Creating a matrix using diag

Hi,

Would you please help me to write a script with "diag" command to create the matrix as shown in image. http://s1.postimage.org/5i4gd1esv/matrix.jpg By they way, it's a 99 X 99 matrix.

Thank you, John

[Matt J]

The toeplitz() command would be quicker.

[John Edward]

Thanks for your reply and I forgot to mention that it's a 99 by 99 matrix. Can you be able to script it for me.

Thank you, John

Answer 1

[Matt J]

>> z=zeros(3,1); z(1)=2; z(2)=-1; A=toeplitz(z)

A =

       2    -1     0
      -1     2    -1
       0    -1     2

[John Edward]

Matt, how would you make it a 99 X 99 matrix. That's where I am stock. Thx man.

[Cedric]

Look at what happens if you replace the 3 in Matt's answer with e.g. 5.

But just to be sure, you want a tri-diagonal 99x99 matrix, right?

[John Edward]

Thank YOU, YOU are a life saver. It worked.

Answer 2

[Walter Roberson]

diag(this, -1) + diag(that) + diag(theother,1)

[John Edward]

Thanks for your reply but it's a 99 by 99 matrix.

Answer 3

[Matt J]

A small example

    >> A=diag([1,1,1])-diag([1,1],1); A=A+A.'

    A =

         2    -1     0
        -1     2    -1
         0    -1     2

[John Edward]

Thanks for your reply but it's a 99 by 99 matrix. How would you script that? Thx a lot.

[Matt J]

If you understood how it works for 3x3, it should be pretty obvious how to extend it to the 99x99 case.

[John Edward]

Honestly, I am new to matlab and I have been working on this for past 4 hours and I can't get the code right. Can you help me on that? Thank you.

[Matt J]

I don't know what more help I can give without just doing the whole thing for you. Are you aware that

 >> help diag

or

 >> doc diag

will display an explanation of the diag command? If you've read that documentation, it should be really easy. Some more examples:

>> diag([1 2 3 4]), diag([10 20 30], 2), diag([5 7 6],-1)

ans =

       1     0     0     0
       0     2     0     0
       0     0     3     0
       0     0     0     4

ans =

       0     0    10     0     0
       0     0     0    20     0
       0     0     0     0    30
       0     0     0     0     0
       0     0     0     0     0

ans =

       0     0     0     0
       5     0     0     0
       0     7     0     0
       0     0     6     0