Euler prob# 10 using sieve of Eratosthenes

project_euler/Problem 10/sol2.c

@@ -0,0 +1,51 @@
+#include <stdio.h>
+#include <math.h>
+#include <stdlib.h>
+
+
+int main(int argc, char* argv[])
+{
+    long n = 100;
+    long long sum = 0;
+    char *sieve = NULL;
+
+    if (argc == 2)  /* if command line argument is provided */
+        n = atol(argv[1]);  /* use that as the upper limit */
+
+    /* allocate memory for the sieve */
+    sieve = calloc(n, sizeof(*sieve));
+    if(!sieve)
+    {
+        perror("Unable to allocate memory!");
+        return -1;
+    }
+
+    /* build sieve of Eratosthenes 
+        In the array, 
+        * if i^th cell is '1', then 'i' is composite
+        * if i^th cell is '0', then 'i' is prime
+    */
+    for (long i = 2; i < sqrtl(n)+1; i++) 
+    {
+        /* if i^th element is prime, mark all its multiples
+        as composites */
+        if (!sieve[i]) 
+        { 
+            for (long j = i * i; j < n + 1; j += i) 
+            {
+                sieve[j] = 1;
+            }
+            sum += i;
+        }
+    }
+
+    for (long i = sqrtl(n)+1; i < n; i++) 
+        if (!sieve[i])
+            sum += i;
+
+    free(sieve);
+    
+    printf("%ld: %lld\n", n, sum);
+
+    return 0;
+}