How to find the position of the elements of a matrix?

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giorgos kazas 2020-2-3

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回答： giorgos kazas 2020-2-4

I have a 3x3 matrix: J=[0 3 4;3 0 8;4 8 0]

and I want to know in which position the elements exist. For example that 8 exists in second row and third column (2,3) and (3,2) too.

After that I want a new matrix with the value (i.e. 8) in one column and their ''coordinates'' (i.e. 2,3) in a column side by side.

Thanks

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Steven Lord 2020-2-3

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Paul Shoemaker's answer will give you the indices of the selected elements in the array, but depending on what you want to do that may not be necessary. For example if you want to get the elements of another array (the same size as J) in the same location as the 8's in J you can do this using logical indexing.

J=[0 3 4;3 0 8;4 8 0]
M = magic(3)
X = M(J == 8)

The vector X contains 9 and 7, the elements from M in locations corresponding to 8's in J. See this documentation page for more information.

Paul Shoemaker 2020-2-3

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Yes, Steven Lord makes a good point: usage may drive you a different way. This is why it's helpful for those posting questions to explain what their end objective is, not just how to perform some specific mechanical action.

Similar to Steven's example, you can also change the original "J" variable where a certain condition is met:

J(J==8) = NaN; % efficient way to change all occurrences of "8" in the original matrix to be NaN's

Paul S

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Answer 1

Paul Shoemaker 2020-2-3

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Check out the ind2sub Matlab function.

Paul Shoemaker

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giorgos kazas 2020-2-3

Thank you very much!

Guillaume 2020-2-3

编辑：Guillaume 2020-2-3

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Hum, this is rather convoluted! For 2D matrices,

[i,j] = ind2sub(size(J),find(J==8));

is simply:

[i, j] = find(J == 8)

For ND matrices, N >= 3, then indeed it makes sense to use ind2sub.

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Answer 2

giorgos kazas 2020-2-4

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Thank you to everyone for the input.

I have also another question.

Do you know how can I get the results from "disp([i j])" into a matrix? Because I want to create a matrix with the values in one column and their position in the second-third column.