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

columnextractMATLABmatrixrowsubmatrix

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

Best Answer

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.

Related Solutions

MATLAB: How to extract value from some row or column in a matrix

By indexing like shown below:

% if A is a matrix
A=rand(100); % fake data
B=A([1:10 25:35],:) % extracts specified rows
B=A(:,[1:10 25:35]) % extracts specified columns
% if A is a vector then
A=1:100; % fake data (row vector) , (1:100).' - column vector
B=A([1:10 25:35]) % extracts specified elements (1D-a vector)

Note: The square brackets [] is used to denote discontinuities thereby extracting the spcified elements, rows or columns.

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

MATLAB: How do i use the indices from [M,I]=max(A,[],3) to find the corresponding values in another 3D array, B, of the same size as A

clc; clear all ;
n = 10 ; m = 7 ; k = 10 ;
A = rand(n,m,k) ; 
B = rand(n,m,k) ;
[M,I]=max(A,[],3) ;
% using loop
iwant1 = zeros(size(M)) ;
for i = 1:n
    for j = 1:m
        iwant1(i,j) = B(i,j,I(i,j)) ;
    end
end
% without using loop 
B2D = reshape(B,n*m,k) ;
B2D = B2D(:,I(:)) ;
B2D = diag(B2D) ;
iwant2 = reshape(B2D,n,m) ;

You can compare iwant1 and iwant2.

Best Answer

Related Solutions

MATLAB: How to extract value from some row or column in a matrix

MATLAB: How do i use the indices from [M,I]=max(A,[],3) to find the corresponding values in another 3D array, B, of the same size as A

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