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Annotation of 43BSDTahoe/usr.lib/learn/C/L43.1a, revision 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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