title: Reading a file and then rewriting it. Cryptography program in C
body: <p>this is a program that is giving me many headaches, but I am tackling it!</p>
  
  <p>I want to create a program that basically reads a file called message.txt which would have some texts with a message, let's say: ''Hello I am a program'' and then encrypts that message and puts it in a file called encryptMessage.txt, additionally it will save the key used for the user in the file key.txt. Now this is what I have done so far.</p>
  
  <p>That part is already finished. There's only one last thing that I need to do.</p>
  
  <p>The file message.txt has more than one line. For example:</p>
  
  <pre><code>hello I like programming
  this is a test
  to see if this program
  can read and encrypt many lines
  </code></pre>
  
  <p>I want the program to read all those lines and then encrypt them and save them in encryptMessage.txt, such as this (let's suppose the key is 3):</p>
  
  <pre><code>khoor L olnh surjudpplqj
  wklv lv d whvw
  wr vhh wklv surjudp
  fdq uhdg dqg hgfu|sw pdq| olqhv
  </code></pre>
  
  <p>However, I do not know how to make it work. I know I need to use a loop and !feof function or something like that. But I sincerely do not know how to implement, do you have any idea? Thank you very much!</p>
  
  <pre><code>#include &lt;stdio.h&gt;
  #include &lt;ctype.h&gt;
  #define MAXSIZE 100
  
  int main(void)
  {
      FILE *message;
      FILE *encryptMessage;
      FILE *key;
  
      message = fopen("message.txt", "r");
      encryptMessage = fopen("encryptMessage.txt", "w");
      key = fopen("key.txt", "w");
  
      if  ((message == NULL) || (encryptMessage == NULL) || (key == NULL))
      {
              printf("Error reading file!!!\n");
              return 1;
      }
  
      int userKey;
      char sentence[MAXSIZE];
      char q[MAXSIZE];
      int i = 0;
  
      printf("Input the text that you want to encrypt:\n&gt; ");
      fgets(sentence, 99, message);
  
  
     // printf("\nThe string that you wrote is:\n%s\n\n", sentence);
  
      printf("Input the key:\n");
      scanf("%d", &amp;userKey);
      fprintf(key, "%d", userKey);
  
      //printf("\nThe key that you selected is: %d\n\n", userKey);
  
  
      for(i = 0; sentence[i] != '\0'; ++i)
      {
          if( ( isupper(sentence[i]) ) || ( islower(sentence[i]) ) )
          {
              q[i] = sentence[i] + (char)userKey;
          }
          else
          {
              q[i] = (sentence[i]);
          }
      }
  
      q[i] = '\0';
      printf("%s", q);
      fprintf(encryptMessage, "%s", q);
  
      fclose(encryptMessage);
      fclose(key);
      fclose(message);
  
      return 0;
  }
  </code></pre>