Extract row elements corresponding to a particular column value and save as new matrix

I have a Mx4 matrix with the following format:

a1 b1 c d1

a2 b2 c d2

a3 b3 c d3

a4 b4 c1 d4

a5 b5 c1 d5

and what i want to do is create new matrices corresponding to the different c values in the third column renaming this matrix matc.

Any help would be really appreciated Sarah

Answer 1

[Azzi Abdelmalek]

b=A(:,3)
b(b==c)=yourvalue
A(:,3)=b

[Sarah]

Thank you! the values of c are increasing but not monotonically, I would like to create a loop to extract these values corresponding to the different c values. Any ideas on how I might do that?

[Azzi Abdelmalek]

How do you want to replace these c numbers?

[Sarah]

I do not want to replace them, I just want to extract the data corresponding to the different groups of c values for example create a matrix containing the data corresponding to c1 values and so on.

[Sarah]

Also in the code you wrote there the last line just rewrites the variable b with the original matrix again?

[Azzi Abdelmalek]

If A=[ 2 3 4 6
       4 8 7 9
       5 2 7 1
       4 2 2 3]

What should be the result?

[Azzi Abdelmalek]

A=[ 2 3 4 6
     4 8 7 9
     5 2 7 1
     4 2 2 3]
A=sortrows(A,3)
b=A(:,3);
c=unique(b);
for k=1:numel(c)
  idx=find(b==c(k));
  assignin('base',sprintf('mat%d',c(k)),A(idx,:))
end

%Check the matrices mat2, mat4 and mat7

[Sarah]

And because some of the c values are not integers is there a way to round the c value before assigning it to the variable name?

[Azzi Abdelmalek]

But if you have three values of c: 1.8, 2 and 2.6 how will you round?

[Azzi Abdelmalek]

What I suggest is using cell array

b=A(:,3);
c=unique(b);
for k=1:numel(c)
  idx=find(b==c(k));
  out{k}=A(idx,:)
end

then check

out{1}
out{2}
out{3}

[Sarah]

I would round to the nearest integer eg 1.8->2 and so on.. Finally to save each matrix, I am unfamiliar with the assignin function. How would I save the individual matrices eg mat11 mat12 and so on..?

[Azzi Abdelmalek]

What about the cell array solution?

[Sarah]

I have looked at the various matrices when I round the value c to the nearest integer, it is just saving the matrices within the loop that I am having some difficulty with now.

[Azzi Abdelmalek]

Did you check the cell array solution? You do not need to create such variables

[Sarah]

You mean I do not need to save the matrices? I would like to save them individually as they are created within the loop.

[Azzi Abdelmalek]

Check this :

clear out
b=A(:,3);
c=unique(b);
for k=1:numel(c)
idx=find(b==c(k));
out{k,2}=A(idx,:)
out{k,1}=c(k)
end

out{1,1},  % is c1
out{1,2}   % corresponding to c1

out{2,1},  % is c2
out{2,2}   % corresponding to c2

and so on

[Sarah]

the matrices are being created correctly with the original code, I only wish to now save each matrix that was created within the loop eg after the assignin('base',sprintf('mat%d',c(k)),A(idx,:)) line. I appreciate your help with this!

[Azzi Abdelmalek]

% Check this:   for c=2.4 the variable will be mat2p4 instead of mat2.4

A= [ 2 3 4 6
     4 8 7 9
     5 2 7 1
     4 2 2.4 3]
A=sortrows(A,3)
b=A(:,3);
c=unique(b);
for k=1:numel(c)
  idx=find(b==c(k));
  s=num2str(c(k))
  s=strrep(s,'.','p')
  assignin('base',['mat' s],A(idx,:))
end

Check the result

mat2p4
mat4
mat7

Answer 2

[Sarah]

So the matrix is sorted according to the third column so

A1=[4 2 2 3

2 3 4 6

4 8 7 9

5 2 7 1];

the answers should be

 mat2=[4 2 2 3];

 mat4=[2 3 4 6];

 mat7=[4 8 7 9

      5 2 9 1];