enable rand(::AbstractArray, [dims])

This is a rewrite of 6d329ce, 9d0282e and d9814ff (from #8309,
which was reverted).

base/random.jl

@@ -369,7 +369,10 @@ function rand{T<:Integer, U<:Unsigned}(g::RandIntGen{T,U})
 end
 
 rand{T<:Union(Signed,Unsigned,Bool,Char)}(r::UnitRange{T}) = rand(RandIntGen(r))
-rand{T}(r::Range{T}) = r[rand(1:(length(r)))]
+
+# Randomly draw a sample from an AbstractArray r
+# (e.g. r is a range 0:2:8 or a vector [2, 3, 5, 7])
+rand(r::AbstractArray) = @inbounds return r[rand(1:length(r))]
 
 function rand!(g::RandIntGen, A::AbstractArray)
     for i = 1 : length(A)
@@ -380,16 +383,19 @@ end
 
 rand!{T<:Union(Signed,Unsigned,Bool,Char)}(r::UnitRange{T}, A::AbstractArray) = rand!(RandIntGen(r), A)
 
-function rand!(r::Range, A::AbstractArray)
+rand!(r::Range, A::AbstractArray) = _rand!(r, A)
+
+# TODO: this more general version is "disabled" until #8246 is resolved
+function _rand!(r::AbstractArray, A::AbstractArray)
     g = RandIntGen(1:(length(r)))
     for i = 1 : length(A)
         @inbounds A[i] = r[rand(g)]
     end
     return A
 end
 
-rand{T}(r::Range{T}, dims::Dims) = rand!(r, Array(T, dims))
-rand(r::Range, dims::Int...) = rand(r, dims)
+rand{T}(r::AbstractArray{T}, dims::Dims) = _rand!(r, Array(T, dims))
+rand(r::AbstractArray, dims::Int...) = rand(r, dims)
 
 ## random BitArrays (AbstractRNG)
 

doc/stdlib/base.rst

@@ -4028,9 +4028,13 @@ A ``MersenneTwister`` RNG can generate random numbers of the following types: ``
 
    Populate the array A with random values.
 
-.. function:: rand(r, [dims...])
+.. function:: rand(coll, [dims...])
 
-   Pick a random element or array of random elements from range ``r`` (for example, ``1:n`` or ``0:2:10``).
+   Pick a random element or array of random elements from the indexable collection ``coll`` (for example, ``1:n`` or ``['x','y','z']``).
+
+.. function:: rand!(r, A)
+
+   Populate the array A with random values drawn uniformly from the range ``r``.
 
 .. function:: randbool([rng], [dims...])
 

test/random.jl

@@ -31,6 +31,12 @@ rand!(MersenneTwister(0), A)
 @test A == [858542123778948672  5715075217119798169;
             8690327730555225005 8435109092665372532]
 
+# rand from AbstractArray
+@test rand(0:3:1000) in 0:3:1000
+coll = Any[2, UInt128(128), big(619), "string", 'c']
+@test rand(coll) in coll
+@test issubset(rand(coll, 2, 3), coll)
+
 # randn
 @test randn(MersenneTwister(42)) == -0.5560268761463861
 A = zeros(2, 2)