MATLAB Answers

Matlab syntax (vectorization)

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Fabian Föll
Fabian Föll on 30 Mar 2020
Edited: Dhananjay Kumar on 2 Apr 2020
Dear all,
i have a simple question that's been bothering me for a while:
For example there is a data array A of size
[size_a,size_b,size_c]=size(A);
and a logical array B with size
[size_a,size_b]=size(B);
Is it possible to query the first two dimensions of Array A with array B without reshaping?
Suppose i want to do something like
C(:,:)=A(B(:),:);
which does not work. Solution would be
D=reshape(A,[size_a*size_b,size_c]);
C(:,:)=D(B(:),:);
Regards

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Mohammad Sami
Mohammad Sami on 31 Mar 2020
You can use the functions ind2sub and sub2ind to convert between subscripts and linear indexing.

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Answers (1)

Dhananjay Kumar
Dhananjay Kumar on 2 Apr 2020
Edited: Dhananjay Kumar on 2 Apr 2020
You should note that logical indexing would select not all the elements from a row(or column) from an array, and that should result in heterogenous array. But MATLAB arrays are homegenous.
So if you do logical indexing, the resulting array flattened and a column of elemnts is returned by MATLAB:
>> a = [1 4 5; 2 0 5];
>> b = a(logical([1 0 0; 1 1 1]))
b =
1
2
0
4
....
Now I will demonstrate the solution you need for your purpose.
You have:
a =
1 4 5
2 0 4
Now do this
>> a2 = num2cell(a)
a2 =
2×3 cell array
{[1]} {[4]} {[5]}
{[2]} {[0]} {[4]}
>> a3 = a2(logical([1 0 0; 1 1 1]))
a3 =
4×1 cell array
{[1]}
{[2]}
{[0]}
{[4]}
Now you can reshape the above 1-dimensional cell-array to a 2x2 cell-array:
>> a4 = reshape(a3, 2,2)
a4 =
2×2 cell array
{[1]} {[0]}
{[2]} {[4]}
Now here is a small hack (there is no function named cell2array) :
>> a5 = table2array(cell2table(a4))
a5 =
1 0
2 4
You can merge all the steps above to make 2 commands:
>> a2 = num2cell(a);
>> a5 = table2array(cell2table(reshape(a2(logical([1 0 0; 1 1 1])),2,2)))
a5 =
1 0
2 4

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