Re: segmentation fault when using matrix uniform with index

[Ethan A M. <me...@uw...>]

On Thursday, 19 March 2020 15:17:21 PDT Juhász Péter wrote:
> Hi,
> 
> Today I've tried to create a heatmap from a CSV-ish file that contained
> more than one blocks of data, and I've naively thought that I can
> combine `plot ... index N` with `plot ... matrix rowheaders
> columnheaders w image`. Admittedly, it's an edge case, but it's not
> explicitly mentioned as illegal either.
> 
> Unfortunately, it doesn't work:
> for small files it produces an empty plot with some error messages, and
> for large files it crashes.
> 
> The following one-liner will create a file that can be used to
> reproduce the crash:
> 
> perl -le '$N=100; $,=v9; for (1..2) {print "x",qw(a b)x$N; print
> "a",(1,2)x$N for 1..$N; print"" for 1..2}' > /tmp/foo
> 
> then in gnuplot,
> 
> plot '/tmp/foo' i 0 matrix rowheaders columnheaders w image
> 
> Decrease the value of $N in the one-liner to generate a file that
> doesn't crash, for me the threshold was 26.
> 
> gnuplot version: 5.5 patchlevel 0 last modified 2019-12-22
> 
> The crash happens in graphics.c:process_image.
> 
> So it appears that `index` doesn't cooperate with `matrix` - I can
> accept that this combination is not supported but then there should be
> a note about the fact in the documentation. And it shouldn't crash in
> any case.

I agree that the program should never crash just because the command
or the data is not exactly as expected.  So there certainly is a bug.
But I don't think it is a question of 'matrix' not recognizing 'index.

I think the problem is that it is not well defined what it means to
have column headers in a file with multiple data blocks.

The program seems to think that there is a single set of column headers
on the first line and does not expect them to reappear in front of each
data block.  If I take the large test file generated by your perl jiffy and
comment out the second row of headers, then the program seems to
operate acceptably.

I will try to figure out why an unexpect line of headers causes
a segfault, but beyond that I don't know which is more common:

    1 line of column headers applying to the entire file
or
  a separate line of column headers before each data block.

Thoughts from anyone who deals with this kind of data file?

	Ethan
> 
> best regards,
> Peter Juhasz