durand_kerner_roots.c File Reference

Compute all possible approximate roots of any given polynomial using Durand Kerner algorithm More...

#include <complex.h>
#include <limits.h>
#include <math.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>

Include dependency graph for durand_kerner_roots.c:

Macros
#define	ACCURACY   1e-10
	maximum accuracy limit

Functions
long double complex	poly_function (long double *coeffs, unsigned int degree, long double complex x)
	Evaluate the value of a polynomial with given coefficients. More...
const char *	complex_str (long double complex x)
	create a textual form of complex number More...
char	check_termination (long double delta)
	check for termination condition More...
int	main (int argc, char **argv)

Detailed Description

Compute all possible approximate roots of any given polynomial using Durand Kerner algorithm

Author

Krishna Vedala

Test the algorithm online: https://gist.github.com/kvedala/27f1b0b6502af935f6917673ec43bcd7

Try the highly unstable Wilkinson's polynomial:

./numerical_methods/durand_kerner_roots.c 1 -210 20615 -1256850 53327946
-1672280820 40171771630 -756111184500 11310276995381 -135585182899530
1307535010540395 -10142299865511450 63030812099294896 -311333643161390640
1206647803780373360 -3599979517947607200 8037811822645051776
-12870931245150988800 13803759753640704000 -8752948036761600000
2432902008176640000

Sample implementation results to compute approximate roots of the equation \(x^4-1=0\):

Function Documentation

check_termination()

char check_termination

(

long double

delta

)

check for termination condition

Parameters

[in]

delta

point at which to evaluate the polynomial

Returns

0 if termination not reached

1 if termination reached

{
    static long double past_delta = INFINITY;
    if (fabsl(past_delta - delta) <= ACCURACY || delta < ACCURACY)
        return 1;
    past_delta = delta;
    return 0;
}

complex_str()

const char * complex_str

(

long double complex

x

)

create a textual form of complex number

Parameters

[in]

x

point at which to evaluate the polynomial

Returns

pointer to converted string

{
    static char msg[50];
    double r = creal(x);
    double c = cimag(x);
 
    sprintf(msg, "% 7.04g%+7.04gj", r, c);
 
    return msg;
}

main()

int main	(	int	argc,
		char **	argv
	)

store intermediate values to a CSV file

{
    long double *coeffs = NULL;
    long double complex *s0 = NULL;
    unsigned int degree = 0;
    unsigned int n, i;
 
    if (argc < 2)
    {
        printf(
            "Please pass the coefficients of the polynomial as commandline "
            "arguments.\n");
        return 0;
    }
 
    degree = argc - 1; /* detected polynomial degree */
    coeffs = (long double *)malloc(
        degree * sizeof(long double)); /* store all input coefficients */
    s0 = (long double complex *)malloc(
        (degree - 1) *
        sizeof(long double complex)); /* number of roots = degree-1 */
 
    /* initialize random seed: */
    srand(time(NULL));
 
    if (!coeffs || !s0)
    {
        perror("Unable to allocate memory!");
        if (coeffs)
            free(coeffs);
        if (s0)
            free(s0);
        return EXIT_FAILURE;
    }
 
#if defined(DEBUG) || !defined(NDEBUG)
    /**
     * store intermediate values to a CSV file
     */
    FILE *log_file = fopen("durand_kerner.log.csv", "wt");
    if (!log_file)
    {
        perror("Unable to create a storage log file!");
        free(coeffs);
        free(s0);
        return EXIT_FAILURE;
    }
    fprintf(log_file, "iter#,");
#endif
 
    printf("Computing the roots for:\n\t");
    for (n = 0; n < degree; n++)
    {
        coeffs[n] = strtod(argv[n + 1], NULL);
        if (n < degree - 1 && coeffs[n] != 0)
            printf("(%Lg) x^%d + ", coeffs[n], degree - n - 1);
        else if (coeffs[n] != 0)
            printf("(%Lg) x^%d = 0\n", coeffs[n], degree - n - 1);
 
        double tmp;
        if (n > 0)
            coeffs[n] /= tmp; /* numerical errors less when the first
                                 coefficient is "1" */
        else
        {
            tmp = coeffs[0];
            coeffs[0] = 1;
        }
 
        /* initialize root approximations with random values */
        if (n < degree - 1)
        {
            s0[n] = (long double)rand() + (long double)rand() * I;
#if defined(DEBUG) || !defined(NDEBUG)
            fprintf(log_file, "root_%d,", n);
#endif
        }
    }
 
#if defined(DEBUG) || !defined(NDEBUG)
    fprintf(log_file, "avg. correction");
    fprintf(log_file, "\n0,");
    for (n = 0; n < degree - 1; n++)
        fprintf(log_file, "%s,", complex_str(s0[n]));
#endif
 
    double tol_condition = 1;
    unsigned long iter = 0;
 
    clock_t end_time, start_time = clock();
    while (!check_termination(tol_condition) && iter < INT_MAX)
    {
        long double complex delta = 0;
        tol_condition = 0;
        iter++;
 
#if defined(DEBUG) || !defined(NDEBUG)
        fprintf(log_file, "\n%ld,", iter);
#endif
 
        for (n = 0; n < degree - 1; n++)
        {
            long double complex numerator =
                poly_function(coeffs, degree, s0[n]);
            long double complex denominator = 1.0;
            for (i = 0; i < degree - 1; i++)
                if (i != n)
                    denominator *= s0[n] - s0[i];
 
            delta = numerator / denominator;
 
            if (isnan(cabsl(delta)) || isinf(cabsl(delta)))
            {
                printf("\n\nOverflow/underrun error - got value = %Lg",
                       cabsl(delta));
                goto end;
            }
 
            s0[n] -= delta;
 
            tol_condition = fmaxl(tol_condition, fabsl(cabsl(delta)));
 
#if defined(DEBUG) || !defined(NDEBUG)
            fprintf(log_file, "%s,", complex_str(s0[n]));
#endif
        }
        // tol_condition /= (degree - 1);
 
#if defined(DEBUG) || !defined(NDEBUG)
        if (iter % 500 == 0)
        {
            printf("Iter: %lu\t", iter);
            for (n = 0; n < degree - 1; n++) printf("\t%s", complex_str(s0[n]));
            printf("\t\tabsolute average change: %.4g\n", tol_condition);
        }
 
        fprintf(log_file, "%.4g", tol_condition);
#endif
    }
end:
 
    end_time = clock();
 
#if defined(DEBUG) || !defined(NDEBUG)
    fclose(log_file);
#endif
 
    printf("\nIterations: %lu\n", iter);
    for (n = 0; n < degree - 1; n++) printf("\t%s\n", complex_str(s0[n]));
    printf("absolute average change: %.4g\n", tol_condition);
    printf("Time taken: %.4g sec\n",
           (end_time - start_time) / (double)CLOCKS_PER_SEC);
 
    free(coeffs);
    free(s0);
 
    return 0;
}

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poly_function()

long double complex poly_function	(	long double *	coeffs,
		unsigned int	degree,
		long double complex	x
	)

Evaluate the value of a polynomial with given coefficients.

Parameters

[in]	coeffs	coefficients of the polynomial
[in]	degree	degree of polynomial
[in]	x	point at which to evaluate the polynomial

Returns

\(f(x)\)

{
    long double complex out = 0.;
    unsigned int n;
 
    for (n = 0; n < degree; n++) out += coeffs[n] * cpow(x, degree - n - 1);
 
    return out;
}