[julia-users] Fastest method to create a matrix of random integers

[David P. Sanders]

Hi,

What is the fastest way to create a matrix containing random integers: 1 or 
-1 with equal probability?
I have tried the following options but was wondering if I am missing 
something.
(I also need 1, 0 or -1 with different probabilities later, which is why I 
don't just use a boolean random matrix.)

Thanks,
David.

julia> function rand_pm1_matrix(L)
           matrix::Array{Int8, 2} = 2 .* randbool(L,L) .- 1
       end
rand_pm1_matrix (generic function with 1 method)

julia> function elementwise(L)
           A = Array(Int8, (L, L))
           for i in 1:length(A)
               if rand() < 0.5
                   A[i] = -1
               else
                   A[i] = 1
               end
           end
           A
       end
elementwise (generic function with 1 method)

julia> function elementwise_random_block(L)
           r = rand(L, L)
           A = Array(Int8, (L, L))
           for i in 1:length(A)
               if r[i] < 0.5
                   A[i] = -1
               else
                   A[i] = 1
               end
           end
           A
       end
elementwise_random_block (generic function with 1 method)

julia> L = 10
10

julia> rand_pm1_matrix(L);

julia> elementwise(L);

julia> elementwise_random_block(L);

julia>

julia> L = 100
100

julia> @time rand_pm1_matrix(L);
elapsed time: 6.5661e-5 seconds (188588 bytes allocated)

julia> @time elementwise(L);
elapsed time: 0.001172332 seconds (10160 bytes allocated)

julia> @time elementwise_random_block(L);
elapsed time: 0.000131455 seconds (90240 bytes allocated)

julia>

julia> L = 1000
1000

julia> @time rand_pm1_matrix(L);
elapsed time: 0.013428217 seconds (18125456 bytes allocated)

julia> @time elementwise(L);
elapsed time: 0.074257183 seconds (1000176 bytes allocated)

julia> @time elementwise_random_block(L);
elapsed time: 0.007721683 seconds (9000288 bytes allocated)

julia>

julia> L = 10000
10000

julia> @time rand_pm1_matrix(L);
elapsed time: 2.75006866 seconds (1812500448 bytes allocated)

julia> @time elementwise(L);
elapsed time: 6.727148454 seconds (100000176 bytes allocated)

julia> @time elementwise_random_block(L);
elapsed time: 2.819847794 seconds (900000288 bytes allocated)

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