converting to lowercase and eliminating doubled-up case labels

ex13.c

@@ -8,37 +8,40 @@ int main(int argc, char *argv[])
     }
 
     int i = 0;
-    for(i = 0; argv[1][i] != '\0'; i++) {
+    char letter;
+    int up_low_sep = (int)('a' - 'A');
+    int upper_floor = (int)'A';
+    int upper_ceil = (int)'Z';
+
+    for(i = 0; '\0' != (letter = argv[1][i]); i++) {
         char letter = argv[1][i];
+        int i_letter = (int)letter;
+        if (upper_floor < i_letter && i_letter < upper_ceil) {
+            letter = (char)(i_letter + up_low_sep);
+        }
 
         switch(letter) {
             case 'a':
-            case 'A':
-                printf("%d: 'A'\n", i);
+                printf("%d: 'a'\n", i);
                 break;
 
             case 'e':
-            case 'E':
-                printf("%d: 'E'\n", i);
+                printf("%d: 'e'\n", i);
                 break;
 
             case 'i':
-            case 'I':
-                printf("%d: 'I'\n", i);
+                printf("%d: 'i'\n", i);
                 break;
 
             case 'o':
-            case 'O':
-                printf("%d: 'O'\n", i);
+                printf("%d: 'o'\n", i);
                 break;
 
             case 'u':
-            case 'U':
-                printf("%d: 'U'\n", i);
+                printf("%d: 'u'\n", i);
                 break;
 
             case 'y':
-            case 'Y':
                 if (i > 2) {
                     printf("%d: 'Y'\n", i);
                 } else {