Matlab syntax (vectorization)

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

[Mohammad Sami]

You can use the functions ind2sub and sub2ind to convert between subscripts and linear indexing.

Answer 1

[Dhananjay Kumar]

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