Re: gnuplot bad-dat parsing bug?

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

On Saturday, 27 January 2024 16:26:09 PST Dima Kogan wrote:
> 
> Hi. I'm observing a data-parsing problem that I think is a bug... but
> looking at the code, maybe it isn't? This has been an issue for a long
> time; it's not something new in 6.0.
> 
> To reproduce, I have this script:
> 
>   set terminal dumb 80,40
>   unset grid
>   plot '-' with linespoints
>   1  -
>   2  10
>   3  5
>   4  13
>   5  4
>   6  13
>   7  7
>   8  15
>   e
> 
> Note the bad-data line "1 -". This produces this plot:
> 
[snip] 
> 
> Note that I would expect an up/down zigzag, which ignores the whole
> bad-data line. Removing that line produces the expected output:

Not a bug.

It would be more obvious what is happening if you replace the
first column of values with some other range that doesn't start at 1.
For instance:

 set terminal dumb 80,10
  unset grid
  plot '-' with linespoints
  11  -
  12  10
  13  5
  14  13
  15  4
  16  13
  17  7
  18  15
  e

  18 +----------------------------------------------------------------------+   
  17 |-+       +         +         +          +         +    *****G****   +-|   
  16 |-+                                           *****G****   '-' ***G***-|   
  15 |-+                      *****G**********G****                       +-|   
  13 |-+            *****G****                                            +-|   
  12 |-+  *****G****     +         +          +         +         +       +-|   
  11 +----------------------------------------------------------------------+   
     0         1         2         3          4         5         6         7   

Note that the x range runs from 0 to 7 rather than from 11 to 18.

That is because the program found only a single number of the first line
of data, so it assumed you implicitly wanted "plot foo using 0:1 with lp".
I.e. if only a single column of data is given it is used as a y value and
the corresponding x value is taken from the row number, a.k.a. column 0.

The same data plotted with the command
plot '-' using 1:2 with linespoints
gives

  16 +----------------------------------------------------------------------+   
  14 |-+         +          *G*          +         *G*          +       ***-|   
  12 |-+                 ***   **                **   ****using 1:2 ***G***-|   
  10 |**              ***        **            **         ****    ***     +-|   
   8 |-+******     ***             ***       **               **G*        +-|   
   6 |-+      ***G*          +        ** + **       +           +         +-|   
   4 +----------------------------------------------------------------------+   
     12          13          14          15         16          17          18  

Both the x range and the shape of the plot is as expected.

> 
> So the first row of data has one valid column, and we then expect
> exactly one valid column from every subsequent row of data as well.
> Checked here:
> 
>   https://urldefense.com/v3/__https://github.com/gnuplot/gnuplot/blob/fbeb88eadedf927a4d778b41dd118e373f33eacb/src/datafile.c*L742__;Iw!!K-Hz7m0Vt54!me4XQdEz6ddDkXcDlBCDA3x0Nh-puPO6RfaqP9afG6Z-DMAiShcRiRh2zLMlBLfTzAetVK89V0o9M82vM3Azbm7y31YbKA$ 
> 
> Is this what we want?

It has always been that way.

If you know what columns you want to take the data from,
that's what "using" is for.
Otherwise the program has to guess, and it uses the first
row of data to make that guess.

	Ethan