Combine matrices using loop and condition in matlab

Question

I have the following two matrices

c=[1 0 1.05 
   1 3 2.05
   1 6 2.52
   1 9 0.88
   2 0 2.58
   2 3 0.53
   2 6 3.69
   2 9 0.18
   3 0 3.22
   3 3 1.88
   3 6 3.98]

f=[1 6 3.9
   1 9 9.1
   1 12 9
   2 0 0.3
   2 3 0.9
   2 6 1.2
   2 9 2.5
   3 0 2.7]

And the final matrix should be

n=[1 6 2.52 3.9
   1 9 0.88 9.1
   2 0 2.58 0.3
   2 3 0.53 0.9
   2 6 3.69 1.2
   2 9 0.18 2.5
   3 0 3.22 2.7]

The code I used gives as a result only the last row of the previous matrix [n].

for j=1
  for i=1:rs1
     for k=1
          for l=1:rs2
               if f(i,j)==c(l,k) && f(i,j+1)==c(l,k+1)                 
                  n=[f(i,j),f(i,j+1),f(i,j+2), c(l,k+2)];
               end
          end
      end 
   end
 end

Can anyone help me on this? Is there something more simple?

Thanks in advance

Eitan T · Accepted Answer · 2013-03-28 00:14:56Z

1

You should learn to use set operations and avoid loops wherever possible. Here intersect could be extremely useful:

[u, idx_c, idx_f] = intersect(c(:, 1:2) , f(:, 1:2), 'rows');
n = [c(idx_c, :), f(idx_f, end)];

Explanation: by specifying the 'rows' flag, intersect finds the common rows in c and f, and their indices are given in idx_c and idx_f respectively. Use vector subscripting to extract matrix n.

Example

Let's use the example from your question:

c = [1 0 1.05;
     1 3 2.05
     1 6 2.52
     1 9 0.88
     2 0 2.58
     2 3 0.53
     2 6 3.69
     2 9 0.18
     3 0 3.22
     3 3 1.88
     3 6 3.98];

f = [1 6 3.9
     1 9 9.1
     1 12 9
     2 0 0.3
     2 3 0.9
     2 6 1.2
     2 9 2.5
     3 0 2.7];

[u, idx_c, idx_f] = intersect(c(:, 1:2) , f(:, 1:2), 'rows');
n = [c(idx_c, :), f(idx_f, end)];

This should yield the desired result:

n =
    1.0000    6.0000    2.5200    3.9000
    1.0000    9.0000    0.8800    9.1000
    2.0000         0    2.5800    0.3000
    2.0000    3.0000    0.5300    0.9000
    2.0000    6.0000    3.6900    1.2000
    2.0000    9.0000    0.1800    2.5000
    3.0000         0    3.2200    2.7000

answered Mar 28, 2013 at 0:14

Eitan T

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1 Comment

user1487735 Over a year ago

That's what I found two days ago and I used this function finally. Very helpful indeed. Thanks anyway!

beaker · Accepted Answer · 2013-03-24 20:55:44Z

1

According to this answer on Mathworks support you can use join from the statistics toolbox, specifically in your case, an inner join.

Unfortunately I don't have access to my computer with matlab on it, but give it a try and let us know how/if it works.

answered Mar 24, 2013 at 20:55

beaker

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Comments

DMR · Accepted Answer · 2013-03-24 21:44:35Z

1

You can reduce the number of loops by comparing both the first and second columns of at once, then using the "all" function to only collapse the values if they both match. The following snippet replicates the "n" array you had provided.

n = [];
for r1 = 1:size(c, 1)
    for r2 = 1:size(f,1)
        if all(c(r1, [1 2]) == f(r2, [1 2]))
            n(end+1, 1:4) = [c(r1,:) f(r2,3)];
        end
    end    
end

answered Mar 24, 2013 at 21:44

DMR

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Comments

bla · Accepted Answer · 2013-03-24 23:37:19Z

1

If you insist on doing this in a loop you need to give n the proper dimension according to the loop counter you are using, or concatenate it to itself of each iteration (this can be very slow for big matrices). For example, writing:

for j=1
  for i=1:rs1
    for k=1
      for l=1:rs2
           m=m+1;
           if f(i,j)==c(l,k) && f(i,j+1)==c(l,k+1)                 
              n(m,:)=[f(i,j),f(i,j+1),f(i,j+2), c(l,k+2)];
           end
       end
    end 
  end
end

will save into the m-th row the for numbers when the loop reaches a counter value of m.

However, just be aware that this can be done also without a nested loop and an if condition, in a vectorized way. For example, instead of the condition if f(i,j)==c(l,k)... you can use ismember etc...

edited Mar 24, 2013 at 23:37

answered Mar 24, 2013 at 21:35

bla

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Comments

dspyz · Accepted Answer · 2013-03-26 18:43:00Z

0

How about without any for loops at all (besides in native code)

mf = size(f,1);
mc = size(c,1);
a = repmat(c(:,1:2),1,mf);
b = repmat(reshape((f(:,1:2))',1,[]),mc,1);
match = a == b;
match = match(:, 1 : 2 : 2*mf) & match(:, 2 : 2 : 2*mf);

crows = nonzeros(diag(1:mc) * match);
frows = nonzeros(match * diag(1:mf));

n = [c(crows,:),f(frows,3)]

edited Mar 26, 2013 at 18:43

answered Mar 26, 2013 at 18:37

dspyz

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