[Octave-bug-tracker] [bug #41305] error: eigs: error -14 in dneupd

[Max G.]

Follow-up Comment #8, bug #41305 (project octave):

I'm amazed about the reception this bug has found here.

Unfortunately, I don't see this beeing resolved by a mere
errno2string-function.

The behaviour of eigs is quite weird, as I try to demonstrate in the command
line excerpt:

address@hidden:~$ octave -q
warning: function /usr/share/octave/packages/statistics-1.2.0/fstat.m shadows
a core library function
octave:1> for it = 115:20000 rand('state',it); W = rand(200); W(W < 0.9) = 0;
W(W != 0) = rand(size(W(W != 0))) - 0.5; eigs(W,1,'LM'); endfor
error: eigs: error -14 in dneupd
octave:1> it
it =  137
octave:2> for it = 136:20000 rand('state',it); W = rand(200); W(W < 0.9) = 0;
W(W != 0) = rand(size(W(W != 0))) - 0.5; eigs(W,1,'LM'); endfor
error: eigs: error -14 in dneupd
octave:2> it
it =  137
octave:3> for it = 137:20000 rand('state',it); W = rand(200); W(W < 0.9) = 0;
W(W != 0) = rand(size(W(W != 0))) - 0.5; eigs(W,1,'LM'); endfor
error: eigs: error -14 in dneupd
octave:3> it
it =  182
octave:4> rand('state',137); W = rand(200); W(W < 0.9) = 0; W(W != 0) =
rand(size(W(W != 0))) - 0.5; eigs(W,1)
ans = -0.12943 - 1.26169i
octave:5> rand('state',136); W = rand(200); W(W < 0.9) = 0; W(W != 0) =
rand(size(W(W != 0))) - 0.5; eigs(W,1)
ans =  1.06808 - 0.73793i
octave:6> rand('state',138); W = rand(200); W(W < 0.9) = 0; W(W != 0) =
rand(size(W(W != 0))) - 0.5; eigs(W,1)
ans =  1.3301
octave:7> address@hidden:~$ 
address@hidden:~$ octave -q
warning: function /usr/share/octave/packages/statistics-1.2.0/fstat.m shadows
a core library function
octave:1> rand('state',138); W = rand(200); W(W < 0.9) = 0; W(W != 0) =
rand(size(W(W != 0))) - 0.5; eigs(W,1)
ans =  1.3301
octave:2> rand('state',137); W = rand(200); W(W < 0.9) = 0; W(W != 0) =
rand(size(W(W != 0))) - 0.5; eigs(W,1)
ans = -0.12943 - 1.26169i
octave:3> rand('state',136); W = rand(200); W(W < 0.9) = 0; W(W != 0) =
rand(size(W(W != 0))) - 0.5; eigs(W,1)
ans =  1.06808 - 0.73793i
octave:4> for it = 136:20000 rand('state',it); W = rand(200); W(W < 0.9) = 0;
W(W != 0) = rand(size(W(W != 0))) - 0.5; eigs(W,1,'LM'); endfor
error: eigs: error -14 in dneupd
octave:4> it
it =  137


I use a for loop to find seeds that trigger the error. If the for loop starts
at it=115 or at it=136, the error occurs at seed 137.  If the for loop starts
at it=137, the error occurs at seed 182. If I try to trigger the error by
hand, knowing the seed, the error does not occur at all.

The errno2string function would be helpful, I'm glad to know a workaround, but
I think there is a real error hidden somewhere.Octave, errors coming from
Arpack dneupd look like:
"error: eigs: error -14 in dneupd" see e.g. [1]

This is not helpful and it would be good to have a convenient way to 
print error messages from Arpack itself?

Maybe a convenience function like "errno2string" or 

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