For loop error, not getting desired output

Question

Tony Stark 2022-9-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1803125-for-loop-error-not-getting-desired-output

编辑： Chris 2022-9-12

I am having trouble finding the issue with my code. I've tried everything and I still cannot diagnose why this is happening.

When I run this code, I get the answer for all_correct to be the following: all_correct = [0;0;4;]

I know this is wrong, the correct answer should be: all_correct = [4;4;4;]

This is because all x elements are exactly the same to all y elements. I'm not too sure what is wrong with my code, in order to achieve my desired output.

I've tried playing around with the indexes, but it doesn't make a difference.

x = [1 2 3 5; 4 3 1 3; 1 3 3 4];
y = [1 2 3 5; 4 3 1 3; 1 3 3 4];
columns = 3;
for i = 1:columns
    correct = 0;
    for j = 1:4
        if x(i,j) == y(i,j)
           correct = correct + 1;
        end
        all_correct(columns,1) = correct;
    end
end

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Answer 1

Chris 2022-9-12

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编辑：Chris 2022-9-12

在 MATLAB Online 中打开

At the end of the inner for loop, you set

all_correct(columns,1) = correct;

Columns == 3 forever, and correct == 4 by that point. So what you are saying is

all_correct(3,1) = 4;

On the other hand, i iterates in each outer loop.

all_correct(i,1) = correct;

Note this is summing rows, not columns.

Alternatively, you could do:

x = [1 2 3 5; 4 3 1 3; 1 3 3 4];
y = [1 2 3 5; 4 3 1 3; 1 3 3 4];
correct = x==y
correct = 3×4 logical array
   1   1   1   1
   1   1   1   1
   1   1   1   1
all_correct = sum(correct,2)
all_correct = 3×1
     4
     4
     4

This is not only more efficient code-wise, but speed-wise (which doesn't make a difference now, but could with larger arrays). Matlab is not all that good at loops.

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Answer 2

David Hill 2022-9-12

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https://ww2.mathworks.cn/matlabcentral/answers/1803125-for-loop-error-not-getting-desired-output#answer_1051505

在 MATLAB Online 中打开

x = [1 2 3 5; 4 3 1 3; 1 3 3 4];
y = [1 2 3 5; 4 3 1 3; 1 3 3 4];
columns = 3;
for i = 1:columns
    correct = 0;
    for j = 1:4
        if x(i,j) == y(i,j)
           correct = correct + 1;
        end
        all_correct(i,1) = correct;%need to index with i
    end
end
all_correct
all_correct = 3×1
     4
     4
     4