In Julia I created a Matrix A and want to set its value at (1, 1) to the value of 5. How do I do that? I tried this:
A = zeros(5, 5)
A[1][1] = 5
It throws the error
MethodError: no method matching setindex!(::Float64, ::Int64, ::Int64)
Stacktrace:
[1] top-level scope
@ In[38]:4
[2] eval
@ .\boot.jl:373 [inlined]
[3] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
@ Base .\loading.jl:1196
DNF's comment already gives you the answer, but I'll elaborate a little.
The basic issue with what you're doing is that A[1] is a valid indexing operation returning the first element of A, and therefore A[1][1] is trying to index into that first element, which is just a number. To see:
julia> A[1]
0.0
julia> A[1][1]
0.0
Essentially what you are doing is equivalent to this:
julia> x = 1
1
julia> x[1] = 2
ERROR: MethodError: no method matching setindex!(::Int64, ::Int64, ::Int64)
Stacktrace:
[1] top-level scope
@ REPL[12]:1
numbers in Julia are immutable, i.e. you cannot "replace" a number with another number in place. Arrays on the other hand are mutable:
julia> y = [1]
1-element Vector{Int64}:
1
julia> y[1] = 2
2
So you could also have done:
julia> A[1] = 5
5
julia> A
5×5 Matrix{Float64}:
5.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
0.0 0.0 0.0 0.0 0.0
Why does A[1] work? A single index will give you the element you arrive at when you count down row-wise, continuing at the next column when you get to the bottom:
julia> z = rand(2, 2)
2×2 Matrix{Float64}:
0.727719 0.983778
0.792305 0.0408728
julia> z[3]
0.9837776491559934