CODEWITHSUNDEEP
multidimensional-arrayjulia
hr0m
hr0m

Reputation: 2805

Find the first occurrence of a given column

In the documentation i have found a function findfirst which is capable of returning the index of the first element, which is equal to the given one.

In my case, i have a vector (or a one dimensional array) and i want to find the first column, which is equal to the vector.

I know how to do it the "hard" way: With findnext iterating over the first row, checking then the whole column. But is there a smarter way, which isn't obvious to me?

Upvotes: 1

Views: 333

Answers (2)

hr0m
hr0m

Reputation: 2805

This is the way i go now. It does not feel very smart. I am somehow still confident that there is a better way, however i don't see it right now.

Coming from C/C++ and Python my look my look somewhat weird. I have no idea about good taste in julia. Suggestions are welcome.

function findfirstcolumn(A, v)
    index = findfirst(A[1,:],v[1])
    found = false
    while index != 0 && found == false
        found = true
        for i = 2:size(v)[1]
            if A[i,index] != v[i]
                found = false
                break
            end
        end
    if found == true
        return index
    end
        index = findnext(A[1,:], v[1], index+1)
    end
    return 0
end

Upvotes: 0

Dan Getz
Dan Getz

Reputation: 18217

Suppose m is your matrix, and v is the vector. Then:

findfirst(c->view(m,:,c)==v,1:size(m,2))

Should return 0 if the vector is not found and the column number if it is. Going down to basic element accesses might be faster, but this should also do the trick.

Upvotes: 5

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