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'

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.

So, it uses fgetc(stdin) and ungetc(stdin). Thanks root4!

On a file stream (not stdin), then mark() is ftell() and reset() is fseek()

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
}

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."

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.

Hmm.. I think there is no way to do this except using buffer. And yet quiet complex.

Thanks in advance, I give up.

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".

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.

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;
  }
}

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.

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.