Week 2-caesar

caesar.c

@@ -0,0 +1,37 @@
+#include <cs50.h>
+#include <stdio.h>
+#include <string.h>
+#include <ctype.h>
+
+int main(int argc, string argv[])
+{
+    string p, c = "";
+    if (argc == 2 && atoi(argv[1]) > 0)// check the arguments
+    {
+        int key = atoi(argv[1]);// use key
+        p = get_string("plaintext:  ");// get the plaintext
+        printf("ciphertext: ");
+        for (int i = 0; i < strlen(p); i++)
+        {
+            if (p[i] >= 'a' && p[i] <= 'z') //check lowercase
+            {
+                printf("%c", (((p[i] - 'a') + key) % 26) + 'a');
+            }
+            else if (p[i] >= 'A' && p[i] <= 'Z')//check uppercase
+            {
+                printf("%c", (((p[i] - 'A') + key) % 26) + 'A');
+            }
+            else 
+            {
+                printf("%c", p[i]); //otherways
+            }
+        }
+        printf("\n");
+    }
+    else
+    {
+        printf("Usage: ./caesar key\n");//when the argc has not key
+        return 1;
+    }
+
+}