Use Indexing for more than 1 dimension of array simultaneously

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deathtime
deathtime 2023-7-25
编辑： James Tursa 2023-7-25
Ran in:
I have a 10 by 3 matrix:
matrix = rand(10, 3);
I have an indexing array which selects a column number for each row:
idx = [1; 2; 2; 3; 1; 2; 1; 2; 3; 3];
I want to produce a 10x1 array which uses the above index array to extract the respective column number element at each row. Without using a for loop.
I have tried:
output = matrix(1:10, idx)
output = 10×10
0.2039 0.6450 0.6450 0.8811 0.2039 0.6450 0.2039 0.6450 0.8811 0.8811 0.3955 0.0347 0.0347 0.1645 0.3955 0.0347 0.3955 0.0347 0.1645 0.1645 0.5710 0.9536 0.9536 0.9736 0.5710 0.9536 0.5710 0.9536 0.9736 0.9736 0.5103 0.9297 0.9297 0.2534 0.5103 0.9297 0.5103 0.9297 0.2534 0.2534 0.0723 0.4962 0.4962 0.0039 0.0723 0.4962 0.0723 0.4962 0.0039 0.0039 0.1273 0.1957 0.1957 0.3339 0.1273 0.1957 0.1273 0.1957 0.3339 0.3339 0.0603 0.7878 0.7878 0.9416 0.0603 0.7878 0.0603 0.7878 0.9416 0.9416 0.0832 0.5866 0.5866 0.9793 0.0832 0.5866 0.0832 0.5866 0.9793 0.9793 0.5630 0.3352 0.3352 0.9332 0.5630 0.3352 0.5630 0.3352 0.9332 0.9332 0.7188 0.8864 0.8864 0.6835 0.7188 0.8864 0.7188 0.8864 0.6835 0.6835
This produces a 10x10 matrix.

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回答（3 个）

Fangjun Jiang
Fangjun Jiang 2023-7-25
编辑：Fangjun Jiang 2023-7-25
matrix(sub2ind(size(matrix),(1:10)',idx))
the cyclist
the cyclist 2023-7-25
Ran in:
You can use linear indexing:
matrix = rand(10, 3);
idx = [1; 2; 2; 3; 1; 2; 1; 2; 3; 3];
linearIndex = sub2ind(size(matrix),1:10,idx');
matrix(linearIndex)
ans = 1×10
0.8064 0.6649 0.2393 0.9216 0.0166 0.3291 0.6957 0.8769 0.4766 0.2352
James Tursa
James Tursa 2023-7-25
编辑：James Tursa 2023-7-25
Ran in:
Yet another way using linear indexing (showing what sub2ind( ) does internally):
matrix = rand(10, 3);
idx = [1; 2; 2; 3; 1; 2; 1; 2; 3; 3];
m = size(matrix,1);
matrix( (1:m)' + m*(idx-1) )
ans = 10×1
0.8923 0.3166 0.8492 0.0101 0.2614 0.4642 0.7455 0.5160 0.9367 0.5664
And to show that it is the same as other solutions:
matrix(sub2ind(size(matrix),(1:m)',idx))
ans = 10×1
0.8923 0.3166 0.8492 0.0101 0.2614 0.4642 0.7455 0.5160 0.9367 0.5664
  1 个评论
Walter Roberson
Walter Roberson 2023-7-25
Note that linear indexing is what happens internally when you use sub2ind: sub2ind is a helper function to make it easier to calculate the correct linear indexing.

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