Possibility of changing nested for into parfor

Question

Jozef Krsacok 2022-3-8

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https://ww2.mathworks.cn/matlabcentral/answers/1666324-possibility-of-changing-nested-for-into-parfor

评论： Raymond Norris 2022-3-9

Hello, i have this piece of code and i'm trying to get rid of nested for and do the following slice of code in parallel. Im also using reshape function to change PCNN_Y and PCNN_S from matrix into a vector. I seem to struggle to understand something, because even though i change the code to work with parfor, it's functionality is missing.

for i = 1:1:x
    for j = 1:1:y
        if PCNN_Y(i,j) == 1
            if position >= sp
                PCNN_S(i,j) = bitand(PCNN_S(i,j),254);
                PCNN_S(i,j) = bitor(PCNN_S(i,j),stego_bin(iv));
                iv = iv + 1;
            end
            position = position +1;
        end
        if iv > iv_max
            break;
        end
    end
    if iv > iv_max
        break;
    end
end 

I understand that there should now be any return or break statements in parfor loop, but is there a possibility to create a restriction for the iv to not exceed iv_max? Also matrices are 512x512 which means when they are reshaped it is a vector of 1:262144 and mostly because of iv_max i get an error it should not exceed the number of array elements. Index must not exceed something around 780.

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Answer 1

Raymond Norris 2022-3-8

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1666324-possibility-of-changing-nested-for-into-parfor#answer_913014

If I have this right, you're only working with PCNN_Y where its value is 1. Then you skip over the first sp instances where it's 1. Then you want to stop after iv_max occourances past sp.

Below is a way to rewrite your loop.

Find all cases where PCNN_Y_r is 1 (_r is the reshaped vector of the matrix PCNN_Y)
Skip over the first sp instances of the value 1 (start)
End after sp+iv_max-1 instances of the value 1 (finish)

This way you don't need to end the parfor loop early. You know the exact start and end of the loop.

Below is an example. I've put dummy values for position, iv, and iv_max -- adjust accordingly.

In this case, we'll assign PCCN_S where position is [3 4 5], but nothing after.

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Jozef Krsacok 2022-3-9

This way looks solid, only problem, im getting this error "The endpoints of the parfor-loop variable range 'i' must evaluate to integers."

Error in Test (line 293)

parfor i = start:finish

Seems that the code has some problems with data types, though i don't know why because both sp and iv_max are double so it should work as datatype with parfor.

Raymond Norris 2022-3-9

在 MATLAB Online 中打开

Yup. There's an error in my code

ridx= PCNN_Y_r==1;

should be

[~,ridx] = find(PCNN_Y_r==1);

So try this

% Find where the reshaped vector has a value of 1
[~,ridx] = find(PCNN_Y_r==1);
start = ridx(sp);
finish = ridx(sp+iv_max-1);
parfor idx = start:finish
    PCNN_S(idx) = bitand();
    PCNN_S(idx) = bitor();
end