Unknown operation performed.

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?

Please help me understand this!

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

Answer 1

[Philippe Lebel]

This is called logical (or boolean) indexing.

https://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

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

[Yulia M]

Hi Philippe,

Thank you for your reply. Does this work even if A is not logical??

And another question as well I am confuysed how it gets the sizes? You can see I have mentioned even the size of the matrix I am wondering how they change? It would be helpful if you share some knowledge on it as well. especially B.

regards,

[Philippe Lebel]

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]

Hi Philippe,

I got it. Thank you very much for your valuable time.

regards,