/*
 *  fpi.c  : Fixpunktiteration zur Lösung von
 *
 *           x = exp( - x / 2 )  =:  phi( x )
 *
 *  x0 ..... Startwert
 *  xacc ... Genauigkeit der  Näherungslösung
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>


#define   MAXITER   30


/*
 *  Iterationsfunktion phi(x)
 */
double phi( double x ) {
    return( exp(-0.5 * x) );
}


int main( void ) {

    double x0, xacc, x, xn;
    int    j;

    printf("x0 : ");
    scanf("%lf", &x0);
    printf("xacc : ");
    scanf("%lf", &xacc);

    x = x0;

    /*  Iterationsschleife  */

    for (j = 1; j <= MAXITER; j++) {

        xn = phi( x );

        printf("x = %14.7e    |xn - x| = %14.7e\n", x, fabs(xn - x));

        if (fabs(xn - x) < xacc) {
           printf("s = %14.7e\n", x);
           break;
        }

        x  = xn;
    }

    if (j > MAXITER) {
       printf("fpi: MAXITER ueberschritten\n");
       exit(1);
    }

    return(0);
}