Combine matrices using loop and condition in matlab - 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

Answer 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

Answer 2

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.

Answer 3

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

Answer 4

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...

Answer 5

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)]