# Unknown operation performed.

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Yulia M on 21 Nov 2019
Edited: Yulia M on 22 Nov 2019
Hi,
I am new to MATLAB. the following is my code I am unable to understand
the operation perfromed B = S(A); and C = S(~A);
B has size 15x1 double
C has size 285x1 double
S is 20x15 double
A is 20x15 logical
my question is what operation has been performed in B and C and how did they get that size?
clear all;
clc;
A = false( 20,9 );
sP = generate_pilots( 15 , 4, 'deterministic' );
G = [4:4:11 12:4:21]';
H = (2:7:20)';
A( G, H-1 ) = true;
S = zeros( size(A) );
B = S(A);
C = S(~A);
D = sP(1:15);
S( A ) = sP(1:15);
Regards,
Gnanesh

Philippe Lebel on 21 Nov 2019
Edited: Philippe Lebel on 21 Nov 2019
This is called logical (or boolean) indexing.
Your A matrix is filled with booleans.
B is now a vector that has all the elements of S that correspond to the same indexes as the Ones in A.
C is now a vector that has all the elements of S that correspond to the same indexes as the Zeros in A.
a = logical([1 0 0 1; 0 0 1 0])
s = [1,2,3,4;5,6,7,8]
s(a)
ans =
1
7
4
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Philippe Lebel on 21 Nov 2019
Edited: Philippe Lebel on 21 Nov 2019
No, it has to be filled with only booleans.
you can make it by different means:
1: automatically
mat = [1,2,3,4,5];
a = mat<3
a =
1 1 0 0 0
2:manually
a = [true true false flase true];
%or
a = logical([1 1 0 0 1]);
For the sizes of the matrix B and C, take the following example:
A = logical([1 0 0 1; 0 0 1 0]);
S = [1,2,3,4;5,6,7,8];
B = S(A)
B =
1
7
4
B is a 3x1 vector because there are only 3 Ones in A. The logical indexing fetches only 3 elements in S to put them in B.
Yulia M on 22 Nov 2019
Edited: Yulia M on 22 Nov 2019
Hi Philippe,
I got it. Thank you very much for your valuable time.
regards,