Strange behaviour computing l1 norm of rows of a matrix

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Christoph Neumann
Christoph Neumann 2020-7-22
评论： James Tursa 2024-1-28
I have a (1102,8232)-matrix B (from Data that I don't have control over).
The query
B(1102,:)
yields
ans = All zero sparse: 1×8232
However, when I compute
beta = sum(abs(B),2);
or equivalently
beta = vecnorm(B,1,2)
I get
beta(1102)
ans = (1,1) 55
On the other hand, the command
norm(B(1102,:),1)
ans = 0
works as expected.
I'm really confused what's happening here. Any help is appreciated.

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Bruno Luong
Bruno Luong 2020-8-13
编辑：Bruno Luong 2020-8-13
Who build this sparse matrix B?
If it's build from external source it can be not-valid. I believe there is a tool on FileExchange to validate the sparse matrix, the name escapes me now.
EDIT: this command crahes my MATLAB
B = B+0
indicating this matrix has serious building flaw.
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Bruno Luong
Bruno Luong 2020-8-13
Yeah but be aware the fixed matrix might not be correct. You better try to fix at gurobi_read or python, whichever step that is responsible for the error.

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