# using a value to find index in a matrix?

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Axel Yantosca on 22 Oct 2020
Commented: Walter Roberson on 22 Oct 2020
How would one find the location inside a matrix? In this case I am looking for x, assuming it exists.
Matrix_A = randi([1 100],100,100)
x = randi ([1,100], 1)

Stephen Cobeldick on 22 Oct 2020
Edited: Stephen Cobeldick on 22 Oct 2020
The simplest solution is to simply check for equality:
idx = x==Matrix_A;
which will return a logical array with true in every location where the value x occurs in that matrix.
If you want to convert that info to subscript indices, then you can use find on that logical array:
[R,C] = find(idx)
In many cases using a logical index is simpler and more efficient, but it really depends entirely on what you are going to do with these indices, which so far you have not explained.

David Hill on 22 Oct 2020
[i,j]=ind2sub(size(Matrix_A),find(Matrix_A==x));

#### 1 Comment

Walter Roberson on 22 Oct 2020
You might as well just use
[i, j] = find(Matrix_A == x)
There is a role for using ind2sub(), but really not until you are working in more than 2 dimensions.

Walter Roberson on 22 Oct 2020
Like I replied to your other question that was essentially the same, use
ismember(x, Matrix_A)
If you are trying to find where in the matrix it occurs, then use the second output of ismember().
This assumes that there is only one match. If there might be multiple matches then you might as well use
[row, column] = find(Matrix_A == x)