Creating a Matrix that is a manipulation of another matrix

Hi, I'm very new to Matlab, and I have a question about creating a matrix. If there is a matrix A = [a b ; c d], how do you create a matrix = [a -b ; c -d]? So the second column entries of A are multiplied by -1.

Thank you.

Answer 1

[dpb]

A(:,2)=-A(:,2);  % negate elements in original

or

A(:,2)=-1*A(:,2);

or

B=[A(:,1) -A(:,2}];  % new array keeping old

Any number of additional ways are possible including, of course linear algebra methods...

B=A*[1 0;0 -1];

Answer 2

[Joseph Cheng]

There are lots of ways how about element by element multiplication like:

x = magic(4)
trans = [ones(4,2) -ones(4,2)]
x.*trans

which if it gets complicated say corners and such you can build the trans matrix to whatever you want.

or by selecting the columns manually:

x(:,3:4) = -x(:,3:4)

Answer 3

[Julian]

Thanks!

Answer 4

[Julian]

I have another question. How do you replace just one element in a column not the entire elements in the column?

[Joseph Cheng]

so the indexing is something you should spend some time to read about to fully understand but its like:

x(row,column) so you can say x(2,3) which will specifically be the element of intersection of row 2 and column 3. so to replace one element you would just specify that one element. the : marker in the examples everyone is giving is the "all" marker for all rows/columns (depending on where it is)

x(:,2) is all rows in column 2

x(2,:) is for all columns for row 2.

Answer 5

[Julian]

Thank you all!