| 1 | ### choose =====================================================================
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| 2 |
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| 3 | Tags
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| 4 | ----
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| 5 | scripting, JS, internal, treenode, general concept
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| 6 |
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| 7 | Parameters
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| 8 | ----------
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| 9 | n,k
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| 10 |
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| 11 | General description
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| 12 | -------------------
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| 13 |
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| 14 | Returns the number of combinations of n items taken k at a time.
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| 15 |
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| 16 | For example, the number of five card hands is choose(52,5)
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| 17 |
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| 18 | ```
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| 19 | n!
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| 20 | choose(n,k) = -------------
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| 21 | k! (n - k)!
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| 22 | ```
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| 23 | ###
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| 24 |
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| 25 |
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| 26 | Eval_choose = ->
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| 27 | push(cadr(p1))
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| 28 | Eval()
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| 29 | push(caddr(p1))
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| 30 | Eval()
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| 31 | choose()
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| 32 |
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| 33 | # Result vanishes for k < 0 or k > n. (A=B, p. 19)
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| 34 |
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| 35 | #define N p1
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| 36 | #define K p2
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| 37 |
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| 38 | choose = ->
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| 39 | save()
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| 40 |
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| 41 | p2 = pop()
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| 42 | p1 = pop()
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| 43 |
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| 44 | if (choose_check_args() == 0)
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| 45 | push_integer(0)
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| 46 | restore()
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| 47 | return
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| 48 |
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| 49 | push(p1)
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| 50 | factorial()
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| 51 |
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| 52 | push(p2)
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| 53 | factorial()
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| 54 |
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| 55 | divide()
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| 56 |
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| 57 | push(p1)
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| 58 | push(p2)
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| 59 | subtract()
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| 60 | factorial()
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| 61 |
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| 62 | divide()
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| 63 |
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| 64 | restore()
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| 65 |
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| 66 | choose_check_args = ->
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| 67 | if (isnum(p1) && lessp(p1, zero))
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| 68 | return 0
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| 69 | else if (isnum(p2) && lessp(p2, zero))
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| 70 | return 0
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| 71 | else if (isnum(p1) && isnum(p2) && lessp(p1, p2))
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| 72 | return 0
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| 73 | else
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| 74 | return 1
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| 75 |
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| 76 |
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