title: warning: implicit declaration
body: <p>I have an assignment I am supposed to turn in for my computer science MOOC CS50. In it I have to turn in the assignment over the Harvard website, but it won't accept my code while it says "Warning: implicit declaration..."</p>
  
  <p>Is there a way to shut that off? </p>
  
  <p>I have two functions that I am using, <code>islower()</code>, and <code>isupper()</code>, and they are what is causing the hangup. </p>
  
  <p>My code seems to work just fine, it compiles and everything. BTW if anyone wants to tell me how crappy my code is that would be appreciated. I don't get a lot (or any) criticism taking classes over the web. </p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include "cs50.h"
  #include &lt;stdio.h&gt;
  #include &lt;string.h&gt;
  
  int main(int argc, string argv[])
  {
      int salt, cipherNum;
      char cipher[40];
      char letter;
  
  
      //// Greeting
      printf("Please enter the text to ceez...\n");
      //// grab text
  
      string txxt = GetString();
  
  
      if (argc == 2) // must have command line argument
      {
  
          salt = atoi(argv[1]) % 26;
          //printf("Salt: %d\n", salt);
      }
  
      else // yell at user if command line arg is not there
      {
          printf("Not cool! I need something to caesariphy...\n");
          return 1;
      }
  
      //~ 
      // This will iterate over each letter in the text
      for (int i = 0, n = strlen(txxt); i &lt; n; i++)
      {
          // int letter = 'A'; i.e. 65 
          // Must Preserve Case
  
          //~ printf("%c---\n", txxt[i]);
  
          //if lower start from 'a'or 97
          if ( islower(txxt[i]) )
          {
              //~ printf("islower\n");
              letter = txxt[i];
              cipherNum = txxt[i];
              //~ printf("%c is the letter\n", letter + salt);
              //~ printf("%d is the cipherNumz\n", cipherNum);
  
              if ((letter + salt) &gt; 122)
              { 
                  //~ printf("letter + salt is &gt; 90: %d \n", letter+salt);
                  cipherNum = (96 + (cipherNum + salt) % 122);
                  //~ printf("%c is the letters", cipherNum); 
              }
              else
              {
                  cipherNum = letter + salt;
              }
  
  
  
              cipher[i] = cipherNum ;
  
          }
          else if ( isupper(txxt[i]))
          {
  
              letter = txxt[i];
              cipherNum = txxt[i];
              //printf("%c is the letter\n", letter + salt);
              //printf("%d is the cipherNumz\n", cipherNum);
  
              if ((letter + salt) &gt; 90)
              { 
                  //printf("letter + salt is &gt; 90: %d \n", letter+salt);
                  cipherNum = (64 + (cipherNum + salt) % 90);
                  //printf("%c is the letters", cipherNum); 
              }
              else
              {
                  cipherNum = letter + salt;
              }
  
              //printf("%c\n", cipherNum);
              cipher[i] = cipherNum ;
              //printf("testing12\n");    
          }
          else
          {
              cipher[i] = txxt[i];
          }
          //~ 
  
      }
      cipher[strlen(txxt) + 1] = '\0';
      printf("%s\n", cipher);
  
  
      return 0;
  }
  </code></pre>