creating a function nonpivotcols that takes a m*n matrix as input and produces a matrix consisting of the nonpivot columns of the input

Hi, I'm trying to create a function nonpivotcols that takes a m*n matrix as input and produces a matrix consisting of the nonpivot columns of the input matrix.

This is the code I have so far, but I'm not sure how to write a code for the (HELP) part..

Can't I just write it as the line %%Getting columns which it fcol ?

%  Returns the nonpivot columns from the matrix A
function npc = nonpivotcols(A)

% determine n the number of columns of A 
[m, n_plus_one] = size(A)
n = n_plus_one-1 

% use rref to obtain a list of the pivot columns of A (pcols)
[R pcols]=rref(A);
A(:,pcols)=[];
%(HELP) from the list of all columns 1:n remove pcols to obtain a list of the nonpivot columns (fcols)
 
%% Getting columns which if fcol
[r,fcols]=size(A); 

% extract the nonpivot columns from A
npc = A(:,fcols);           
end

And this is the code to call the function:

A = [ 1 1 2 2 3 3; 4 4 5 5 6 6; 7 7 8 8 9 9]     
[R, pcols] = rref(A)
nonpivotcols(A)

I'm getting an error for the third test code, with the variable size issue.

Please help!

Thank you!

Answer 1

[Walter Roberson]

[R pcols]=rref(A);
A(:,pcols)=[];

You deleted the pivot columns out of A. Whatever is left must be non-pivot columns.

[r,fcols]=size(A); 

fcols will be the number of columns in the portion of A that remains -- at this point, the number of non-pivot columns. fcols will be a scalar such as 6 if A were 4 x 6 at that point

npc = A(:,fcols); 

A indexed with a scalar column index is going to return something that has exactly one column (provided that A is not empty... you would get an error if A were empty.)

But surely you want all of what remains of A, not just the last non-pivot column.

[Seungryul Lee]

So do you mean I don't have any problem with the code?

I'm sure why I'm getting an error for this test.

Can you explain why?

[Walter Roberson]

No, I am pointing out that your code is always extracting the last non-pivot column, when your task is to extract all of the non-pivot columns.