How can I mange the index of the loop in julia?

Question

It is the section of the Genetic algorithm that is coding in Julia. the code was wrote as follow:

popc= [individual(rand(0:1,nvar),[]) for i in 1:nc/4,j in 1:2];
for k=1:nc/4
    #select firdt parent
    i1=rand(1:npop);
    p1=pop[i1];
    #select second parent
    i2=rand(1:npop);
    if i1==i2
        i2=rand(1:npop);
    end
    p2=pop[i2]
    #apply crossover
    m=singlepointcrossover(p1.position,p2.position);
    append!(popc[k,1].position, m[1]);
    append!(popc[k,2].position, m[2]);
end

function singlepointcrossover(x1,x2)

    nvar=length(x1);

    cutpoint=rand(1:nvar-1);

    y1=append!(x1[1:cutpoint],x2[cutpoint+1:end]);

    y2=append!(x2[1:cutpoint],x1[cutpoint+1:end]);

    return y1,y2
end

but it has this error. would you pleas help me?. why is it happened?

ArgumentError: invalid index: 1.0
getindex(::Array{individual,2}, ::Float64, ::Int64) at abstractarray.jl:883
macro expansion at GA.juliarc.jl:87 [inlined]
anonymous at <missing>:?
include_string(::String, ::String) at loading.jl:522
include_string(::String, ::String, ::Int64) at eval.jl:30
include_string(::Module, ::String, ::String, ::Int64, ::Vararg{Int64,N} where N) at eval.jl:34
(::Atom.##102#107{String,Int64,String})() at eval.jl:82
withpath(::Atom.##102#107{String,Int64,String}, ::String) at utils.jl:30
withpath(::Function, ::String) at eval.jl:38
hideprompt(::Atom.##101#106{String,Int64,String}) at repl.jl:67
macro expansion at eval.jl:80 [inlined]
(::Atom.##100#105{Dict{String,Any}})() at task.jl:80

hckr · Accepted Answer · 2019-05-05T13:55:00.760

The problem is / operator gives floating-point results for integer arguments and floating point results cannot be used for indexing an Array. You can index an Array with an Integer.

/(x, y)

Right division operator: multiplication of x by the inverse of y on the right. Gives floating-point results for integer arguments.

for k=1:nc/4

1:nc/4 will create a Float64 range and k, a Float64, is later used in indexing in your code at append!(popc[k,1].position, m[1]);. You should, therefore, make k an Integer.

If nc is an integer, you should use Euclidean division with div(nc, 4) or simply nc ÷ 4, or bit shift operators nc >> 2 and nc >>> 2 (for Euclidean division by 2^n you should shift by n). They will all give integer results for integer arguments.

If nc itself is a floating-point number, you should probably use one of the options pointed out by @Colin T Bowers.

popc= [individual(rand(0:1,nvar),[]) for i in 1:nc/4,j in 1:2];

You do not have an error on the first line, since you do not use i for indexing here. It is still better to replace nc/4 with one of the options I listed above.

score 1 · Answer 2 · answered May 05 '19 at 11:48

Fractions in Julia always output Float64, even if the answer can be exactly converted to Int.

Importantly, in your case, note that Int can be used to index arrays, but Float64 cannot. So you'll need to adjust:

for k=1:nc/2

to

for k=1:Int(nc/2)

so that your index k is will be of type Int, not Float64.

If nc is not guaranteed to be an even integer, then you may need to use floor(Int, nc/2) or ceil(Int, nc/2), depending on which is more appropriate.

How can I mange the index of the loop in julia?

2 Answers2