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Text File  |  1987-12-09  |  1KB  |  69 lines
  1. Class DiscreteProbability
  2.     | randnum |
  3. [
  4.     initialize
  5.         randnum <- Random new
  6.  
  7. |    next
  8.         ^ self inverseDistribution: randnum next
  9.  
  10. |    computeSample: m outOf: n    
  11.         m > n ifTrue: [^ 0.0]
  12.         ^ n factorial / (n - m) factorial
  13. ]
  14.  
  15. Class Geometric    :DiscreteProbability
  16.     | prob |    
  17.  
  18. [
  19.     mean: m
  20.         prob <- m
  21.  
  22. |    mean
  23.         ^ 1.0 / prob
  24.  
  25. |    variance
  26.         ^ (1.0 - prob) / prob * prob
  27.  
  28. |    density: x
  29.         x > 0 ifTrue: [^prob * ((1.0-prob) raisedTo: x-1)]
  30.               ifFalse: [^1.0]
  31.  
  32. |    inverseDistribution: x
  33.         ^ (x ln / (1.0 - prob) ln) ceiling
  34. ]
  35.  
  36. Class Binomial    :DiscreteProbability
  37.     | number prob |
  38. [
  39.     events: num mean: p
  40.         (p between: 0.0 and: 1.0)
  41.            ifFalse: [self error: 'mean must be > 0'].
  42.         number <- num.
  43.         prob <- p
  44.  
  45. |    mean
  46.         ^ prob
  47.  
  48. |    variance
  49.         ^ prob * (1 - prob)
  50.  
  51. |    density: x
  52.         (x between: 0.0 and number)
  53.            ifTrue: [^((self computeSample: x outOf: number)
  54.             / (self computeSample: x outOf: x))
  55.             * (prob raisedTo: x) * ((1 - prob) raisedTo: number - x)]
  56.            ifFalse: [^0.0]
  57.  
  58. |    inverseDistribution: x
  59.         x <= prob
  60.             ifTrue: [^ 1]
  61.             ifFalse: [^ 0]
  62.  
  63. |    next
  64.     | t |
  65.         t <- 0.
  66.         number timesRepeat: [t <- t + super next].
  67.         ^ t
  68. ]
  69.