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samples/sieve.c

@@ -0,0 +1,67 @@
+/*
+ * Calculate all primes up to a specific number.
+ */
+
+
+
+#include <stdio.h>
+#include <conio.h>
+
+
+
+/*****************************************************************************/
+/*     	      	    	  	     Data				     */
+/*****************************************************************************/
+
+
+
+#define COUNT		8192		/* Up to what number? */
+#define SQRT_COUNT	91		/* Sqrt of COUNT */
+
+static unsigned char Sieve[COUNT];
+
+
+
+/*****************************************************************************/
+/*     	      	    	  	     Code				     */
+/*****************************************************************************/
+
+
+
+int main (void)
+{
+    /* This is an example where register variables make sense */
+    register unsigned char* S;
+    register unsigned 	    I;
+    register unsigned 	    J;
+
+    /* Execute the sieve */
+    I = 2;
+    while (I < SQRT_COUNT) {
+	if (Sieve[I] == 0) {
+	    /* Prime number - mark multiples */
+	    S = &Sieve[J = I*2];
+       	    while (J < COUNT) {
+	      	*S = 1;
+	      	S += I;
+	      	J += I;
+	    }
+	}
+	++I;
+    }
+
+    /* Print the result */
+    for (I = 2; I < COUNT; ++I) {
+	if (Sieve[I] == 0) {
+	    printf ("%4d\n", I);
+	}
+	if (kbhit() && cgetc() == 'q') {
+	    break;
+	}
+    }
+
+    return 0;
+}
+
+
+