BSD 4_3_Tahoe release

[unix-history] / usr / lib / learn / C / L43.1a

#print
Write a function 
       cubrt(x)
that returns the cube root of a floating point number.
Put it on a file named "cubrt.c"; compile and test it,
then type "ready".
(If you don't know how to compute cube roots, try Newton's method).
#once #create reldif.c
double reldif(a,b)
double a,b;
{
double c,d;
if (a==0. && b==0.) return(0.);
c = a>0 ? a : -a;
d = b>0 ? b : -b;
c = c>d ? c : d;
return( (a-b)/c );
}
#once #create tzaqc.c
main()
{
       double cubrt();
       double reldif();
       double a, b, eps;

       a = 8.;
       b = 2.;
       eps = 1e-5;
       if (reldif(cubrt(a), b) > eps)
               exit(1);

       a = 0.;
       b = 0.;
       if (reldif(cubrt(a), b) > eps)
               exit(1);

       a = 1e6;
       b = 1e2;
       if (reldif(cubrt(a), b) > eps)
               exit(1);
       exit(0);
}
#user
cc tzaqc.c cubrt.o reldif.c
a.out
#succeed
/* one way */
double cubrt(x)
double x;
{
       /* Newton's method:    x <- x - (x**3-a)/(3*x*x) */
       double y, yn, dabs();
       y = 0.;
       yn = x;
       while (dabs(y-yn) > y*1.e-8) {
               y = yn;
               yn = y - (y*y*y-x)/(3*y*y);
       }
       return(yn);
}

double dabs(x)
double x;
{
       return(x>0 ? x : -x);
}
#log
#next
50.1a 10
43.1b 5