task_sem3_4.10

#include <stdio.h>
#include <math.h>
#include "ode.h"

#define ALPHA   0.01
#define TOL     1e-6
#define REL     1
#define DX      0.1

void f(double x, double y[], double yp[]) {
    yp[0] = 2.0 * y[0] - ALPHA * y[0] * y[1];
    yp[1] = -y[1] + ALPHA * y[0] * y[1];
}

int main(int argc, char *argv[]) {
    double y[2]; // y[0] - r; y[1] - f
    double x;
    double step = 1e-3;
    FILE *fdat;

    // first situation
    fdat = fopen("sit1.dat", "w");
    x = 0.0;
    y[0] = 10.0;
    y[1] = 30.0;
    fprintf(fdat, "%.1lf\t%.10e\t%.10e\n", 0.0, y[0], y[1]);
    //printf("x = %.1lf\tr = %.10e\tf = %.10e\n", 0.0, y[0], y[1]);

    while (x <= 20.0) {
        autoint(2, &x, x + DX, y, &step, TOL, REL, f);
        fprintf(fdat, "%.1lf\t%.10e\t%.10e\n", x, y[0], y[1]);
        //printf("x = %.1lf\tr = %.10e\tf = %.10e\n", x, y[0], y[1]);
    }
    fclose(fdat);

    // second situation
    fdat = fopen("sit2.dat", "w");
    x = 0.0;
    y[0] = 30.0;
    y[1] = 10.0;
    fprintf(fdat, "%.1lf\t%.10e\t%.10e\n", 0.0, y[0], y[1]);

    while (x <= 20.0) {
        autoint(2, &x, x + DX, y, &step, TOL, REL, f);
        fprintf(fdat, "%.1lf\t%.10e\t%.10e\n", x, y[0], y[1]);
    }
    fclose(fdat);

    // third situation
    fdat = fopen("sit3.dat", "w");
    x = 0.0;
    y[0] = 300.0;
    y[1] = 100.0;
    fprintf(fdat, "%.1lf\t%.10e\t%.10e\n", 0.0, y[0], y[1]);

    while (x <= 20.0) {
        autoint(2, &x, x + DX, y, &step, TOL, REL, f);
        fprintf(fdat, "%.1lf\t%.10e\t%.10e\n", x, y[0], y[1]);
    }
    fclose(fdat);

    return 0;
}