01-03-2010
Originally C had almost no float support at all. Arithmetic was done in double, arguments to function were double, etc. When you used a float it was "promoted" to a double. It was like an object n that was a float was treated as "(double) n" except as an lvalue. So you could store data as float in an array or struct to save space, but it doubled when you used it. I'm not sure what the motivation was especially since I came from a Fortran environment. I heard some expanations (excuses?), but Fortran faced the same issues and made multiple precisions available anyway. Much of this changed when Ansi-C was introduced. But constants stayed double by default to save existing code. The Ansi comittee wanted to avoid breaking code where they could.
Look at your compiler's docs. You may have an option to make your constants floats.
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J0(3) Linux Programmer's Manual J0(3)
NAME
j0, j0f, j0l, j1, j1f, j1l, jn, jnf, jnl, y0, y0f, y0l, y1, y1f, y1l, yn, ynf, ynl - Bessel functions
SYNOPSIS
#include <math.h>
double j0(double x);
double j1(double x);
double jn(int n, double x);
double y0(double x);
double y1(double x);
double yn(int n, double x);
float j0f(float x);
float j1f(float x);
float jnf(int n, float x);
float y0f(float x);
float y1f(float x);
float ynf(int n, float x);
long double j0l(long double x);
long double j1l(long double x);
long double jnl(int n, long double x);
long double y0l(long double x);
long double y1l(long double x);
long double ynl(int n, long double x);
DESCRIPTION
The j0() and j1() functions return Bessel functions of x of the first kind of orders 0 and 1, respectively. The jn() function returns the
Bessel function of x of the first kind of order n.
The y0() and y1() functions return Bessel functions of x of the second kind of orders 0 and 1, respectively. The yn() function returns the
Bessel function of x of the second kind of order n.
For the functions y0(), y1() and yn(), the value of x must be positive. For negative values of x, these functions return -HUGE_VAL.
The j0f() etc. and j0l() etc. functions are versions that take and return float and long double values, respectively.
CONFORMING TO
The functions returning double conform to SVID 3, BSD 4.3, XPG4, POSIX 1003.1-2001. The other functions exist by analogy, and exist on sev-
eral platforms.
BUGS
There are errors of up to 2e-16 in the values returned by j0(), j1() and jn() for values of x between -8 and 8.
2002-08-25 J0(3)