MATLAB: Taking all values of a row if matrix fulfills vector=1

columMATLABrowselecting

Hello,

I got a matrix and a vector like this:

>> a= rand(5)

>> a= rand(5)
a =
    0.8147    0.0975    0.1576    0.1419    0.6557
    0.9058    0.2785    0.9706    0.4218    0.0357
    0.1270    0.5469    0.9572    0.9157    0.8491
    0.9134    0.9575    0.4854    0.7922    0.9340
    0.6324    0.9649    0.8003    0.9595    0.6787
   
    
    
   vector=[1; 0;0; 1; 1]
vector =
     1
     0
     0
     1
     1

If I write this, I will only get the values of the first colum.

 b=a(vector==1)
b =
    0.8147
    0.9134
    0.6324

But I would like to have the whole row instead of the values of the first colum. Like this:

  b=  
  0.8147    0.0975    0.1576    0.1419    0.6557
  0.9134    0.9575    0.4854    0.7922    0.9340
  0.6324    0.9649    0.8003    0.9595    0.6787

Thanks!

Best Answer

b=a(vector==1,:)

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David - if you want to convert your (say) 4x6 matrix into a 4x4 by removing the first two columns, then you could do something like

 B = A(:,3:end);

In the above, we use the colon to mean "all rows", and the 3:end to mean "get columns 3 through to the end". You can extend this to other (different sized) matrices.

EDIT

In your code where you try to set the mth row to the empty matrix, and the nth column to the empty matrix

 newA(m,:) = [];
 newA(:,n) = [];

what you really want to do is to set the first two columns to be the empty matrix which has the effect of removing them. Try

 newA = A;
 newA(:,1:2) = [];

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It will be much easier if we can turn your 'A', 'B', 'C', 'D' into 1, 2, 3, 4. For example,

>> coef = [1;2;3;4];
>> combin = nchoosek(coef, 3)
combin =
       1     2     3
       1     2     4
       1     3     4
       2     3     4

Also assume that we can put vectors {A, B, C, D} into one big matrix (say R). For example,

>> R = rand(10,4)
R =
      0.8147    0.1576    0.6557    0.7060
      0.9058    0.9706    0.0357    0.0318
      0.1270    0.9572    0.8491    0.2769
      0.9134    0.4854    0.9340    0.0462
      0.6324    0.8003    0.6787    0.0971
      0.0975    0.1419    0.7577    0.8235
      0.2785    0.4218    0.7431    0.6948
      0.5469    0.9157    0.3922    0.3171
      0.9575    0.7922    0.6555    0.9502
      0.9649    0.9595    0.1712    0.0344

Then the following one line will create vector mod1,

>> mod1 = R(:, combin(1,:))
mod1 =
      0.8147    0.1576    0.6557
      0.9058    0.9706    0.0357
      0.1270    0.9572    0.8491
      0.9134    0.4854    0.9340
      0.6324    0.8003    0.6787
      0.0975    0.1419    0.7577
      0.2785    0.4218    0.7431
      0.5469    0.9157    0.3922
      0.9575    0.7922    0.6555
      0.9649    0.9595    0.1712

Best Answer

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