how to delete certain columns and rows from matrix

I have the matrix

M = [1000 -1000 0 0;

-1000 10000 0 -9000;

0 0 5000 -5000;

0 -9000 -5000 14000];

and I want to get the matrix when the 1st column and row and the 3rd column and row are gone, so that Mnew = [10000 -9000;

-9000 14000];

Any help?

Answer 1

[William Rose]

@Antrea Plastira,

M = [1000       -1000           0           0;
    -1000       10000           0       -9000;
    0           0        5000       -5000;
    0       -9000       -5000       14000];
Mnew=M([2,4],[2,4])

Mnew = 2×2

10000 -9000 -9000 14000

The line above keeps specifies the rows and columns to keep.

[Antrea Plastira]

thank you!!

[William Rose]

@Antrea Plastira, you're welcome.

Answer 2

[Steven Lord]

Do you know which rows/columns you want to delete or which ones you want to keep? If to keep:

M = magic(4)

M = 4×4

16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1

toKeep = [2 4];
A = M(toKeep, toKeep)

A = 2×2

11 8 14 1

If to delete:

M = magic(4)

M = 4×4

16 2 3 13 5 11 10 8 9 7 6 12 4 14 15 1

toDelete = [1 3];
M(toDelete, :) = []

M = 2×4

5 11 10 8 4 14 15 1

M(:, toDelete) = []

M = 2×2

11 8 14 1

Answer 3

[William Rose]

If you want to sepcify the rows and columns to delete:

M = [1000       -1000           0           0;
    -1000       10000           0       -9000;
    0           0        5000       -5000;
    0       -9000       -5000       14000];
Mnew=M;
rows2delete=[1,3];
cols2delete=rows2delete;
Mnew(rows2delete,:)=[];
Mnew(:,cols2delete)=[]

Mnew = 2×2

10000 -9000 -9000 14000

Try it.