How to write a diagonal rectangular matrix

I have to insert on matlab this matrix:

n = 100;

A = diag(21*ones(n,1)) + diag(-4*ones(n-1,1),-1)...

+ diag(-20*ones(n-1,1),1) + diag(ones(n-2,1),-2)...

+ diag(2*ones(n-2,1), 2);

I can generate it when it is squared, but now the exercise tells me to use the same numbers in a rectangular matrix having dimensions 100x98: how do I create the new matrix?

Answer 1

[Chunru]

You can use toeplitz function.

% Smaller n, m here
n = 10;
m = 8; 

A = diag(21*ones(n,1)) + diag(-4*ones(n-1,1),-1)...
    + diag(-20*ones(n-1,1),1) + diag(ones(n-2,1),-2)...
    + diag(2*ones(n-2,1), 2);
whos

  Name         Size            Bytes  Class     Attributes

  A           10x10              800  double              
  cmdout       1x33               66  char                
  m            1x1                 8  double              
  n            1x1                 8  double              

A

A = 10×10

21 -20 2 0 0 0 0 0 0 0 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 0 0 0 0 0 0 0 1 -4 21

c = zeros(n, 1);  c(1:3)=[21 -4 1];   % 1st column
r = zeros(m, 1);  r(1:3)=[21 -20 2];  % 1st row
B = toeplitz(c, r)

B = 10×8

21 -20 2 0 0 0 0 0 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 0 0 0 0 0 1 -4 21 0 0 0 0 0 0 1 -4 0 0 0 0 0 0 0 1

[Dyuman Joshi]

Why not just remove the last 2 columns?

[Leonardo]

How can I remove the last two columns easily? @Dyuman Joshi

[Dyuman Joshi]

% Smaller n, m here
n = 10;
m = 8; 
A = diag(21*ones(n,1)) + diag(-4*ones(n-1,1),-1)...
    + diag(-20*ones(n-1,1),1) + diag(ones(n-2,1),-2)...
    + diag(2*ones(n-2,1), 2)

A = 10×10

21 -20 2 0 0 0 0 0 0 0 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 2 0 0 0 0 0 0 1 -4 21 -20 0 0 0 0 0 0 0 1 -4 21

%Remove the last n-m columns
A(:,end-(1:n-m)) = []

A = 10×8

21 -20 2 0 0 0 0 0 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 2 0 0 0 0 1 -4 21 -20 0 0 0 0 0 1 -4 21 0 0 0 0 0 0 1 -4 2 0 0 0 0 0 0 1 -20 0 0 0 0 0 0 0 21