A-M baby

Task/Middle-three-digits/C/middle-three-digits.c

@@ -0,0 +1,47 @@
+#include <stdio.h>
+
+#define E_SMALL 1
+#define E_EVEN  2
+static const char *errors[] = {"too small", "even length"};
+
+int mid3(int num, char *buf)
+{
+    int i, mag = 1;
+    int tmp = num = num < 0 ? -num : num; /* Discard sign. */
+
+    /* A number's magnitude can be used to determine both
+     * its length and, in this case, the oddness thereof. */
+    while (tmp /= 10) mag *= 10;
+
+    if (mag < 100) return E_SMALL;
+    if (mag & 128) return E_EVEN;
+
+    while (num > 1000) {
+        num -= num / mag * mag; /* Chop left digit. */
+        num /= 10; /* Now the right. */
+        mag /= 100;
+    }
+
+    /* Populate a character buffer to handle zeroes. */
+    for (i = 2; i >= 0; num /= 10, --i)
+        buf[i] = num % 10 + '0';
+
+    return 0;
+}
+
+int main(void)
+{
+    int num;
+    char buf[4] = "";
+
+    while (scanf("%d", &num) != EOF) {
+        printf("%d: ", num);
+
+        if ((num = mid3(num, buf)) == 0)
+            printf("%s\n", buf);
+        else
+            puts(errors[--num]);
+    }
+
+    return 0;
+}