修改文件结构。

POP/02/Optional-2.c

@@ -0,0 +1,52 @@
+#include <stdio.h>
+
+// Code for encrypting
+void encryptDecrypt(int mode) {
+    char input[100] = {0}; //Maximum length of the string is 100 characters
+    short pass; //Password should be within range 0~25
+    if (mode == 0) {
+        printf("Enter the string to be encrypted (no longer than 100 characters):\n");
+    }
+    else {
+        printf("Enter the string to be decrypted (no longer than 100 characters):\n");
+    }
+    scanf("%s",input);
+    printf("Enter your password (0-25): ");
+    scanf("%hd", &pass);
+    // Go through every character in input
+    for (int i = 0; i < 100; i++){
+        if (input[i] == '\0') {
+            // '\0' means we have reached the string's end.
+            break;
+        }
+        else {
+            input[i] -= 'A'; //Calculate the chacter's place in the alphabet, from 0~25
+            if (mode == 0) {
+                input[i] += pass; //Add the offset of pass
+            }
+            else {
+                input[i] -= pass; //Subtract the offset of pass
+                input[i] += 26; //Avoid negative number
+            }
+            input[i] %= 26; //Prevent the number from overflow
+            input[i] += 'A'; //Turn the number into ASCII again.
+            printf("%c", input[i]);
+        }
+    }
+}
+
+int main() {
+    char mode;
+    printf("Slect Mode: (1) Encypt (2) Decrypt: ");
+    mode = getchar();
+    if (mode == '1') {
+        encryptDecrypt(0);
+    }
+    else if (mode == '2') {
+        encryptDecrypt(1);
+    }
+    else {
+        printf("Unrecognized command.");
+    }
+    return 0;
+}