Julia: Append to an array

Question

Someone please help me understand this. I have the following code below. I am trying to append index[i]-1 to an empty array. However I am getting this error: "BoundsError: attempt to access 0-element Array{Any,1} at index [1]" :

sample_size_array = [9,5,6,9,2,6,9]
n_minus_1 = []
array_length = length(sample_size_array)
for i in 1:array_length
    n_minus_1[i].append(sample_size_array[i] -1)
end
println(n_minus_1)

If Julia does not understand array[0] then why is i starting at 0 and not at 1?

Answer 1

Your code has two problems:

• in the first iteration you are trying to access n_minus_1 array at index 1 while this array is still empty (has 0 length) - this throws you an error;
• in Julia you do not invoke methods using a . (this symbol is used for different purposes - in this case it is parsed as field access and also would throw an error later)

To solve both those problems use push! function which appends an element at the end of an array. The code could look like this:

sample_size_array = [9,5,6,9,2,6,9]
n_minus_1 = []
array_length = length(sample_size_array)
for i in 1:array_length
    push!(n_minus_1, sample_size_array[i]-1)
end
println(n_minus_1)

However in this case the whole operation can be written even simpler as:

n_minus_1 = sample_size_array .- 1

and you do not need any loop (and here you see another use of . in Julia - in this case we use it to signal that we want to subtract 1 from every element of sample_size_array).