Re: [Gnuplot-bugs] Plot of constant function causes gnuplot to 'hang'

[Hans-Bernhard B. <br...@ph...>]

Mikkelsen, Christian Ingemann wrote:

> On a Ubuntu 5.10 machine plotting of a simple (but constant) function
>  causes gnuplot to hang and use all available CPU resources:

The problem is that, contrary your expectations, this function is *not*
actually constant.  You're expecting that an equality found in
infinite-precision arithmetic will be preserved in finite-precision
arithmetic as done by your computers.  Well, it's not.  As the saying
goes: "In computing 10 times 0.1 is hardly ever 1.0".

The hang is caused indirectly by this: this function is so close to 
being constant, that it drives the algorithm trying to determine a 
suitable axis range and tic interval face-first into the wall.  The 
actual range of values will be something like [1-a*DBL_EPS:1+b*DBL_EPS], 
for small integers a and b.

If you specify a suitable, fixed axis range, it will no longer hang, and 
you'll see the problem:

	set yrange [1-1e-15 : 1+1e-15]
	plot sin(x/2)**4 + sin(x)**2/2 + cos(x/2)**4

Or you can plot the difference between the supposedly constant result 
and the expected, actual constant:

	set autoscale y
	plot sin(x/2)**4 + sin(x)**2/2 + cos(x/2)**4 - 1