Initial data commit

Task/Untouchable-numbers/C/untouchable-numbers.c

@@ -0,0 +1,70 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <locale.h>
+
+bool *primeSieve(int limit) {
+    int i, p;
+    limit++;
+    // True denotes composite, false denotes prime.
+    bool *c = calloc(limit, sizeof(bool)); // all false by default
+    c[0] = true;
+    c[1] = true;
+    for (i = 4; i < limit; i += 2) c[i] = true;
+    p = 3; // Start from 3.
+    while (true) {
+        int p2 = p * p;
+        if (p2 >= limit) break;
+        for (i = p2; i < limit; i += 2 * p) c[i] = true;
+        while (true) {
+            p += 2;
+            if (!c[p]) break;
+        }
+    }
+    return c;
+}
+
+int main() {
+    const int limit = 1000000;
+    int i, j, n, uc = 2, p = 10, m = 63, ul = 151000;
+    bool *c = primeSieve(limit);
+    n = m * limit + 1;
+    int *sumDivs = (int *)calloc(n, sizeof(int));
+    for (i = 1; i < n; ++i) {
+        for (j = i; j < n; j += i) sumDivs[j] += i;
+    }
+    bool *s = (bool *)calloc(n, sizeof(bool)); // all false
+    for (i = 1; i < n; ++i) {
+        int sum = sumDivs[i] - i; // proper divs sum
+        if (sum <= n) s[sum] = true;
+    }
+    free(sumDivs);
+    int *untouchable = (int *)malloc(ul * sizeof(int));
+    untouchable[0] = 2;
+    untouchable[1] = 5;
+    for (n = 6; n <= limit; n += 2) {
+        if (!s[n] && c[n-1] && c[n-3]) untouchable[uc++] = n;
+    }
+    setlocale(LC_NUMERIC, "");
+    printf("List of untouchable numbers <= 2,000:\n");
+    for (i = 0; i < uc; ++i) {
+        j = untouchable[i];
+        if (j > 2000) break;
+        printf("%'6d ", j);
+        if (!((i+1) % 10)) printf("\n");
+    }
+    printf("\n\n%'7d untouchable numbers were found  <=     2,000\n", i);
+    for (i = 0; i < uc; ++i) {
+        j = untouchable[i];
+        if (j > p) {
+            printf("%'7d untouchable numbers were found  <= %'9d\n", i, p);
+            p *= 10;
+            if (p == limit) break;
+        }
+    }
+    printf("%'7d untouchable numbers were found  <= %'d\n", uc, limit);
+    free(c);
+    free(s);
+    free(untouchable);
+    return 0;
+}