
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Exercise 3.16: Ben Bitdiddle decides to write a
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procedure to count the number of pairs in any list structure. “It’s easy,”
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he reasons. “The number of pairs in any structure is the number in the
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car plus the number in the cdr plus one more to count the current
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pair.” So Ben writes the following procedure:
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(define (count-pairs x)
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(if (not (pair? x))
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0
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(+ (count-pairs (car x))
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(count-pairs (cdr x))
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1)))
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Show that this procedure is not correct. In particular, draw box-and-pointer
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diagrams representing list structures made up of exactly three pairs for which
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Ben’s procedure would return 3; return 4; return 7; never return at all.
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