l want to create an array having this structure
k[1]= 1
k[2]= 2
k[3]= 3
k[4]= 4
l tried it in this way but it's not working
n= 10
for i in 1:n
k[i]= i
end
any suggestions ?
4 Answers
You havent initialized the array, so calling k[1] or k[2] or k[n] wont return an error
You should either:
n= 10
k = Array(Int64, n) #or Float64 or Any instead of Int64
for i in 1:n
k[i]= i
end
or you could
n= 10
k = []
for i in 1:n
push!(k,i)
end
I suggest the former, the other method would be more suited if you woulnt be able to determine the size of the array beforehand
2 Comments
David P. Sanders
I like the second method even when I do know the size. However, you should define the array initially with the correct argument type, e.g.
k = Int[ ] in this example.
postylem
It seems the first version you give doesn't work in Julia 1.6+ (maybe earlier versions too; gives a MethodError). Instead, one can use the
Array{Int64}(undef, n) as described in the docs, to make an array of set dimensions (1 dimension, size n) and type (Int64), with uninitialized values.The simplest way to get the specific array you want is this:
k = collect(1:10)
If you want to define an array in a loop, then you first need to preallocate the array, e.g. using k = zeros(10) and then specify the values:
n = 10
k = zeros(n)
for i = 1:n
k[i] = i
end
Comments
See: https://en.wikibooks.org/wiki/Introducing_Julia/Arrays_and_tuples#Uninitialized
"Using comprehensions and generators to create arrays"
k = [i for i in 1:10]
Comments
How do you know it's not working? Try evaluating k:
n= 10
for i in 1:n
k[i]= i
end
k
10-element Array{Int64,1}: 1 2 3 4 5 6 7 8 9 10
A more succinct way of doing the same thing:
collect(1:10)
2 Comments
RedPointyJackson
In 0.4.6 this snippet will throw an UndefVarError, and I suspect that in other versions too. The basic asumption is that the array isn't initialized in the question, and the author is just testing Matlab behaivour, where this works.
bit-player
Thanks for the explanation. Embarrassing mistake. I ran the code in a Jupyter notebook -- where
k was in fact defined in another cell.
k = -5:5. Then you can dok[3]and get2back, which is the third element of the range.