matlab - How can I create an array of matrices from a single matrix by using first column as index? -


suppose, have following matrix,

1   2   3   4   5   6   7   8 2   3   4   5   6   7   8   1  3   4   5   6   7   8   1   2  4   5   6   7   8   1   2   3 1   8   7   6   5   4   3   2 2   7   6   5   4   3   2   9 3   6   5   4   3   2   9   8  4   5   4   3   2   9   8   7

i want create array of 4 matrices, classified according column # 1.

for instance, output should following,

[    2 3 4 5 6 7 8   8 7 6 5 4 3 2    3 4 5 6 7 8 1   7 6 5 4 3 2 9     4 5 6 7 8 1 2   6 5 4 3 2 9 8    5 6 7 8 1 2 3   5 4 3 2 9 8 7  ]

my target apply this parzen function each of them.

is following?

function [retval] = bayes (train, test)      classcounts = rows(unique(train(:,1)));     pdfmx = ones(rows(test), classcounts);      variance = 0.25;     pdf = parzen(train(:,2:end), test(:,2:end), variance);      cl=1:classcounts         clidx = train(:,1) == cl;         mu(:,cl) = train(clidx,2:end);           end     retval = mu; endfunction

this code generating following error,

>> bayes(mat, mat) error: bayes: a(i,j,...) = x: dimensions mismatch error: called     bayes @ line 11 column 12 >>

that's classic job accumarray. outputs cell array, recommend continue 3d-matrix.

%// data = [1   2   3   4   5   6   7   8      2   3   4   5   6   7   8   1       3   4   5   6   7   8   1   2       4   5   6   7   8   1   2   3      1   8   7   6   5   4   3   2      2   7   6   5   4   3   2   9      3   6   5   4   3   2   9   8       4   5   4   3   2   9   8   7]  %// input groupid = a(:,1);      %// group identifier rowidx = 1:size(a,1);  %// row index  %// accumarray cellarray = accumarray(groupid(:),rowidx (:),[],@(x) {a(x,2:end)})  %// transform cell array 3d-matrix threedarray = cat(3,cellarray{:})

explanation

what accumarray?

  • it takes elements of vals = rowidx same subs = groupid, groups , performs action.
  • as want perform action on rows of matrix , not single vector, vector input required, "trick" accumarray introducing row indices input, use within function applied
  • a(x,2:end) means take rowidx same groupid stored in x , use access matrix a, put {} around cell array output
cellarray{1} =      8     7     6     5     4     3     2      2     3     4     5     6     7     8 cellarray{2} =      7     6     5     4     3     2     9      3     4     5     6     7     8     1 cellarray{3} =      6     5     4     3     2     9     8      4     5     6     7     8     1     2  cellarray{4} =      5     4     3     2     9     8     7      5     6     7     8     1     2     3 
threedarray(:,:,1) =      8     7     6     5     4     3     2      2     3     4     5     6     7     8   threedarray(:,:,2) =      7     6     5     4     3     2     9      3     4     5     6     7     8     1 threedarray(:,:,3) =      6     5     4     3     2     9     8      4     5     6     7     8     1     2 threedarray(:,:,4) =      5     4     3     2     9     8     7      5     6     7     8     1     2     3

if order of rows in output important, need "stable version" of accumarray (inspired this answer:

%// stabilize accumarray sz = max(groupid,[],1); [~, i] = sort(groupid*cumprod([1,sz(1:end-1)]).');  %// stable accumarray cellarray = accumarray(groupid(i,:),rowidx(i),[],@(x) {a(x,2:end)}) 
cellarray{1} =      2     3     4     5     6     7     8      8     7     6     5     4     3     2 cellarray{2} =      3     4     5     6     7     8     1      7     6     5     4     3     2     9 cellarray{3} =      4     5     6     7     8     1     2      6     5     4     3     2     9     8 cellarray{4} =      5     6     7     8     1     2     3      5     4     3     2     9     8     7 
threedarray(:,:,1) =      2     3     4     5     6     7     8      8     7     6     5     4     3     2 threedarray(:,:,2) =      3     4     5     6     7     8     1      7     6     5     4     3     2     9 threedarray(:,:,3) =      4     5     6     7     8     1     2      6     5     4     3     2     9     8 threedarray(:,:,4) =      5     6     7     8     1     2     3      5     4     3     2     9     8     7

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