Find the indicies of the top 250 values of a Matrix

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Emma Davis
Emma Davis 2023-4-5
评论： Emma Davis 2023-4-5
I performed a spearman correlation analysis that produced a Rho variable. The size of the Rho variable is:
sizeRho = size(Rho) = [983, 27471]
Now, I am trying to find the the indicies for the top 250 values of Rho.
Here is some of what I have tried:
sizeRho = size (Rho);
top = maxk(Rho,250);
[B,I] = maxk(Rho,250, 'ComparisonMethod', 'abs');
[top, ixt, jxt] = prctile(Rho, [90])
top = prctile(Rho, [99])
largest_vals = maxk(Rho(:), 250);
[largest_vals, indicies_t] = maxk(Rho(:), 250);
Any help and/or advice would be greatly appreciated. Thank you.
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Torsten
Torsten 2023-4-5
Why ? maxk returns a matrix whose columns contain the k largest elements of each column of A.
Thus the matrix has dimension 250x27471.
Emma Davis
Emma Davis 2023-4-5
Yes. I believe I was using the wrong function by using maxk, which is why I also tried using the prctile function. I did not want the 250 largest elements of each column, but rather the 250 largest elements of the entire matrix. I hope this is clearer now.

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采纳的回答

the cyclist
the cyclist 2023-4-5
编辑：the cyclist 2023-4-5
Ran in:
[Sorry if you saw the incorrect solution I posted and then deleted.]
If you want the max values and indices for the matrix overall (rather than each column), you need this syntax:
[B,I] = maxk(Rho(:),250, 'ComparisonMethod', 'abs');
Note the use of Rho(:) as an input, rather than just Rho.
The output I has the linear indices into M.
For example
M = magic(4)
M = 4×4
16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1
[B,I] = maxk(M(:),3, 'ComparisonMethod', 'abs')
B = 3×1
16 15 14
I = 3×1
1 12 8

更多回答（1 个）

Chunru
Chunru 2023-4-5
Ran in:
Not very clear on what you are asking. Here is a guess.
Rho = rand(5, 6) % small matrix for illustration
Rho = 5×6
0.6787 0.1810 0.3615 0.8210 0.5301 0.1768 0.6672 0.1126 0.0277 0.1434 0.8380 0.4849 0.4192 0.4177 0.3058 0.5544 0.7236 0.3155 0.7835 0.1233 0.4060 0.2886 0.9057 0.2207 0.4596 0.8407 0.9688 0.6253 0.7939 0.7684
[B, I] = maxk(Rho(:), 7, 'ComparisonMethod', 'abs') % top 7 of all Rho (not columnwise)
B = 7×1
0.9688 0.9057 0.8407 0.8380 0.8210 0.7939 0.7835
I = 7×1
15 24 10 22 16 25 4
[row, col] = ind2sub(size(Rho), I);
[row col]
ans = 7×2
5 3 4 5 5 2 2 5 1 4 5 5 4 1

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