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I am migrating from MATLAB to Julia and I am trying to generate a random integer in range 1:n.

For n < 21,

rand(r[1:n]) works.

However for n > 20, for example, rand(r[1:21]), I get this message:

ERROR: BoundsError() in getindex at range.jl:121
Benoit Pasquier
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user3411759
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1 Answers1

78

You can give a range as the first argument to rand, as in rand(1:n):

julia> rand(1:10)
7

julia> rand(1:10,10,10)
10x10 Array{Int64,2}:
 10   2  5  8   5   5  3   7   1   3
  5   1  4  2   4   4  1   6   6   9
  8   1  3  9   4   8  7   8   7  10
  3   8  1  5   7   9  7   8  10   7
  5   8  5  6   6   2  2   7   4   3
 10   4  8  8  10   5  1  10   5   1
  6   1  8  1   6   5  7  10   6  10
  5  10  2  5   4   5  4   1   3   9
  5   4  6  4   4   1  7   8   1   5
 10   2  6  4   3  10  7   3   8   7

The first argument to the general rand function usually gives a "thing to sample from", be it a range of values or a distribution object as defined in Distributions.jl.

StefanKarpinski
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    Does `rand(S)` use uniform sampling or other probability distribution over `S`? I mean if I would like to generate uniform random integer I usually do `rand(Uniform(1, n))`, then I apply `ceil(Int, ...)` or `floor(Int, ...)`. But I think that writing `rand(1:n)` is much more correct _if the sampling is done uniformly_. – Ribz Nov 25 '17 at 22:28