Fourier

#include <iostream>
#include <math.h>
#include <fstream>
#include <cmath>

using namespace std;

const double PI = 3.1415926535897932384626433832795;
const double step_x = 0.001;
const int max_number = 100;
const double step_t = 0.00001;
const double length = 100;

double q_array[max_number];
////////////////////////////////////////////////////////
//ищем q и в масив
////////////////////////////////////////////////////////
double f(double x)
{
    return (cos(x) / sin(x)) - x;
}

double bissection (int number)
{
    double x_1, x_2, half_x_1_x_2 ;
    x_1 = (number * PI) + step_x;
    x_2 = (number * PI) + PI - step_x;
    while (abs(x_1 - x_2) > step_x) {
        half_x_1_x_2 = (x_1 + x_2) / 2;
        if (f(x_1) * f(half_x_1_x_2) < 0)
            x_2 = half_x_1_x_2;
        else
            x_1 = half_x_1_x_2;
    }
    return abs(x_1 + x_2) / 2;
}

void finding_qk () {
    for (int i = 0; i < max_number; i++) {
        ::q_array[i] = 0;
    }
    for (int i = 0; i < max_number; i++) {
        ::q_array[i] = bissection(i);
    }
}

double calculate(double q, double x, double t) {
    double result = 0;
    result = (4 / (2 * q + sin(2 * q)))*(sin(q) + 2 * sin(q / 2) * sin(q / 2) / q + 2 * sin(q / 10) * cos(q / 3)) * exp(-t * q * q) * cos(q * x);
    return result;
}

void calculation() {
    double result = 0;
    fstream f;
    ///////////////// 2T
    f.open("C://Users//User//Documents//2T.txt", fstream::trunc | fstream::out);
    for (double j = 0; j < 1; j+= step_x){
        for (int i = 0; i < max_number; i++) {
            result += calculate(q_array[i], j, 2);
        }
        result += -2 + j;
        f << result << endl;
        result = 0;
    }
    f.close();
    /////////////////////////
    //////////////////////5T
    f.open("C://Users//User//Documents//5T.txt", fstream::trunc | fstream::out);
    for (double j = 0; j < 1; j += step_x) {
        for (int i = 0; i < max_number; i++) {
            result += calculate(q_array[i], j, 5);
        }
        result += -2 + j;
        f << result << endl;
        result = 0;
    }
    f.close();
    /////////////////////////
    //////////////////////l/3
    f.open("C://Users//User//Documents//l.txt", fstream::trunc | fstream::out);
    for (double j = 0; j < 5; j += step_t) {
        for (int i = 0; i < max_number; i++) {
            result += calculate(q_array[i], (double)1/3, j);
        }
        result += -2 + (double)1/3;
        f << result << endl;
        result = 0;
    }
    f.close();
    /////////////////////////
    //////////////////////T= 0
    f.open("C://Users//User//Documents//0.txt", fstream::trunc | fstream::out);
    for (double j = 0; j < 1; j += step_x) {
        for (int i = 0; i < max_number; i++) {
            result += calculate(q_array[i], j, 0);
        }
        result += -2 + j;
        f << result << endl;
        result = 0;
    }
    f.close();
}



int main() {
    finding_qk();
    calculation();
    fstream f;
    f.open("C://Users//User//Documents//step_x.txt", fstream::trunc | fstream::out);
    for (double i = 0; i < 1; i+=step_x) {
        f << i << endl;
    }
    f.close();
    f.open("C://Users//User//Documents//step_t.txt", fstream::trunc | fstream::out);
    for (double i = 0; i < 5; i += step_t) {
        f << i << endl;
    }
    f.close();
    return 0;
}