Object oriented programming

#include <stdio.h>

struct date {
    int year;
    int month;
    int day;
};
typedef struct date date;
// 1 d1 > d2
// 0 d1 == d2
// -1 d1 < d2
int compare(date d1, date d2) {
    if (d1.year > d2.year) return 1;
    else if (d1.year < d2.year) return -1;
    else { // d1.year == d2.year
        if (d1.month > d2.month) return 1;
        else if (d1.month < d2.month) return -1;
        else { // d1.month == d2.month
            if (d1.day > d2.day) return 1;
            else if (d1.day < d2.day) return -1;
            else return 0;
        }
    }
}
/**
* This function is aproximation
*/
int days(date d1, date d2) {
    int days = d1.day - d2.day;
    days += (d1.month - d2.month) * 30;
    days += (d1.year - d2.year) * 365;
    return days;
}

void read(date *d) {
    scanf("%d.%d.%d", &d->day, &d->month, &(*d).year);
}

void print(date *d) {
    printf("%02d.%02d.%d\n", d->day, d->month, d->year);
}

int main() {
    date d1;
    date d2;
    read(&d1);
    read(&d2);

    int res = compare(d1, d2);
    if (res == 0) {
        printf("Dates are equal\n");
    } else if (res > 0) {
        printf("The difference in days is %d days.\n", days(d1, d2));
    } else {
        printf("The difference in days is %d days.\n", days(d2, d1));
    }
    return 0;
}

#include <stdio.h>

struct vector {
    float x;
    float y;
    float z;
};

typedef struct vector vector;

float scalar_product(vector v1, vector v2) {
    return v1.x * v2.x + v1.y + v2.y + v1.z * v2.z;
}

vector vector_product(vector v1, vector v2) {
    vector v;
    v.x = v1.y * v2.z - v1.z * v2.y;
    v.y = v1.z * v2.x - v1.x * v2.z;
    v.z = v1.x * v2.y - v1.y * v2.x;
    return v;
}


int main () {
    vector v1 = { 2, 4, 6 };
    vector v2 = { 3, 5, 9 };
    vector v = vector_product(v1, v2);
    printf("v1 * v2 = %.2f\n", scalar_product(v1, v2));
    printf("v1 x v2 = [%.2f, %.2f, %.2f]\n", v.x, v.y, v. z);
    return 0;
}

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

typedef struct complex_number {
    float real;
    float imag;
} comp;

comp add(comp a, comp b) {
    comp c = a;
    c.real += b.real;
    c.imag += b.imag;
    return c;
}

comp subtract(comp *pok1, comp *pok2) {
    comp c = *pok1;
    c.real -= (*pok2).real;
    c.imag -= (*pok2).imag;
    return c;
}

void multiply(comp a, comp b, comp *c) {
    c->real = a.real * b.real - a.imag * b.imag;
    c->imag = a.real * b.imag + a.imag * b.real;
}

void print(comp *pok) {
    printf("%.2f", pok->real);
    if (pok->imag >= 0)
        printf("+j%.2f\n", pok->imag);
    else
        printf("-j%.2f\n", fabs(pok->imag));
}

int main() {
    comp a, b, c;
    scanf("%f %f", &a.real, &a.imag);
    scanf("%f %f", &b.real, &b.imag);
    print(&a);
    print(&b);
    printf("a + b\n");
    c = add(a, b);
    print(&c);
    printf("a - b\n");
    c = subtract(&a, &b);
    print(&c);
    printf("a * b\n");
    multiply(a, b, &c);
    print(&c);
    return 0;
}

#include <stdio.h>
#include <string.h>

struct student {
    char first_name[15];
    char last_name[20];
    int number;
    int points;
};

void norm(char *s) {
    // First letter uppercase, others lowercase
    *s = toupper(*s);
    while (*(++s) != '\0')
        *s = tolower(*s);
}

void sort(struct student a[], int n) {
    int i, j;
    struct student s;
    for (i = 0; i < n; i++)
        for (j = 0; j < n - i - 1; j++)
            if (a[j].points < a[j + 1].points) {
                s = a[j];
                a[j] = a[j + 1];
                a[j + 1] = s;
            }
}

int main() {
    struct student st[50];
    int i, n;
    scanf("%d", &n);
    for (i = 0; i < n; ++i) {
        scanf("%s", &st[i].first_name);
        scanf("%s", &st[i].last_name);
        scanf("%d", &st[i].number);
        int j, zadaca;
        st[i].points = 0;
        for(j = 0; j < 4; j++) {
            scanf("%d", &zadaca);
            st[i].points += zadaca;
        }
        norm(st[i].first_name);
        norm(st[i].last_name);
    }
    sort(st, n);
    for (i = 0; i < n; i++) {
        printf("%d. %s %s\t%d\t%d\n", i + 1, st[i].first_name, st[i].last_name, st[i].number, st[i].points);
    }
    return 0;
}

#include<stdio.h>

typedef struct city {
        char name[30];
        long population;
} city;

typedef struct president {
        char name[20];
        char party[20];
} pres;

typedef struct country {
        char name[30];
        pres president;
        long population;
        city capital;
} country;

int main() {
        country d[20];
        int n, i, maxi, max;
        scanf("%d", &n);
        for (i = 0; i < n; ++i) {
                scanf("%s", &d[i].name);
                printf("president:\n");
                scanf("%s", &d[i].president.name);
                scanf("%s", &d[i].president.party);
                scanf("%d", &d[i].population);
                scanf("%s", &d[i].capital.name);
                scanf("%d", &d[i].capital.population);
        }
        maxi = 0;
        max = d[maxi].capital.population;
        for (i = 0; i < n; ++i)
                if (d[i].capital.population > max) {
                        max = d[i].capital.population;
                        maxi = i;
                }
        printf(
                        "Name of the president of the country with the largest capital is: %s\n",
                        d[maxi].president.name);
        return 0;
}