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# Editing entries of matrix

I'm looking at a matrix, say adjacency matrix of a graph, g.am(). I want to change some of the entries now. I am told it's a vector and it's immutable. What can I do?

One thought I had is adding some other matrix to it with only one nonblank entry. But, is there some easy way to do that?

Another thought I had was turning it into a list, list(g.am()). Maybe that's acceptable. I'm not 100% sure yet for what I'm working on. If I go that way, is there a way to print it in a matrix form again, i.e., entries print in a square shape.

Thanks for any help

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## 2 Answers

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I'm confused. At least in Sage 4.7.1, the adjacency matrix returned by .am() isn't immutable.

sage: G = graphs.DiamondGraph()
sage: m = G.am()
sage: m[2,3] = 99
sage: m
[ 0  1  1  0]
[ 1  0  1  1]
[ 1  1  0 99]
[ 0  1  1  0]
sage: m[:,1] = 17
sage: m
[ 0 17  1  0]
[ 1 17  1  1]
[ 1 17  0 99]
[ 0 17  1  0]
sage: m[:,0] = vector([6,5,4,3])
sage: m
[ 6 17  1  0]
[ 5 17  1  1]
[ 4 17  0 99]
[ 3 17  1  0]


Even assuming it were immutable, making a new copy of it would be a natural thing to do. Could you cut-and-paste exactly what you did and the error message it generated?

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## Comments

You're a genius. I was doing m[2][3] = 99 and it called the error I mentioned above. Thanks for your help.

( 2011-10-09 22:07:21 +0100 )edit

To simplify the process of getting help, always cut and paste exactly what you did and the error message rather than paraphrasing it.

( 2011-10-09 23:50:17 +0100 )edit
1

m[2] returns a vector (which is immutable, IIRC), so m[2][3] is trying to set the entry of an immutable vector.

( 2011-10-10 07:15:15 +0100 )edit

@DSM Yes, a good point, thanks. @Jason Okay, thanks for explaining it.

( 2011-10-10 10:37:12 +0100 )edit

To answer myself, matrix(whatever_the_name_of_the_list_that_came_from_the_original_matrix_is) works. This might be all I need. But, this still isn't perfect, because I'm wanting to look at columns of the matrix and I don't know a good way to do that with lists... matrix[a] gives the row a... well okay, I can just make a new list and do a for loop that loops through the rows and picks of the ath entries in each row and appends it to the list, which would give me the ath column. But, that's not as easy as matrix.column(a). Is there a way with lists other than what I just said?

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## Comments

Wow, thanks me. That's very helpful. But, not sure if it's a perfect answer.

( 2011-10-09 16:29:39 +0100 )edit

@G-Sage No problem, my pleasure. Hopefully someone else can help you out some more.

( 2011-10-09 16:29:46 +0100 )edit
1

You can see the 4th column of M with M[:,3] or M.column(3) or M.transpose()[3].

( 2011-10-09 16:39:38 +0100 )edit

Okay, now that's what I'm talking about. Actually, the last two options don't work because I'm taking a matrix and making it into a list (so I can edit it). But, the first one you "list"ed does work if you do M[:][3]. So, thanks very much.

( 2011-10-09 16:52:03 +0100 )edit

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Asked: 2011-10-09 16:15:34 +0100

Seen: 8,680 times

Last updated: Oct 09 '11