How to create a submatrix from a matrix, keeping all columns and a certain number of rows?

Hello. I have a big matrix, called "A", of which I want to extract a submatrix "B" consisting off all the columns but only a certain number of consecutive rows, let's say from row "b" to row "c" ("b" and "c" are obtained from an if statement). What I've tried is:

B = A([b:c],:);

But I get the message

"Subscript indices must either be real positive integers or logicals."

How should I do it? Thanks a lot

Answer 1

[Guillaume]

The error message clearly tells you what the problem is, the b:c vector contains values that are either non-integer or smaller than 1. Clearly, your if statement that calculates these values does not do what you want.

What are the actual values of b and c and what is the code used to generate them?

Note that you can use the debugger to help you find out what is going wrong. If you issue

dbstop if error

before running your code, then it will pause at the line causing the error and you can look at the values of b and c.

[Matias]

Thank you very much. Clearly it was the value of b that was equal to zero. Very simple error that I wasn't able to see.

Answer 2

[KSSV]

A = rand(100) ;  % create a random matrix 
iwant1 = A(5:10,:) ;  % picking columns 5 to 10
idx = [3 4 7 9 20] ;   % pick these columns
iwant2 = A(idx,:) ;