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34 lines
793 B
34 lines
793 B
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Exercise 1.6: Alyssa P. Hacker doesn’t see why
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if needs to be provided as a special form. “Why can’t I just define it
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as an ordinary procedure in terms of cond?” she asks. Alyssa’s friend
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Eva Lu Ator claims this can indeed be done, and she defines a new version of
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if:
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(define (new-if predicate
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then-clause
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else-clause)
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(cond (predicate then-clause)
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(else else-clause)))
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Eva demonstrates the program for Alyssa:
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(new-if (= 2 3) 0 5)
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5
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(new-if (= 1 1) 0 5)
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0
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Delighted, Alyssa uses new-if to rewrite the square-root program:
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(define (sqrt-iter guess x)
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(new-if (good-enough? guess x)
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guess
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(sqrt-iter (improve guess x) x)))
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What happens when Alyssa attempts to use this to compute square roots?
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Explain.
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