Hi,
I was trying to make a figure of the Bessel function
with Microsoft Visual C++ 6.0 compiler.
My code is a simple combination of bessel.h and psplot.h
from NR3 as follows:
***********
#include "nr3.h"
#include "psplot.h"
#include "bessel.h"

Int main(void) {
const Int RES=500;
const Doub XLL=75., XUR=525., YLL=250., YUR=700.;
char *plotfilename = tmpnam(NULL);
VecDoub xx(RES), yy(RES);
Doub x1,x2;
Doub x3;
Int i;
Bessjy mybessel;
FILE *fp;

fp=fopen("test.txt","w");
for (;;) {
Doub ymax = -9.99e99, ymin = 9.99e99, del;
cout << endl << "Enter x1 x2 (x1=x2 to stop) :" << endl;
cin >> x1 >> x2;
if (x1==x2) break;
for (i=0;i<RES;i++) {
xx[i] = x1 + i*(x2-x1)/(RES-1.);
x3=xx[i];
yy[i] = mybessel.jn(0, x3);

fprintf(fp,"%f %f \n",xx[i],yy[i]);

if (yy[i] > ymax) ymax=yy[i];
if (yy[i] < ymin) ymin=yy[i];
}
del = 0.05*((ymax-ymin)+(ymax==ymin ? abs(ymax) : 0.));


PSpage pg(plotfilename);
PSplot plot(pg,XLL,XUR,YLL,YUR);
plot.setlimits(x1,x2,ymin-del,ymax+del);
plot.frame();
plot.autoscales();
plot.xlabel("x");
plot.ylabel("Bessel J_0(x)");
plot.setlinewidth(1.);
plot.lineplot(xx,yy);
if (ymax*ymin < 0.) plot.lineseg(x1,0.,x2,0.);
plot.display();
}

remove(plotfilename);

fclose(fp);
return 0;
}
**************
This code is not optimal. Anyway it was successfully compiled.
The figure showed nice up-down curve between 0 and 8.
But around x=8, there are abrupt changes.
In addition, if I increase the order n, the graph showed like a histogram.
I hope you have an answers for this.
Would you show me a track?
Thanks in advance.
jdkim

...I hope you have an answers for this...

The problem is not with bessel.h; it's in the way that your particular compiler treats psplot.h.


The old, creaky, cranky MSVC++ Version 6 has a bug that shows up in psplot.h. The following thread shows how to change nr3.h to work around that particular problem: http://www.nr.com/forum/showthread.php?t=1459

On Windows xp compiling with Visual C++ Version 6 and using the original nr3.h from my Version 3.0 CD I see exactly what you describe. With the fixes in nr3.h, it looks OK.


Regards,

Dave

Footnote: I notice that the link given in the above thread to download nr3.h actually gives me the original distribution's nr3.h, not the fixed nr3.h, so here's what I recommend:

1. Make a copy of your nr3.h (just in case...). Call it something like "nr3_distribution.h"

2. Apply the changes to the nr3.h that you will be using.

3. Compile again.

With the original distribution's nr3.h, using Windows XP and compiling from a command line with Visual C++ Version 6, I got the following compiler messages:

./nr3.h(43) : warning C4172: returning address of local variable or temporary
./psplot.h(165) : see reference to function template instantiation 'const double &__cdecl MIN(const double &,const double &)' being compiled
./nr3.h(33) : warning C4172: returning address of local variable or temporary
./psplot.h(166) : see reference to function template instantiation 'const double &__cdecl MAX(const double &,const double &)' being compiled

With the fixed version, there were no compiler messages.

Bottom line: Sometimes compiler warnings are significant. They don't necessarily mean there is a bug in the program, but in this case they definitely mean that the compiler is not doing the Right Thing.

Hi, Dave
Thanks for your comments.
My problem was solved and now I feel the NR much more comfortably. Maybe I should change my C compiler.
JD