[Octave-bug-tracker] [bug #54100] fread using SKIP larger than zero is extremely slow

[Michael Leitner]

Follow-up Comment #2, bug #54100 (project octave):

Wouldn't it be even better to just do a large binary read into an existing
buffer buf1 and then pick the values that you need, that is, in C it would
look somehow like


char * buf1;
double * buf2;

for (i=0;i<N;i++) buf2[i]=*(double *)(buf1+(sizeof(double)+skip)*i);


for the case of output in doubles. Of course, for very large skip-values one
should fall back to the present implementation, and for very large N you would
want to do this piece-meal in order not to explode your memory requirements.
If you think that this step of copying is inefficient, remember that in the
present case you even have some function calls per value, and that whenever
you have a non-starred precision argument (other than double) you have to do
the casting and copying in any case. 


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