Extract data from cell array and include index of cell for identification

Question

I have a 69x1cell array. Each row contains 3 columns of double but with variable numbers of rows.

In each row there are, for example;

4x3 double 13x3 double ... etc.

I extract all the data from the cell array into one matrix using;

result = vertcat(data{:})

What I would like to do is to give them an identify based on which cell they are extracted from in the data cell array. For example, the 4 rows extracted from the 1st row in the data cell array would have 1 in front of them...so i would get something like;

1,x,y,z
1,x,y,z
1,x,y,z
1,x,y,z (as row 1 in data has 4 rows)
2,x,y,z
2,x,y,z (for the 13 rows)

Robert Seifert · Accepted Answer · 2015-02-24 11:44:51Z

2

This is one way:

data = { rand(2,3);
         rand(5,3) }

%// create identity vector
out1 = cell2mat(arrayfun(@(x) x.*ones(1,size(data{x},1)), 1:numel(data),'uni',0)).' %'
%// convert data from cell to double matrix
out2 = cell2mat(data)

%// concatenate output
out = [ out1 , out2 ]

or directly

out = cell2mat([arrayfun(@(x)x.*ones(1,size(data{x},1)).',1:numel(data),'uni',0).',data])

or a little more fancy and a bit less costly (untested):

%// create identity vector
A(cumsum([1; cellfun(@(x) size(x,1),data)])) = 1

%// concatenate output
out = [cumsum(A(1:end-1)).' cell2mat(data)]

gives:

data = 
    [2x3 double]
    [5x3 double]


out =
            1      0.88441     0.018613      0.43851
            1      0.72086      0.67478      0.43782
            2      0.11704      0.37569      0.51537
            2      0.81468      0.54655      0.65753
            2      0.32486      0.56192      0.95092
            2      0.24623      0.39582      0.72235
            2      0.34271      0.39813      0.40008

edited Feb 24, 2015 at 11:44

answered Feb 24, 2015 at 11:09

Robert Seifert

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2 Comments

user3644826 Over a year ago

Perfect! @thewatwewalk I just tested your "untested" method, and it works perfectly too.

Robert Seifert Over a year ago

@user3644826 the "untested" comment was regarding the "a bit less costly" statement :)

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