Select elements of a matrix based on the values of a logical array

I have a 23x4 matrix A and a 23x1 logical array B

A = [
   2619834.52               1169193.206               2622778.731                1168042.07
   2622778.731                1168042.07               2623330.871               1167853.553
   2623330.871               1167853.553               2623895.366               1167330.066
   2623895.366               1167330.066               2623752.209               1167450.319
   2623895.366               1167330.066               2623752.209               1167450.319
   2623900.076                1167326.26               2624169.326               1167219.875
   2624169.326               1167219.875                2624253.37               1167198.916
    2624253.37               1167198.916                2624535.39               1167048.569
    2624253.37               1167198.916                2624535.39               1167048.569
   2625504.044               1167095.529               2626025.669               1167318.065
   2625552.809               1167096.398               2625957.908               1167292.096
    2625592.67               1167098.709               2625720.968               1167147.911
   2625637.183               1167100.023               2625720.968               1167147.911
   2625720.968               1167147.911               2625749.984               1167166.109
   2625739.693               1167164.891               2625957.908               1167292.096
   2625749.984               1167166.109               2625880.835               1167248.686
   2625880.835               1167248.686               2625990.736               1167298.256
   2625552.809               1167096.398               2625957.908               1167292.096
   2625880.835               1167248.686               2625990.736               1167298.256
   2626025.669               1167318.065               2625957.908               1167292.096
   2628613.339               1167794.423               2628793.988               1167686.086
   2628793.988               1167686.086               2629072.311               1167703.565
   2629072.311               1167703.565               2631163.404               1169347.638];
           
B = [
   0
   0
   0
   1
   0
   0
   0
   0
   1
   0
   0
   0
   0
   0
   0
   0
   0
   1
   1
   0
   0
   0
   0];

I need to create a 2-column matrix C, where each row of C is a row of A, but picking up only the elements of both the 1st and 2nd columns of A, when the corrisponding element of B = 0, and taking only the elements of both the 3rd and 4th columns of A, when the corresponding element of B = 1. My current "solution" is...

tic
if B==0
    C = A(:,1:2) 
else
    C = A(:,3:4)
end
toc

% Result
C =

               2622778.731                1168042.07
               2623330.871               1167853.553
               2623895.366               1167330.066
               2623752.209               1167450.319
               2623752.209               1167450.319
               2624169.326               1167219.875
                2624253.37               1167198.916
                2624535.39               1167048.569
                2624535.39               1167048.569
               2626025.669               1167318.065
               2625957.908               1167292.096
               2625720.968               1167147.911
               2625720.968               1167147.911
               2625749.984               1167166.109
               2625957.908               1167292.096
               2625880.835               1167248.686
               2625990.736               1167298.256
               2625957.908               1167292.096
               2625990.736               1167298.256
               2625957.908               1167292.096
               2628793.988               1167686.086
               2629072.311               1167703.565
               2631163.404               1169347.638

Elapsed time is 0.000728 seconds.

...I would like to avoid the "if", and therefore to use a more compact code, maybe only one line (and not slower than now, obviously). Any idea?

Answer 1

[Guillaume]

First, have you noticed that your code doesn't work at all? It either copy all of columns 3 and 4 of A into C if B is all 1s, and copy all of columns 1 and 2 of A otherwise. Note that B == 0 is a logical vector.

C = zeros(size(A, 1), 2); %preallocate destination
C(B, :) = A(B, [3, 4]);  %copy columns 3 and 4 where B is true
C(~B, :) = A(~B, [1, 2]);  %copy columns 1 and 2 where B is false

edit: fixed embarassing typo in the code

[Guillaume]

Do'h! Fixed the typo in my answer. It should have been C(B, :) = ... and C(~B, :) = ...

I'll repeat, your code doesn't do what you want. It has nothing to do with the version of matlab or conversion to logical.

if B == 0

is only true if all the elements of B are equal to 0. Therefore in your example it is not true and your code simply does:

C = A(:,3:4)

Notice that the 4th row of C is still the 3 and 4th column of the 4th row of A despite B(4) being equal to 1.

That is because you're passing a logical vector (B == 0) to if. When you pass a vector to if, the if branch is only true if all the elements of the vectors are true (and the vector is not empty).

[Sim]

You are right, I did a mistake... and I think I need more coffee to be more awake :)

Thank you very much! Your solution is perfectly working

B = logical(B);
C = zeros(size(A, 1), 2); %preallocate destination
C(B,:) = A(B, [3, 4]);  %copy columns 3 and 4 where B is true
C(~B,:) = A(~B, [1, 2]);  %copy columns 1 and 2 where B is false