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Annotation of 42BSD/usr.lib/learn/C/L43.1a, revision 1.1.1.1

1.1       root        1: #print
                      2: Write a function 
                      3:        cubrt(x)
                      4: that returns the cube root of a floating point number.
                      5: Put it on a file named "cubrt.c"; compile and test it,
                      6: then type "ready".
                      7: (If you don't know how to compute cube roots, try Newton's method).
                      8: #once #create reldif.c
                      9: double reldif(a,b)
                     10:  double a,b;
                     11: {
                     12: double c,d;
                     13: if (a==0. && b==0.) return(0.);
                     14: c = a>0 ? a : -a;
                     15: d = b>0 ? b : -b;
                     16: c = c>d ? c : d;
                     17: return( (a-b)/c );
                     18: }
                     19: #once #create tzaqc.c
                     20: main()
                     21: {
                     22:        double cubrt();
                     23:        double reldif();
                     24:        double a, b, eps;
                     25: 
                     26:        a = 8.;
                     27:        b = 2.;
                     28:        eps = 1e-5;
                     29:        if (reldif(cubrt(a), b) > eps)
                     30:                exit(1);
                     31: 
                     32:        a = 0.;
                     33:        b = 0.;
                     34:        if (reldif(cubrt(a), b) > eps)
                     35:                exit(1);
                     36: 
                     37:        a = 1e6;
                     38:        b = 1e2;
                     39:        if (reldif(cubrt(a), b) > eps)
                     40:                exit(1);
                     41:        exit(0);
                     42: }
                     43: #user
                     44: cc tzaqc.c cubrt.o reldif.c
                     45: a.out
                     46: #succeed
                     47: /* one way */
                     48: double cubrt(x)
                     49: double x;
                     50: {
                     51:        /* Newton's method:    x <- x - (x**3-a)/(3*x*x) */
                     52:        double y, yn, dabs();
                     53:        y = 0.;
                     54:        yn = x;
                     55:        while (dabs(y-yn) > y*1.e-8) {
                     56:                y = yn;
                     57:                yn = y - (y*y*y-x)/(3*y*y);
                     58:        }
                     59:        return(yn);
                     60: }
                     61: 
                     62: double dabs(x)
                     63: double x;
                     64: {
                     65:        return(x>0 ? x : -x);
                     66: }
                     67: #log
                     68: #next
                     69: 50.1a 10
                     70: 43.1b 5

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