Add sieve of eratosthenes

primes.h

@@ -1,3 +1,8 @@
+#include <stdlib.h>
+
+#define	TRUE	1
+#define FALSE	0
+
 int is_prime(long num)
 {
 	int i = 5;
@@ -19,3 +24,25 @@ int is_prime(long num)
 
 	return 1;
 }
+
+char* eratosthenes(long num)
+{
+	long i, j;
+	char* primes;
+
+	primes = (char*)malloc((num+1)*sizeof(char));
+	if (primes == NULL) {
+		return NULL;
+	}
+
+	memset(primes, TRUE, num);
+	primes[1] = FALSE;
+
+	for (i=2; i<=num; i++)
+		if (primes[i])
+			/* Mark factors as composite */
+			for (j=i*i; j<num; j+=i)
+				primes[j] = FALSE;
+
+	return primes;
+}

problem-112.c

@@ -0,0 +1,61 @@
+#include <stdio.h>
+
+#define	TRUE	1
+#define FALSE	0
+
+int is_increasing(long num)
+{
+	int digit;
+	int prev = num % 10;
+	num /= 10;
+
+	while (num>0) {
+		digit = num % 10;
+		if (digit>prev)
+			return FALSE;
+		num /= 10;
+		prev = digit;
+	}
+	return TRUE;
+}
+
+
+int is_decreasing(long num)
+{
+	int digit;
+	int prev = num % 10;
+	num /= 10;
+
+	while (num>0) {
+		digit = num % 10;
+		if (digit<prev)
+			return FALSE;
+		num /= 10;
+		prev = digit;
+	}
+	return TRUE;
+}
+
+
+int is_bouncy(long num)
+{
+	if (is_increasing(num) || is_decreasing(num))
+		return FALSE;
+	return TRUE;
+}
+
+
+int main(int argc, char** argv)
+{
+	int i = 99;
+	int count = 0;
+
+	while((count*100) != (i*99)) {
+		i++;
+		if (is_bouncy(i))
+			count++;
+	}
+
+	printf("The solution is %d / %d = %.9f%%\n", count, i, (double)count/(double)i);
+	return 0;
+}

problem-206.c

@@ -0,0 +1,39 @@
+#include <stdio.h>
+
+#define	TRUE	1
+#define	FALSE	0
+
+#define	MIN_NUM	1020304050607080900
+#define MAX_NUM 1929394959697989990
+
+int digits[] = {0, 9, 8, 7, 6, 5, 4, 3, 2, 1};
+
+int check(long num)
+{
+	int rem;
+	int i = 0;
+
+	while (num>0) {
+		rem = num % 10;
+		if (rem != digits[i])
+			return FALSE;
+		num /= 100;
+		i++;
+	}
+
+	return TRUE;
+}
+
+
+int main(int argc, char **argv)
+{
+	long i = 1010101011; /* while ((i*i) < MIN_NUM) i++; */
+
+	while ((i*i) < MAX_NUM) {
+		if (check(i*i))
+			break;
+		i++;
+	}
+	printf("The unique integer is %ld and it's square is %ld\n", i, i*i);
+	return 0;
+}

problem-357.c

@@ -0,0 +1,53 @@
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "primes.h"
+
+#define	MAX_NUM	100000000
+#define	TRUE	1
+#define	FALSE	0
+
+int main(int argc, char **argv)
+{
+	int i, j, match, midpoint;
+	long sum = 0;
+	char *primes;
+
+	/*
+	primes = malloc((MAX_NUM+1)*sizeof(char));
+	for (i=1; i<MAX_NUM+1; i++) {
+		if (is_prime(i)) {
+			primes[i] = TRUE;
+		} else {
+			primes[i] = FALSE;
+		}
+	}
+	*/
+	primes = eratosthenes(MAX_NUM);
+	if (primes == NULL) {
+		printf("Error");
+		return 1;
+	}
+
+	for(i=1; i<MAX_NUM; i++) {
+		match = TRUE;
+		if (!(primes[i+1])) continue;
+
+		midpoint = (int)sqrt(i) + 1;
+		for (j=1; j<midpoint; j++) {
+			if ((!(i%j)) && (!primes[j+i/j])) {
+				match = FALSE;
+				break;
+			}
+		}
+
+		if (match) {
+			sum += i;
+		}
+	}
+
+	printf("The sum of prime-generating numbers less than %d is %ld\n", MAX_NUM, sum);
+	free(primes);
+	return 0;
+}