setindex! error in Julia

Question

I have been writing scripts in Julia recently, and have run across a problem using the setindex! function that I cannot find an answer to in any documentation (I have also searched stackoverflow, but could not find an answer - my apologies if my search was not good enough and I am repeating a question).

I am getting a MethodError relating to set index with code similar to the following (the error also appears in this code, which is altered simply to make it simpler):

a = 0:0.01:1
a = 2 * pi * (a - 0.4)
a[abs(a) .> pi] += - sign(a[a .> pi]) * 2 * pi

I realize that in the above code I could achieve a similar effect by simply changing the initial expression used to generate a so that it is never greater than pi in magnitude, but in the original code this would be much less readable due to intermediate steps that are not included - additionally, regardless of whether that is possible with this particular problems, there will be other instances using setindex! similarly which I would like to have a solution to.

I have tried using integer indexes instead of logical indexes and have tried storing the logical or integer index as another value. Neither has worked. I would guess this is coming from a fairly basic misunderstanding on my part, but thought this would be a good resource for help.

• If this post is not-standard for stackoverflow in any way, I apologize, this is my first (and I did read the guidelines, but may have perfectly implemented them).

Thanks in advance

Answer 1

You haven't materialized the FloatRange into an Array, so there aren't really any indices to play with yet. It's just a rangelike object:

julia> a = 0:0.01:1
0.0:0.01:1.0

julia> a = 2 * pi * (a - 0.4)
-2.5132741228718345:0.06283185307179587:3.769911184307752

julia> dump(a)
FloatRange{Float64} 
  start: Float64 -251.32741228718345
  step: Float64 6.283185307179586
  len: Float64 101.0
  divisor: Float64 100.0

Compare with:

julia> a = [a]
101-element Array{Float64,1}:
 -2.51327
 -2.45044
 -2.38761
[...]
  3.64425
  3.70708
  3.76991

after which

julia> maximum(a)
3.769911184307752

julia> a[abs(a) .> pi] += - sign(a[a .> pi]) * 2 * pi;

julia> maximum(a)
3.141592653589793

It's the difference between

julia> 1:2:9
1:2:9

julia> [1:2:9]
5-element Array{Int32,1}:
 1
 3
 5
 7
 9