Creating a matrix using diag

2013 1 27
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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

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Matt J
Matt J 2013-1-27
编辑：Matt J 2013-1-27
The toeplitz() command would be quicker.
John Edward
John Edward 2013-1-27
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

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Matt J
Matt J 2013-1-27

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>> z=zeros(3,1); z(1)=2; z(2)=-1; A=toeplitz(z)
A =
2 -1 0
-1 2 -1
0 -1 2

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John Edward
John Edward 2013-1-27
Matt, how would you make it a 99 X 99 matrix. That's where I am stock. Thx man.
Cedric
Cedric 2013-1-27
编辑：Cedric 2013-1-27
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
John Edward 2013-1-27
Thank YOU, YOU are a life saver. It worked.

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更多回答（2 个）

Walter Roberson
Walter Roberson 2013-1-27

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diag(this, -1) + diag(that) + diag(theother,1)
Matt J
Matt J 2013-1-27
编辑：Matt J 2013-1-27

0 个投票

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

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John Edward
John Edward 2013-1-27
Thanks for your reply but it's a 99 by 99 matrix. How would you script that? Thx a lot.
Matt J
Matt J 2013-1-27
If you understood how it works for 3x3, it should be pretty obvious how to extend it to the 99x99 case.
John Edward
John Edward 2013-1-27
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
Matt J 2013-1-27
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

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