Roll

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Roll (?) is a monadic scalar function which returns random or pseudo-random numbers. The name Roll comes from the analogy of rolling dice. Each scalar in the argument must be a positive integer, n, and Roll chooses one of the first n indices. Some dialects allow n to be zero, in which case Roll chooses a floating-point number between 0 and 1. Roll shares its glyph with the other random-result function, Deal.

Examples

      ? 4 ⍴ 6                   ⍝ Roll four six-sided dice
1 4 1 1
      ? 3 2 1 0                 ⍝ The result for 1 is always 1
2 2 1 0.9637543707
      {⍵[?8⍴≢⍵]} 'Hello'        ⍝ Choose 8 random letters
eeHHHolo
Works in: Dyalog APL, NARS2000, dzaima/APL

Description

Each scalar in the argument to Roll must be a positive integer.

The result of Roll on each number n is either

  • One of the elements of ⍳n chosen uniformly at random; or
  • A real number between 0 and 1 chosen uniformly at random, if n is 0 and the dialect has this extension.

Because Iota depends on index origin, Roll depends on index origin unless every number in the argument is 0.

The choices made by Roll do not have to be truly random: they may be pseudorandom (generated by a deterministic but difficult to predict algorithm) or taken from the operating system. They way random numbers are generated is controlled by the random link ⎕RL. Traditionally, APL uses the Lehmer random number generator, but Dyalog APL defaults to the allow choosing a different random number generator and defaults to the Mersenne Twister.

External links

Lessons

Documentation

APL built-ins [edit]
Primitives (Timeline) Functions
Scalar
Monadic Conjugate ∙ Negate ∙ Signum ∙ Reciprocal ∙ Magnitude ∙ Exponential ∙ Natural Logarithm ∙ Floor ∙ Ceiling ∙ Factorial ∙ Not ∙ Pi Times ∙ Roll ∙ Type ∙ Imaginary ∙ Square Root ∙ Round
Dyadic Add ∙ Subtract ∙ Times ∙ Divide ∙ Residue ∙ Power ∙ Logarithm ∙ Minimum ∙ Maximum ∙ Binomial ∙ Comparison functions ∙ Boolean functions (And, Or, Nand, Nor) ∙ GCD ∙ LCM ∙ Circular ∙ Complex ∙ Root
Non-Scalar
Structural Shape ∙ Reshape ∙ Tally ∙ Depth ∙ Ravel ∙ Enlist ∙ Table ∙ Catenate ∙ Reverse ∙ Rotate ∙ Transpose ∙ Raze ∙ Mix ∙ Split ∙ Enclose ∙ Nest ∙ Cut (K) ∙ Pair ∙ Link ∙ Partitioned Enclose ∙ Partition
Selection First ∙ Pick ∙ Take ∙ Drop ∙ Unique ∙ Identity ∙ Stop ∙ Select ∙ Replicate ∙ Expand ∙ Set functions (Intersection ∙ Union ∙ Without) ∙ Bracket indexing ∙ Index ∙ Cartesian Product ∙ Sort
Selector Index generator ∙ Grade ∙ Index Of ∙ Interval Index ∙ Indices ∙ Deal ∙ Prefix and suffix vectors
Computational Match ∙ Not Match ∙ Membership ∙ Find ∙ Nub Sieve ∙ Encode ∙ Decode ∙ Matrix Inverse ∙ Matrix Divide ∙ Format ∙ Execute ∙ Materialise ∙ Range
Operators Monadic Each ∙ Commute ∙ Constant ∙ Replicate ∙ Expand ∙ Reduce ∙ Windowed Reduce ∙ Scan ∙ Outer Product ∙ Key ∙ I-Beam ∙ Spawn ∙ Function axis ∙ Identity (Null, Ident)
Dyadic Bind ∙ Compositions (Compose, Reverse Compose, Beside, Withe, Atop, Over) ∙ Inner Product ∙ Determinant ∙ Power ∙ At ∙ Under ∙ Rank ∙ Depth ∙ Variant ∙ Stencil ∙ Cut ∙ Direct definition (operator) ∙ Identity (Lev, Dex)
Quad names Index origin ∙ Comparison tolerance ∙ Migration level ∙ Atomic vector
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