| 1 | # Binomial coefficient
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| 2 | #
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| 3 | # Input: tos-2 n
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| 4 | #
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| 5 | # tos-1 k
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| 6 | #
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| 7 | # Output: Binomial coefficient on stack
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| 8 | #
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| 9 | # binomial(n, k) = n! / k! / (n - k)!
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| 10 | #
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| 11 | # The binomial coefficient vanishes for k < 0 or k > n. (A=B, p. 19)
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| 12 |
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| 13 |
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| 14 |
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| 15 |
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| 16 | Eval_binomial = ->
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| 17 | push(cadr(p1))
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| 18 | Eval()
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| 19 | push(caddr(p1))
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| 20 | Eval()
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| 21 | binomial()
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| 22 |
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| 23 | binomial = ->
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| 24 | save()
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| 25 | ybinomial()
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| 26 | restore()
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| 27 |
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| 28 | #define N p1
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| 29 | #define K p2
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| 30 |
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| 31 | ybinomial = ->
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| 32 | p2 = pop()
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| 33 | p1 = pop()
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| 34 |
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| 35 | if (BINOM_check_args() == 0)
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| 36 | push(zero)
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| 37 | return
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| 38 |
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| 39 | push(p1)
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| 40 | factorial()
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| 41 |
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| 42 | push(p2)
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| 43 | factorial()
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| 44 |
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| 45 | divide()
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| 46 |
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| 47 | push(p1)
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| 48 | push(p2)
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| 49 | subtract()
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| 50 | factorial()
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| 51 |
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| 52 | divide()
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| 53 |
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| 54 | BINOM_check_args = ->
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| 55 | if (isnum(p1) && lessp(p1, zero))
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| 56 | return 0
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| 57 | else if (isnum(p2) && lessp(p2, zero))
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| 58 | return 0
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| 59 | else if (isnum(p1) && isnum(p2) && lessp(p1, p2))
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| 60 | return 0
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| 61 | else
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| 62 | return 1
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| 63 |
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