=====
ASCII
=====

Characters are essentially just numerical values interpreted as characters. For
instance, `'A'` has the value `65`, and `'B'` has the value `66`. This encoding
can be looked up in an ASCII table. Alternatively, one can also write a program
to output the numerical value!

Tasks
=====
- Modify the existing program as follows:

  ---- CODE(file=session01/page04/ex1.abc, type=abc,linenumbers) ---------------
  @ <stdio.hdr>
                                                                                  
  fn main()
  {
      while (true) {
          local ch: int = getchar();
          printf("ch = %d\n", ch);
      }
  }
  ------------------------------------------------------------------------------
  
  Translate the program and test it. If you now input `'a'`, `'b'`, `'c'`, and
  then `Return`, you should get:
  
  
  ---- CODE (type=txt) ---------------------------------------------------------
  MCL:session1 lehn$ ./a.out 
  abc
  ch = 97
  ch = 98
  ch = 99
  ch = 10
  ------------------------------------------------------------------------------
  
- So, the ASCII value of `'a'` is `97`, that of `'b'` is `98`, that of `'c'` is
  `99`, and that of `Return` is `10`. Determine the ASCII values of the digits
  `'0'`, `'1'`, ...,`'9'`.
  
- It is unpleasant that our program needs to be terminated with `Control-C` each
  time. Modify the program as follows:
  
  ---- CODE(file=session01/page04b/ex1.abc, type=abc,linenumbers) --------------
  @ <stdio.hdr>
  
  fn main()
  {
      while (true) {            
          local ch: int = getchar();
          if (ch == '.') {       
              break;           
          } else {
              printf("ch = %d\n", ch);
          }   
      }
  }
  ------------------------------------------------------------------------------
  
  Test it with an input that includes a `'.'`, for example, `abc.`. Now
  the program terminates when the `'.'` character is read.
  
- It would be even better if the program terminates when no further input from
  the user is expected. Change the if-condition in the program as follows:
  
  ---- CODE(type=abc) ------------------------------------------------------------
  if (ch == EOF) {
  --------------------------------------------------------------------------------
  
  Now you can terminate the program by entering `Control-D`. It doesn't seem
  better than `Control-C` at first, but wait.
  
- `EOF` stands for "End Of File". Try the following (if the file with your ABC
  program is not named ex1.abc, adjust accordingly):

  ---- SHELL (path=session01/page04b,hide) -------------------------------------
  abc ex1_eof.abc
  ------------------------------------------------------------------------------

  ---- SHELL (path=session01/page04b,fold) -------------------------------------
  ./a.out < ex1.abc
  ------------------------------------------------------------------------------

  With the `<` we tell the shell that `a.out` should get its input by reading
  file `ex1.abc`, and not by reading characters from the keyboard. So what you
  see is the ASCII code of the characters in file `ex1.abc`.
  
- The compiler replaces `EOF` with a numerical value. The value is
  platform-dependent, but typically it is `-1`. Test whether the program behaves
  the same if you use `-1` instead of EOF. However, then revert to using `EOF`
  instead of `-1`. It's more readable.
  
- Change the format string and the printf statement as follows:
  
  ---- CODE(file=session01/page04c/ex1.abc,type=abc,linenumbers) ---------------
  @ <stdio.hdr>
  
  fn main()
  {
      while (true) {            
          local ch: int = getchar();
          if (ch == EOF) {       
              break;           
          } else {
              printf("ch = %d -> '%c'\n", ch, ch);
          }   
      }
  }
  ------------------------------------------------------------------------------
  
  Now the ASCII value (with the placeholder `%d`) and the corresponding
  character (with the placeholder `%c`) are output once each.
  
- Modify the program as follows:
  
  ---- CODE(file=session01/page04d/ex1.abc,type=abc,linenumbers) ---------------
  @ <stdio.hdr>
  
  fn main()
  {
      while (true) {            
          local ch: int = getchar();
          if (ch >= '0' && ch <= '9') {
              printf("digit '%c'\n", ch);
          } else if (ch == EOF) {   
              break;           
          } else {
              printf("ch = %d -> '%c'\n", ch, ch);
          }   
      }
  }
  ------------------------------------------------------------------------------
  
  With this modification, the input digits '0' to '9' are treated specially.
  Test the program, for example, with an input like `a1cd4`. Then you should
  get the following output:
  
  ---- CODE(type=txt) ----------------------------------------------------------
  MCL:session1 lehn$ ./a.out
  a1cd4
  ch = 97 -> 'a'
  digit '1'
  ch = 99 -> 'c'
  ch = 100 -> 'd'
  digit '4'
  ch = 10 -> '
  '
  ------------------------------------------------------------------------------
  
- Now, in addition to the ASCII value of a digit, the numeric value of the digit
  is also output. By "numeric value of the digit", we mean the following:
  
  - The character `'0'` has the ASCII value `48`, but it should output `0`.
  - The character `'1'` has the ASCII value `49`, but it should output `1`.
  - ...
  - The character `'9'` has the ASCII value `57`, but it should output `9`.
  
  This can be achieved by subtracting the ASCII value of `'0'` from each digit.
  Modify the line with the printf statement as follows:
  
  ---- CODE(type=abc) ------------------------------------------------------------
  printf("digit '%c', value = %d\n", ch, ch - '0');
  --------------------------------------------------------------------------------
  
  Test the program again with an input like `a1cd4`.