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23 lines
991 B
23 lines
991 B
2 years ago
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Exercise 2.77: Louis Reasoner tries to evaluate
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the expression (magnitude z) where z is the object shown in
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Figure 2.24. To his surprise, instead of the answer 5 he gets an error
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message from apply-generic, saying there is no method for the operation
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magnitude on the types (complex). He shows this interaction to
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Alyssa P. Hacker, who says “The problem is that the complex-number selectors
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were never defined for complex numbers, just for polar and
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rectangular numbers. All you have to do to make this work is add the
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following to the complex package:”
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(put 'real-part '(complex) real-part)
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(put 'imag-part '(complex) imag-part)
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(put 'magnitude '(complex) magnitude)
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(put 'angle '(complex) angle)
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Describe in detail why this works. As an example, trace through all the
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procedures called in evaluating the expression (magnitude z) where
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z is the object shown in Figure 2.24. In particular, how many
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times is apply-generic invoked? What procedure is dispatched to in each
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case?
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