Euler Project (in C)

euler.c

/*
    Copyright 2016 strupo
    
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>

/* Euler Project - Problem 1 */

int main (void)
{
    int i; /* counter */
    int start = 0; /* start summing from here */
    int stop = 1e3; /* stop summing when you reach this */
    int mult[2]; /* this array contains the numbers whose multiples will be summed */
    long sum = 0L; /* will contain the sum; initialized to zero */
    
    mult[0] = 3;
    mult[1] = 5;
    
    for (i = start; i < stop; ++i) {
        if (!(i % mult[0] && i % mult[1])) { /* :DDD */
            sum += (long) i;
        }
    }
    
    printf("Sum of multiples of %d and %d from %d to %d = %ld\n",
            mult[0], mult[1], start, stop, sum);
    
    return 0;
}

/* vim: set et sts=4 sw=4 ts=8 cin fenc=ascii : */

euler2.c

/*
    Copyright 2016 strupo
   
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>

/* Euler Project - Problem 2 */

const long int *fibonacci(const long int max);

int main (void)
{
    printf("Sum = %ld\n", *(fibonacci(4e6)));

    return 0;
}

const long int *fibonacci(const long int max)
{
    static long int sum = 0;

    int i, i1, i2;
#ifdef DEBUG
    int c;

    c = 0;

    printf("fib[%*d]\t=\t%*d\n", 4, c++, 6, 0);
#endif /* DEBUG */

    i2 = 0;
    i1 = 1;
    i = 0;

    while (i < max) {
        i2 = i1;
        i1 = i;
        i = i1 + i2;

#ifdef DEBUG
        printf("fib[%*d]\t=\t%*d\n", 4, c, 6, i);
        ++c;
#endif /* DEBUG */
        if (!(i % 2)) {
            sum += i;
        }
    }

    return (const long int *) &sum;
}

/* vim: set et sts=4 sw=4 ts=8 cin fenc=ascii : */

euler3.c

/*
    Copyright 2016 strupo
    
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

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

/* Project Euler - Problem 3 */

int is_prime(unsigned long n);

int main (void)
{
    unsigned long num = 600851475143UL;
    unsigned long i;
    unsigned long max;

    max = 1;
    for (i = 2; i <= floor(sqrt((double) num)); ++i) {
        if (is_prime(i) && !(num % i)) {
            max = i;
        }
    }

    printf("Largest prime factor for %ld = %lu\n", num, max);

    return 0;
}

int is_prime(unsigned long n)
{
    unsigned long i;
    int p; /* p != 0 <=> n is not prime */

    p = 1;

    for (i = 2; i <= floor(sqrt(n)); ++i) {
        if (!(n % i)) {
            p = 0;
            break;
        }
    }

    return p;
}

/* vim: set et sts=4 sw=4 ts=8 cin fenc=ascii : */

euler4.c

/*
    Copyright 2016 strupo

    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/

#include <stdio.h>

#define MAX_STRING 1<<3

/* Project Euler - Problem 4 */

int is_palindrome(const int n);

char *int_to_string(const int n);

int length_of_string(const char *s);

int find_largest_palindrome(void);

int main (void)
{
    int lp; /* largest palindrome */

    lp = find_largest_palindrome();

    printf("Largest palindrome made from multiplying 3-digit numbers = %d\n", lp);

    return 0;
}

int is_palindrome(const int n)
{
    int p; /* p = 1 <=> n is a palindrome */
    int i; /* counter */
    char *s; /* string containing n */
    int l; /* length of string s */

    s = int_to_string(n);
    l = length_of_string(s) - 1;
    p = 1; /* Assuming n is a palindrome */

    for (i = 0; i <= l / 2; ++i) {
        if (s[i] != s[l - i]) {
            p = 0;
            break;
        }
    }

    return p;
}

char *int_to_string(const int n)
{
    int i; /* Number of digits in n */
    int l;
    int m;
    static char s[MAX_STRING] = {'\0'};

    m = n;
    i = 0;

    while (m > 0) {
        s[i] = m % 10 + '0';
        m /= 10;
        ++i;
    }
    l = i - 1;

    /* reversing s */
    for (i = 0; i < l / 2; ++i) {
        int tmp;
        tmp = s[i];
        s[i] = s[l - i];
        s[l - i] = tmp;
    }

    return s;
}

int length_of_string(const char *s)
{
    int i; /* counter */
    int l; /* length */

    l = i = 0;

    while (s[i++]) {
        ++l;
    }

    return l;
}

int find_largest_palindrome(void)
{
    int i; /* c   u   t   r   */
    int j; /*   o   n   e   s */
    int m; /* max */

    m = 0;
    for (i = 0; i < 1e3; ++i) {
        for (j = 0; j < 1e3; ++j) {
            if (is_palindrome(i * j)) {
                    m = i * j > m ? i * j : m;
            }
        }
    }

    return m;
}

/* vim: set et sts=4 sw=4 ts=8 cin fenc=ascii : */