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# how to form a new matrix (i.e. B) from two different matrices (i.e. a1, a2) knowing the indices of its origional matrix (indices of a)?

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Megha on 25 Feb 2021
Commented: KALYAN ACHARJYA on 25 Feb 2021
% a = [1 2 3 4 5 6 7; 2 4 6 8 10 12 14]; % dimension: 2x7
a1 = [2 6 12]; % index in a = [1 3 6] % dimension: 1x3
a2 = [4 8 10 14]; % index in a = [2 4 5 7] % dimension: 1x4
% Now how to form a new matrix "B" such that B = [2 4 6 8 10 12 14]; using a1 and a2. % dimension: 1x7
% Here we know the position of each index in a1 and a2 to be put in new matrix B.
So, how to form a new matrix (i.e. B) from two different matrices (i.e. a1, a2) knowing the indices of its origional matrix (indices of a)?
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### Accepted Answer

Stephen Cobeldick on 25 Feb 2021
Edited: Stephen Cobeldick on 25 Feb 2021
You can use those indices on the left-hand side. For example:
a1 = [2,6,12];
x1 = [1,3,6];
a2 = [4,8,10,14];
x2 = [2,4,5,7];
% Try
b = [a1,a2];
b([x1,x2])= b
b = 1×7
2 4 6 8 10 12 14
% Or
b = nan(1,7);
b(x1) = a1;
b(x2) = a2
b = 1×7
2 4 6 8 10 12 14
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KALYAN ACHARJYA on 25 Feb 2021

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