stdin question

This is a discussion on stdin question within the C Programming forums, part of the General Programming Boards category; Code: Hello world! ^ getchar(); <-- returns 'H' Hello world! ^ getchar(); <-- returns 'e' Hello world! ^ getchar(); <-- ...

  1. #1
    Ugly C Lover audinue's Avatar
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    Talking stdin question

    Code:
    Hello world!
    ^
    getchar(); <-- returns 'H'
    
    Hello world!
     ^
    getchar(); <-- returns 'e'
    
    Hello world!
      ^
    getchar(); <-- returns 'l'

    1. How about:

    Code:
    Hello world!
     ^
    undochar() <-- returns 'e'
    
    Hello world!
    ^
    undochar() <-- returns 'H'

    2. Or something like:

    Code:
    Hello world!
        ^
    nextchar(5) <-- returns 'o'
    
    Hello world!
     ^
    prevchar(3) <-- returns 'e'


    3. Or something like Java:

    Code:
    Hello world!
    ^
    getchar(); <-- returns 'H'
    
    Hello world!
     |
    mark();
    
    Hello world!
     ^
    getchar(); <-- returns 'e'
    
    Hello world!
      ^
    getchar(); <-- returns 'l'
    
    Hello world!
     |
    reset();
    
    Hello world!
     ^
    getchar(); <-- returns 'e'
    Last edited by audinue; 01-10-2009 at 04:51 AM.
    Just GET it OFF out my mind!!

  2. #2
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    ungetc() allows (at least) one pushbask to the input stream.
    If it does not reply to the question, maybe it's because there is no explicit question...
    fseek(), fgetpos(), fsetpos() might do the work for the rest.
    Last edited by root4; 01-10-2009 at 05:00 AM.

  3. #3
    Ugly C Lover audinue's Avatar
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    So, it uses fgetc(stdin) and ungetc(stdin). Thanks root4!
    Just GET it OFF out my mind!!

  4. #4
    and the hat of wrongness Salem's Avatar
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    On a file stream (not stdin), then mark() is ftell() and reset() is fseek()
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  5. #5
    Ugly C Lover audinue's Avatar
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    Whoops! Something doesn't work here, could you give me some guide please?

    Code:
    char look;
    
    nextChar() {
      look = getchar();
    }
    
    prevChar() {
      //ungetc(look, stdin);
      look = ungetc(look, stdin);
    }
    
    main() { //for instance we will input abc
      nextChar();
      printf("%c\n", look); //print out a
      nextChar();
      printf("%c\n", look); //print out b
      prevChar();
      printf("%c\n", look); //print out b, it should print out a by the way
    }
    Last edited by audinue; 01-10-2009 at 10:23 AM.
    Just GET it OFF out my mind!!

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by audinue
    Something doesn't work here, could you give me some guide please?
    The 1999 edition of the C standard states that: "The ungetc function returns the character pushed back after conversion, or EOF if the operation fails."
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  7. #7
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    starting from 'abc', in your code, you read 'a', then 'b', then you apply ungetc(), which pushback 'b', so next time you read the stream you'll get 'b', in fact you should push back 'b' and 'a' to actually get 'a' but you're not allowed to do that (only one char can be pushed back). So switch to plan B and use fseek() or its friends.

  8. #8
    Ugly C Lover audinue's Avatar
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    Hmm.. I think there is no way to do this except using buffer. And yet quiet complex.

    Thanks in advance, I give up.
    Just GET it OFF out my mind!!

  9. #9
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    What you are getting is correct: ungetc() on success returns the byte that was pushed back onto the stream which in this case is "b" not "a".

  10. #10
    and the Hat of Guessing tabstop's Avatar
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    It's not clear (to me, anyway) why you expected to get 'a' out of that code. ungetc doesn't undo any assignments you made or anything; it just puts the most recent character back in the input stream.

  11. #11
    Ugly C Lover audinue's Avatar
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    It's not clear (to me, anyway) why you expected to get 'a' out of that code. ungetc doesn't undo any assignments you made or anything; it just puts the most recent character back in the input stream.
    However this one done with file stream but not stdin so I can't test it on the way.

    Code:
    void scan(void) {
      char c = fgetc(file); //fetch
      switch(c) {
        case '+':
          putchar(c); //collect
          c = fgetc(file);
          switch(c) {
            case '=':
            case '+':
              putchar(c); 
              putchar('\n'); return; //emit
            default:
              //for instance we support +?+ token
              c = fgetc(file);
              if(c == '+') {
                putchar(c);
                putchar('\n'); return;
              } else {
                fseek(file, -2, SEEK_CUR); //undo 2 step
              }
          }
          putchar('\n'); return;
        case '-':
          putchar(c);
          c = fgetc(file);
          switch(c) {
            case '=':
            case '-':
              putchar(c);
              putchar('\n'); return;
            default:
              fseek(file, -1, SEEK_CUR); //undo 1 step
          }
          putchar('\n'); return;
      }
    }
    Last edited by audinue; 01-11-2009 at 01:38 AM.
    Just GET it OFF out my mind!!

  12. #12
    and the Hat of Guessing tabstop's Avatar
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    So do the exact same thing with stdin and be done with it:
    Code:
    void scan(void) {
        char c = fgetc(stdin);
        switch(c) {
            case '+':
                putchar(c);
                c = fgetc(stdin);
                switch(c) {
                    case '=':
                    case '+':
                        putchar(c);
                        break;
                    default:
                        ungetc(c, stdin);
                }
                break;
            case '-':
                putchar(c);
                c = fgetc(stdin);
                switch(c) {
                    case '=':
                    case '-':
                        putchar(c);
                        break;
                    default:
                        ungetc(c, stdin);
                }
        }
        putchar('\n');
    }
    Exactly the same code, except using stdin. Works exactly the same way, gives exactly the same results, etc.

  13. #13
    Ugly C Lover audinue's Avatar
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    Oh! Awesome! Thanks tabstop I didn't notice it.

    Code:
    void *stream; //init stream on main()
    
    #define fetch   c = fgetc(stream);
    #define collect putchar(c);
    #define emit    putchar('\n'); return;
    #define undo    ungetc(c, stream);
    
    void scan(void) {
      char fetch;
      switch(c) {
        case '+':
          collect;
          fetch;
          switch(c) {
            case '=':
            case '+':
              collect;
              fetch;
              switch(c) {
                case '+':
                  collect;
                  emit;
                default:
                  undo;
              }
              emit;
              break;
            default:
              undo;
          }
          emit;
        case '-':
          collect;
          fetch;
          switch(c) {
            case '=':
            case '-':
              collect;
              emit;
              break;
            default:
              undo;
          }
          emit;
      }
    }
    
    #undef undo
    #undef emit
    #undef collect
    #undef fetch
    EDIT: Do you have another proper name instead of undo, emit, collect and fetch?
    They looks ugly for me.
    Just GET it OFF out my mind!!

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