mercredi 25 novembre 2009

A warning on behavior of gnuplot with numerical values

My favorite plotting software is gnuplot. Not that I am an expert in plotting software, it was just the first I really invested time into, after getting tired of ugly Excel plots... But I got used to it, and I like the idea of script-based plotting.

However, I must say it can be quite tricky, and I often stumble upon some things difficult to achieve, or to make them work. Yesterday, after stumbling for several hours (!) through a function plot that did not work as expected, I finally found out why, and discovered a strange "feature" of gnuplot(4.3): it does not treat integer values as floating-point values !

While this seems quite obvious in a programming language (C), I don't understand why it is so in gnuplot. As far as I can see, a plotting app should treat all numbers as "real" numbers (understand "floating-point"). But this seems to be an opinion I don't have in common with the designers of gnuplot.

To make it clear, what I mean is that the value 10*k/3 is NOT the same as k/3*10, if you happen to give k=2
(just plot these two expressions, you'll see what I mean)
To have it correct, you have to type k=2.0
If not, then 10*k/3 will be equal to 6, and k/3*10 will be equal to 0, while you expected to be 6.66. Yep, you got it, integer division striked again...

Maybe this feature is useful in some situations, I have no clue. After searching the manual, this is indeed explained in section 13 ("Expressions"), page 15 for 4.2 version manual. Of course, I discovered this after trying several hours to make the damn thing work...

It must be pointed out that in this case, I knew what the plot was supposed to look like. So I was able to track down where the problem was. In most situations, such an error is most likely to stay undetected most of the time, until one day, with one particular case of data, you get a plot full of nonsense...

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