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Sort the array by template

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I have two arrays A1 and B1 in which elements correspond:
A1 = [a1 a2 a3 a4 a5];
B1 = [b1 b2 b3 b4 b5];
then I change order of elements of the first array anyhow (they are all unique), for exaple like this:
A2 = [a4 a1 a3 a5 a2];
The question is how to get array B2 sorted in the same way? It should be
B2 = [b4 b1 b3 b5 b2];
I tried to write function which is:
B2 = zeros(length(B1),1);
for i = 1:length(B2)
ind = find( A1 == A2(i) );
B2(i) = B1(ind);
end
but as it uses for loops speed is not fast. Maybe there is a way to do it by using MatLab builtin functions?

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Accepted Answer

Ameer Hamza
Ameer Hamza on 23 May 2018
Edited: Ameer Hamza on 23 May 2018
For a generalized matrix a1 and a2, you can use the following code
[~,index] = ismember(a2', a1', 'row');
b2 = b1(index)
This will work on R2016b and later.

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Ameer Hamza
Ameer Hamza on 23 May 2018
You are welcome.
Rostislav Teryaev
Rostislav Teryaev on 23 May 2018
How did you train yourself to invent such elegant solutions?
Ameer Hamza
Ameer Hamza on 23 May 2018
You will get hold of such solution once you start to develop some intuition about these frequently used MATLAB functions. Of course, that comes with time, experience and reading a lot of documentation :)

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More Answers (1)

sloppydisk
sloppydisk on 23 May 2018
a1 = [1 4 2 5 7];
b1 = 1:5;
mySecretOrder = [1 3 2 5 4];
a2 = a1(mySecretOrder);
[~, order] = ismember(a2, a1)
b1 = b1(order)

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Rostislav Teryaev
Rostislav Teryaev on 23 May 2018
nice! But what if the a1 and a2 are matrices? Real example:
a1 = [5 2 3 4 2 3 6 6 6
1 1 1 3 4 5 4 5 2]
b1 = [19 28 19 38 24 20 27 11 16]
a2 = [2 2 3 3 4 5 6 6 6
1 4 1 5 3 1 2 4 5]
b2 = [28 24 19 20 38 19 16 27 11]
There are columns (which stand for directions from 5 to 1, from 2 to 1 and so on) then they change their order in matrix and become a2. So I need to sort b2 which is also in the order of a1 columns and turn it into b2 which is in a2 order.
Sorry for asking not very this question in the first time. I supposed I would manage to extrapolate solution of easy case for more complicated case like this.
sloppydisk
sloppydisk on 23 May 2018
Please see Ameer's answer.

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