error code "Index in position 2 exceeds array bounds (must not exceed 2)."

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tyler hollings 2020-9-10

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/591910-error-code-index-in-position-2-exceeds-array-bounds-must-not-exceed-2

评论： Walter Roberson 2020-9-10

i keep getting this error code, i know something in my matrix must be wrong, also my output must be a matrix

function z = matrix_mult(X,Y)
[x,y]=size(X)
[x1,y1]=size(Y)
for i=1:y % col
    for j=1:x1 %row
      z(i,j)=X(i,j)*Y(i,j)+X(i,j+1)*Y(i+1,j)
    end
end 
end 

im trying to do matrix multiplication with my own funtion instead of the built in funtion in matlab, i get this first number right and then it pops up with this error

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Answer 1

Walter Roberson 2020-9-10

1
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https://ww2.mathworks.cn/matlabcentral/answers/591910-error-code-index-in-position-2-exceeds-array-bounds-must-not-exceed-2#answer_492829

在 MATLAB Online 中打开

[x,y]=size(X)

so y will be the number of columns in X.

for i=1:y % col

so the variable referring to columns in X will be i

z(i,j)=X(i,j)*Y(i,j)+X(i,j+1)*Y(i+1,j)

but i is being used as a row index, not as a column index.

Your code does not compute matrix multiplication, unless perhaps your matrices are size 2 (and even then it is likely to get indices out of range.)

For matrix multiplication, z(i,j) should be the sum of the products of row i of X together with the transpose of column j of Y.

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tyler hollings 2020-9-10

what does the error mean though ?

Walter Roberson 2020-9-10

Suppose your X is 2 x 3. Then y is going to be 3 because of the way you assigned the output of size(X) . Now you are going to iterate for i = 1 : 3 . You proceed to use X(i,j) . But when i reaches 3, that would be X(3,something) . Which is a problem because X only has 2 rows. The error would then say that you attempted to access past the bounds of the array, stating that the bound was 2. You ran past the end of the array.

You need to pick a variable that will be used to access columns of X and you need to use it to access the columns of X instead of using it to access the rows. You would probably use the same variable to access the rows of Y.

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