Label Data for Classification in Matlab

Question

I have 2 sets of train data, A and B with different sizes, which I want to use for training the classifier, and I have 2 labels in 2 char variables like,

L1 = 'label A';
L2 = 'label B';

How can I produce appropriate labels ?

I will use cat(1,A,B); to merge data first.

Depending on the size(A,1) and size(B,1), It should be something like,

label = ['label A'
         'label A'
         'label A' 
         .
         .
         'label B'
         'label B'];

Answer 1

Assuming the following:

na = size(A,1);
nb = size(B,1);

Here are a few ways to create the cell-array of labels:

1. repmat

   labels = [repmat({'label A'},na,1); repmat({'label B'},nb,1)];
   

2. cell-array filling

   labels = cell(na+nb,1);
   labels(1:na)     = {'label A'};
   labels(na+1:end) = {'label B'};
   

3. cell-array linear indexing

   labels = {'label A'; 'label B'};
   labels = labels([1*ones(na,1); 2*ones(nb,1)]);
   

4. cell-array linear indexing (another)

   idx = zeros(na+nb,1); idx(nb-1)=1; idx = cumsum(idx)+1;
   labels = {'label A'; 'label B'};
   labels = labels(idx);
   

5. num2str

   labels = cellstr(num2str((1:(na+nb) > na).' + 'A', 'label %c'));
   

6. strcat

   idx = [1*ones(na,1); 2*ones(nb,1)];
   labels = strcat({'label '}, char(idx+'A'-1));
   

... you get the idea :)

---

Note that it's always easy to convert between a cell-array of strings and a char-matrix:

% from cell-array of strings to a char matrix
clabels = char(labels);

% from a char matrix to a cell-array of strings
labels = cellstr(clabels);

Answer 2

If the label names have the same length, you create an array like so:

L = [repmat(L1,size(A,1),1);repmat(L2,size(B,1),1)];

Otherwise, you need to use a cell array:

L = [repmat({L1},size(A,1),1);repmat({L2},size(B,1),1)];

Answer 3

label = [repmat('label A',size(A,1),1); repmat('label B',size(B,1),1) ];

This will create the label matrix you are looking for. You need to use repmat. Repmat helps you to repeat a certain value many times