Re: [Gnuplot-info] log(xy) not turning out to be log(x) + log(y)

[Thomas S. <t.s...@fz...>]

Anirban Gangopadhyay <gangopadhyay.anirban <at> gmail.com> writes:

> 
> Hi,
> 
> I used the following simple gnuplot script written below :
> 
>  set terminal postscript enhanced color 'Helvetica' 20
>  p(x,y,n1,n2) = log10((x**n1) * (abs(y)**n2)) + log10(x)
>  q(x,y,n1,n2) = log10((x**n1) * x * (abs(y)**n2))
>  set output 'plot499.ps'
>  plot 'tsorted499diffVsB.dat' u (p(499,$1,3,0.66)):(p(499,$5,2,1)) every
> ::27::44 w p ls 1 t 'N=499(add)', 'tsorted499diffVsB.dat' u
> (q(499,$1,3,0.66)):(q(499,$5,2,1)) every ::27::44 w p ls 2 t
> 'N=499(multiply)'
>  set terminal x11
> 
> Note that the functions p and q are the same function using the idea that
> log(product of two numbers) = sum of log of the two numbers, but the plot
> command does not result in overlapping plots. I have attached the data
> file  'tsorted499diffVsB.dat' and the output file 'plot499.ps' here. Any
> help/clue as to what might be happening here will be appreciated.
> 
> Anirban

if your numbers get too large,

log10(x)+log10(y)

will work perfectly, but

log10(x*y)

will fail because x*y is out of range.

maybe this is happening with your data?