Is it possible to extract the values with a vector of indices for each row without using the for statement from the matrix?

Consider the following example.

A = [1, 2, 3; 4, 5, 6; 7, 8, 9; 10, 11, 12]; % reference matrix
b = [2; 1; 1; 3]; % index for each row that I want to extract
for i=1:size(A,1)
    y(i,1) = A(i,b);
end

I am using the above code to extract values that I want.

Is there any simple function to implement the above script fast?

Answer 1

[Ameer Hamza]

see sub2ind()

A = [1, 2, 3; 4, 5, 6; 7, 8, 9; 10, 11, 12]; % reference matrix
b = [2; 1; 1; 3]; % index for each row that I want to extract

idx = sub2ind(size(A), 1:size(A,1), b.');
A(idx)

Result

>> A(idx)
ans =
     2     4     7    12

[Danny]

Oh, this is what I exactly want. Thank you.

[Ameer Hamza]

I am glad to be of help!

Answer 2

[KSSV]

May be you are looking for

A(b,:)

The other

A(:,b)

will work, but in your case b has number 4 and in A there are only 3 columns.

To extract a complete row or column, : can be use

A(1,:)   % picks the 1st row and all columns 
A(:,3)   % picks the allrows and third column
A(2,3)   % pciks the second row and third column

[Danny]

That is my typo, so I correct it.

However, I've already done above, but this is not working in my the latest version of Matlab.

>> piset
piset =
     9    10     6     7     8     4     5     3     2     1
     7     8     5    10     6     4     9     3     2     1
     9    10     8     6     7     4     3     5     2     1
     7     9     8     6     5    10     3     4     2     1
     7     6     9    10     5     8     3     4     2     1
     7    10     9     4     5     6     8     2     3     1
    10     5     9     6     7     4     8     2     3     1
    10     9     8     7     4     5     6     3     2     1
     8    10     6     9     7     5     4     2     3     1
     9     7    10     6     4     5     8     1     2     3
     8     5     9     7     4    10     3     6     2     1
     7     9     6    10     8     3     4     5     2     1
     6     8    10     7     9     4     5     3     2     1
     7     8     6     3     2    10     9     5     4     1
     9     5     7     6    10     8     2     3     4     1
>> lset
lset =
     8
     7
     7
     2
     3
     3
     9
     2
     4
    10
     3
     6
     5
     7
     1
>> piset(:,lset)
ans =
     3     5     5    10     6     6     2    10     7     1     6     4     8     5     9
     3     9     9     8     5     5     2     8    10     1     5     4     6     9     7
     5     3     3    10     8     8     2    10     6     1     8     4     7     3     9
     4     3     3     9     8     8     2     9     6     1     8    10     5     3     7
     4     3     3     6     9     9     2     6    10     1     9     8     5     3     7
     2     8     8    10     9     9     3    10     4     1     9     6     5     8     7
     2     8     8     5     9     9     3     5     6     1     9     4     7     8    10
     3     6     6     9     8     8     2     9     7     1     8     5     4     6    10
     2     4     4    10     6     6     3    10     9     1     6     5     7     4     8
     1     8     8     7    10    10     2     7     6     3    10     5     4     8     9
     6     3     3     5     9     9     2     5     7     1     9    10     4     3     8
     5     4     4     9     6     6     2     9    10     1     6     3     8     4     7
     3     5     5     8    10    10     2     8     7     1    10     4     9     5     6
     5     9     9     8     6     6     4     8     3     1     6    10     2     9     7
     3     2     2     5     7     7     4     5     6     1     7     8    10     2     9
>>