Index a Y = MxN, with X=Mx5 (where elements in X are column IDs for values to extract from Y)

Question

Andy 2023-12-6

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2057294-index-a-y-mxn-with-x-mx5-where-elements-in-x-are-column-ids-for-values-to-extract-from-y

编辑： Voss 2023-12-6

采纳的回答： Voss

在 MATLAB Online 中打开

Having a Y = MxN matrix of data, and a X = Mx5 (or other fixed nr. < than N) matrix obtained as:

[~, X] = maxk(Z,5,2); % where Z has the same size as Y

is there a non-FOR way to extract the values from Y coresponding to positions as expressed in X? Current solution is:

for line = 1:size(Y,1)
    X1(line,:) = Y(line, X(line,:))
end

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Voss 2023-12-6

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2057294-index-a-y-mxn-with-x-mx5-where-elements-in-x-are-column-ids-for-values-to-extract-from-y#answer_1367049

编辑：Voss 2023-12-6

在 MATLAB Online 中打开

You can use sub2ind for that.

https://www.mathworks.com/help/matlab/ref/sub2ind.html

Example:

Y = randi(50,6,10)
Y = 6×10
    16    15     5     1     3    27    32    25    19    34
    48    32    21    13    39    24    10    10    21    43
    19    42    38    40    24    37    32     1     2    23
    41    41    13    26    46    24     1    31     9    48
     4    39    17    29     4    43     5    27    42    38
     5    17    18     9    14    29    41    23    32    49
Z = Y/100;
[~,X] = maxk(Z,5,2)
X = 6×5
    10     7     6     8     9
     1    10     5     2     6
     2     4     3     6     7
    10     5     1     2     8
     6     9     2    10     4
    10     7     9     6     8
% non for-loop method:
[M,N] = size(Y);
rows = repmat((1:M).',1,size(X,2));
cols = X;
X1 = Y(sub2ind([M N],rows,cols))
X1 = 6×5
    34    32    27    25    19
    48    43    39    32    24
    42    40    38    37    32
    48    46    41    41    31
    43    42    39    38    29
    49    41    32    29    23
% for-loop method:
X1_for = zeros(size(Y,1),size(X,2));
for line = 1:size(Y,1)
    X1_for(line,:) = Y(line, X(line,:));
end
X1_for
X1_for = 6×5
    34    32    27    25    19
    48    43    39    32    24
    42    40    38    37    32
    48    46    41    41    31
    43    42    39    38    29
    49    41    32    29    23
% both methods produce the same result:
isequal(X1,X1_for)
ans = logical
   1

2 个评论
显示无隐藏无

Andy 2023-12-6

编辑：Andy 2023-12-6

yes, sub2ind & repmat do show up in other links ... and it's a tad better than a for - still, not a one liner ... surprised there isn't one word function for this

Voss 2023-12-6

编辑：Voss 2023-12-6

在 MATLAB Online 中打开

Y = randi(50,6,10)
Y = 6×10
    43    10    39    13    47    41    21    29     3    45
    12    33    12    18    49    22    38     7    47    31
    46    13    14    13    27    23     8     2    22    17
    12    33    35    15     7    17    42    32    33     1
    19     2    44    46    32    18     7    32    19     5
     5     9    25    25    35    10     2    42    32    24
Z = Y/100;
[~,X] = maxk(Z,5,2)
X = 6×5
     5    10     1     6     3
     5     9     7     2    10
     1     5     6     9    10
     7     3     2     9     8
     4     3     5     8     1
     8     5     9     3     4
% one-liner:
X1 = Y((X-1)*size(Y,1)+(1:size(Y,1)).')
X1 = 6×5
    47    45    43    41    39
    49    47    38    33    31
    46    27    23    22    17
    42    35    33    33    32
    46    44    32    32    19
    42    35    32    25    25
% for-loop method:
X1_for = zeros(size(Y,1),size(X,2));
for line = 1:size(Y,1)
    X1_for(line,:) = Y(line, X(line,:));
end
X1_for
X1_for = 6×5
    47    45    43    41    39
    49    47    38    33    31
    46    27    23    22    17
    42    35    33    33    32
    46    44    32    32    19
    42    35    32    25    25
% both methods produce the same result:
isequal(X1,X1_for)
ans = logical
   1