Force an entry in an array to be an integer in julia

user_19583053 0
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In the julia code below, the first entry of a_b will be updated in a for-loop. However, I want it to always stay an integer regardless of what it gets updated to, because I get the error: "ERROR: LoadError: ArgumentError: invalid index: 7.0 of type Float64" when I do the final line of my code as it is currently written below (the 7.0 is because ultimately the j in which we terminally assign a_b[1] to is indeed 7.0).

How does one accomplish this? Is there a way to preset the types for the entries in an array analogous to how one can set types when you define a dictionary in julia?

        a_b = [0, 1.0]  
        for j in 1 : 7     
            if T_w[j] > a_w_comma_T_w[2] && T_w[j] < a_b[2]     
                a_b[1] = j  
                a_b[2] = T_w[j]
            end
        end
        println("This is a_b: ", a_b)

        T_prime = copy(T_w)   
        T_prime[a_b[1]] = a_w_comma_T_w[2]

I think the problem is that the second entry in the array is a float (which I want to keep), and so it makes the whole array a float. However, I'm still not sure how to fix it.

julia integer type-conversion
2 Answers

For each element to have a separate type but also be mutable, you'll need something other than an Array. Don't have something off the type of my head that is indexed by integers, though. I normally go for mutable structs (accessed by fields) or separate variables that I keep independently type-stable.

You could do a_b = Union{Int, Float64}[0, 1.0], though. Each element can be either an Int or a Float64 instead of being converted to only a Float64. And since the Union is small, the Union-splitting optimization results in little performance loss.

user_41321440

You can narrow the element type when defining the Vector using the T[] syntax such as:

julia> a_b = Int[0, 1.0]
2-element Vector{Int64}:
 0
 1

Now you have a Vector of Ints and mutating it will keep Int values:

julia> a_b[1] = 77.0
77.0

julia> a_b
2-element Vector{Int64}:
 77
  1

Of course, rounding will not happen automatically:

julia> a_b[1] = 77.1
ERROR: InexactError: Int64(77.1)
user_99577100
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