15 lines
477 B
Racket
15 lines
477 B
Racket
#lang racket
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(define (middle x)
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(cond
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[(negative? x) (middle (- x))]
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[(< x 100) "error: number too small"]
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[else
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(define s (number->string x))
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(define l (string-length s))
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(cond [(even? l) "error: number has even length"]
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[else (define i (quotient l 2))
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(substring s (- i 1) (+ i 2))])]))
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(map middle (list 123 12345 1234567 987654321 10001 -10001 -123 -100 100 -12345))
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(map middle (list 1 2 -1 -10 2002 -2002 0))
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