What's wrong with case 0?

Xiangbuilder · 08-28-2003, 10:14 PM

In chapter 6 of <<The c++ programming language>>, Stroustrup said: " By default >> skips whitespaces (that is, spaces, tabs, newlines, etc.) and leave the value of ch unchanged if the input operation failed. Consequently, ch==0 indicates end of input.
So, I write the program:

Code:

#include<iostream>
#include<istream>
#include<string>
#include<map>

using namespace std;

enum Token_value {
   END, PRINT=';'
};
Token_value curr_tok=PRINT;

Token_value get_token(); //the form of the function.

int main()
{
   while (cin) {
   get_token(); //call the funcion get_token() within the funcion int main().
   if (curr_tok==END) break;
   if (curr_tok==PRINT) continue;
   }
}

Token_value get_token() //define the funcion.
{
   char ch=0;
   cin>>ch;
   switch (ch) {
   case 0:
      return curr_tok=END;
   case ';':
      return curr_tok=Token_value (ch);
   default:
      return curr_tok=PRINT; 
   }
}

But, when I run the porgram, I can't exit it, even if I type spaces, tabs, etc.
What is wrong?
Thank you.

shishir · 08-29-2003, 12:12 AM

0 as in null should represent end of input and not '0' as in character which has an ASCII value of 48... if you replace the line case 0 with case '0', this would work just fine...

Xiangbuilder · 08-29-2003, 01:29 AM

Thank you.
I guess the statement

Code:

case (constant-expression) statement

means if the program can find something accord with case 0,
then it will invoke

Code:

case 0:
      return curr_tok=END;

and implement the clause:

Code:

if (curr_tok==END) break;

Then the user can exit the program.

when I type spaces, tabs, etc, I guess they accord with case 0, but why the program don't do so, I exit the program?

Thank you.

Hko · 08-29-2003, 06:54 AM

Quote:

Originally posted by Xiangbuilder

Stroustrup said:" By default >> skips whitespaces (that is, spaces,
tabs, newlines, etc.) and leave the value of ch unchanged if the input
operation failed. Consequently, ch==0 indicates end of input.

[..snip..]

But, when I run the porgram, I can't exit it, even if I type spaces,
tabs, etc. What is wrong?

There's nothing wrong at all. The book says that when >> reads
whitespaces, it will be skipped. i.e. it will immediately read the next
character without ever returning a whitesoace character. This
does not mean that entering whitespace will end the input. So
entering whitespace will not cause cin to return 0, and will not
have your program exit.

Your program will exit when the "cin >>" reaches the end of
input. In other words, it will return 0 when it reads an
EOF (End Of File) character.

You can send a EOF character through the keyboard by hitting
CTRL-d, like you would do when creating a file with:

cat >myfile.txt

(in MS-DOS sending an EOF was done by hitting CTR-z)

Xiangbuilder · 08-29-2003, 09:13 AM

Thank you teach me many things.

I guess in the clause

Code:

   case 0:
      return curr_tok=END;

"return curr_tok=END;"
will never be invoked.

Hko · 08-29-2003, 09:34 AM

Sure it does!
When you hit CTRL-d to send an EOF character...

Try it. Insert a command that prints something:

Code:

    case 0:
        cout << "Boo!" << endl;
        return curr_tok=END;

Compile, run, type some text, hit ENTER.
Finally hit CTRL-d and "Boo!" will be printed!

P.S.
Be sure to use "case 0:" or "case '\0':"
Not "case '0': ".

Xiangbuilder · 08-29-2003, 11:05 PM

Thank you.
I tried to use Ctrl-d in some of my programs,
I feel it is magic.

Hko · 08-30-2003, 07:26 AM

You're welcome.