Extract rows in a Matrix based on column values of another Matrix.

I want to ask for help with constructing a matrix based on the values of another matrix.

I should to take the values of the matrix q based on the values in matrix p. I want to keep the same order or matrix p, but not successfully extracting the values in q. I should look for the value in the first column in q and then take all the values of that row.

I was using q(p), but this takes the values of p as the location in q, and when to try to extract them, it did not work as I do not have so many values in q. I could not find another similar example in the post, so ask you guys if one of you can help me.

Thanks for your help!

Answer 1

[Stephen23]

The standard MATLAB approach to this common task is to use the second output of ismember, e.g.:

>> [X,Y] = ismember(p,q(:,1));
>> out = q(Y(X),:)

which returns a 177550x5 output matrix:

out =
      181159           8          18          19           7
      181160          29          56          57          27
      181171          18          54          44          19
      181172          56         145         120          57
      181183          54         178         124          44
      181184         145         347         285         120
      181195         178         400         401         124
      181196         347         649         660         285
      181207         400         583         577         401
      181208         649         877         882         660
      181219         583         652         460         577
... lots more rows here
      277980       83878       82141       85350       85440
      277981       91191       88183       91390       92407
      277992       82141       83366       85108       85350
      277993       88183       89522       90415       91390
      278004       83366       84276       84891       85108
      278005       89522       90423       89846       90415

Note that you can also confirm that all p values were matched:

>> all(X)
ans =
     1

[Juan Pablo]

Thanks for your help! I was trying with ismember but not in the right way.

Answer 2

[David Hill]

count=1;
for k=p
    a=find(q(:,1)==k);%assume there is only one match
    if ~isempty(a)
        newMatrix(count,:)=q(a,:);
        count=count+1;
    end
end

[Juan Pablo]

The dimesions of the matrix q and p are not the same. I could not use your approach.