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help with an assignement please

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Join Date: Feb 2007

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loslos

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help with an assignement please

Feb 4th, 2007

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <iostream>
#include <fstream>
#include <cctype>
using std::fstream;
 
using namespace std;
 
int count[26];
int main(int argc, char *argv[])
{
  char ch;
 
  ifstream infile("c:\\a.txt") ;
  if ( !infile )
  {
        cerr << "File could not be opened" << endl;
        exit( 1 );
  }
  int i;
  for(i = 0; i <26; i++) count[ i ] = 0;
 
  while (! infile.eof())
  {
      ch = cin.get();
      if(isalpha(ch)) 
      {
          ch = toupper(ch); 
          count[ch-'A']++;
      }
  };
  for(i = 0; i <26; i++) {
    cout << (char) ('A'+ i) << ": " << count[ i ] << '\n';
  }
 
  return 0;
}
#include <iostream>
#include <fstream>
#include <cctype>
using std::fstream;

using namespace std;

int count[26];
int main(int argc, char *argv[])
{
  char ch;
  
  ifstream infile("c:\\a.txt") ;
  if ( !infile )
  {
        cerr << "File could not be opened" << endl;
        exit( 1 );
  }
  int i;
  for(i = 0; i <26; i++) count[ i ] = 0;
 
  while (! infile.eof())
  {
      ch = cin.get();
      if(isalpha(ch)) 
      {
          ch = toupper(ch); 
          count[ch-'A']++;
      }
  };
  for(i = 0; i <26; i++) {
    cout << (char) ('A'+ i) << ": " << count[ i ] << '\n';
  }

  return 0;
}

what i wanted to do with this code was input a stream of alphabetical characters and output the amount of each alphabetical letter. for ex. if the text said aaabbccccc.....yyzzzz i want to output
a:3
b:2
c:5 etc etc

but even this compiles alrigght and file was opened all i get is a blank screen. i'm really stumped and since i'm such a newbie i really don't know what im doing wrong. ang suggestions or comments will be useful.

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Re: help with an assignement please

Feb 4th, 2007

>> ch = cin.get();
you should use infine, not cin

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
      ch = infile.get();
      ch = infile.get();

And thanks for taking the time to read the board's rules to use code tags correctly. Not many first-time posters do that.

Don't PM me with questions -- you might get a nasty PM in response. If you have a question then post it in one of the forums.

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Join Date: Jul 2005

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Lerner

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Re: help with an assignement please

Feb 4th, 2007

Off topic, but in the spirit of trying not to learn bad habits, and trying to avoid bugs that can be difficult to find when trying to determine why you don't get the results you expect on trial runs with known input as you test your code, the following

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
while (! infile.eof())
  {
      ch = cin.get();
while (! infile.eof())
  {
      ch = cin.get();

could be changed to

while(infile >> ch)

You should avoid using the return value of eof() as the controlling condition for a loop. There are two reasons I know of to want to do this.

First, using the return value of eof() to terminate the loop is likely to cause a double counting of the last char in the file. If that char is a newline, then the double count will go unnoticed in your current code. But if the file terminates with an alphabetical char, it may, or may not, be counted twice. Inmy experience, trying to figure out why the code works appropriately sometimes and not others is probably the hardest bug to find.

Second, using the return value of the input statement to control the loop allows the loop to terminate appropriately, whether it terminates becuase it read the entire file, or whether it terminates because of some other cause. The loop might not terminate appropriately if the control condition is only whether the EOF marker was found but the linput stream fails for some other reason.

Last edited by Lerner; Feb 4th, 2007 at 11:12 am.

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Re: help with an assignement please

Feb 4th, 2007

•

Originally Posted by Lerner

You should avoid using the return value of eof() as the controlling condition for a loop. There are two reasons I know of to want to do this.

First, using the return value of eof() to terminate the loop is likely to cause a double counting of the last char in the file. If that char is a newline, then the double count will go unnoticed in your current code. But if the file terminates with an alphabetical char, it may, or may not, be counted twice. Inmy experience, trying to figure out why the code works appropriately sometimes and not others is probably the hardest bug to find.

That would be feof( ) you are talking about which returns zero only *after* the end of file is reached and not *when* the end of file is reached. eof( ) is actually good and returns a boolean value when the end of file is reached.

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Ravalon

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Re: help with an assignement please

Feb 4th, 2007

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Originally Posted by ~s.o.s~

That would be feof( ) you are talking about which returns zero only *after* the end of file is reached and not *when* the end of file is reached. eof( ) is actually good and returns a boolean value when the end of file is reached.

I don't get it.

eof() returns true if the eofbit is set, but the eofbit doesn't get set until you hit end of file while reading. Unless you test for end of file after the read, the order is wrong and you'll loop one too many times. Here's an example that shows the problem.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <fstream>
#include <iostream>
#include <string>
 
 
int main()
{
  using namespace std;
 
  // Build a test file
  {
    ofstream os( "test.txt" );
 
    os << "12345";
  }
 
  // Test the test file :)
  ifstream is( "test.txt" );
  int loopCount = 0;
  char ch;
 
  while ( !is.eof() ) {
    ++loopCount;
    is.get( ch );
    cout << ch << '\n';
  }
 
  cout << "Loop count: " << loopCount << '\n';
 
  return 0;
}
#include <fstream>
#include <iostream>
#include <string>


int main()
{
  using namespace std;

  // Build a test file
  {
    ofstream os( "test.txt" );

    os << "12345";
  }

  // Test the test file :)
  ifstream is( "test.txt" );
  int loopCount = 0;
  char ch;

  while ( !is.eof() ) {
    ++loopCount;
    is.get( ch );
    cout << ch << '\n';
  }

  cout << "Loop count: " << loopCount << '\n';

  return 0;
}

eof() and feof() have the same problem because they work the same way.

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iamthwee

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Re: help with an assignement please

Feb 4th, 2007

•

Originally Posted by ~s.o.s~

That would be feof( ) you are talking about which returns zero only *after* the end of file is reached and not *when* the end of file is reached. eof( ) is actually good and returns a boolean value when the end of file is reached.

No I believe eof has similar problems when the file contains extra newlines.

Like lerner and ravalon said best to avoid it altogether, especially in c++. I'm surprised your little google groups didn't tell you that sos. Ha ha.

Last edited by iamthwee; Feb 4th, 2007 at 1:22 pm.

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Re: help with an assignement please

Feb 4th, 2007

•

Originally Posted by Ravalon

I don't get it. eof() returns true if the eofbit is set, but the eofbit doesn't get set until you hit end of file while reading. Unless you test for end of file after the read, the order is wrong and you'll loop one too many times.

My bad, I should have been more explicit. Yes, I agree, feof( ) and eof( ) are not that great when compared to checking the stream state while reading files. But for small fixes I normally use:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <fstream>
#include <iostream>
#include <string>
 
int main()
{
  using namespace std;
 
  // Build a test file
  {
    ofstream os( "test.txt" );
 
    os << "12345";
  }
 
  // Test the test file :)
  ifstream is( "test.txt" );
  int loopCount = 0;
  char ch;
 
  while (1 ) {
    is.get( ch );
    if( is.eof() ) break ;
    ++loopCount;
    cout << ch << '\n';
  }
 
  cout << "Loop count: " << loopCount << '\n';
  getchar( ) ;
  return 0;
}
#include <fstream>
#include <iostream>
#include <string>

int main()
{
  using namespace std;

  // Build a test file
  {
    ofstream os( "test.txt" );

    os << "12345";
  }

  // Test the test file :)
  ifstream is( "test.txt" );
  int loopCount = 0;
  char ch;

  while (1 ) {
    is.get( ch );
    if( is.eof() ) break ;
    ++loopCount;
    cout << ch << '\n';
  }

  cout << "Loop count: " << loopCount << '\n';
  getchar( ) ;
  return 0;
}

•

Originally Posted by Iamthwee

I'm surprised your little google groups didn't tell you that sos. Ha ha.

*sigh* Btw when are you turning 14 ?

I don't accept change; I don't deserve to live.

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Dave Sinkula Dave Sinkula is offline

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Re: help with an assignement please

Feb 4th, 2007

•

Originally Posted by ~s.o.s~

But for small fixes I normally use:
Help with Code Tags

C++ Syntax (Toggle Plain Text)
  while ( 1 ) {
    is.get( ch );
    if( is.eof() ) break ;
    ++loopCount;
    cout << ch << '\n';
  }
while ( 1 ) { is.get( ch ); if( is.eof() ) break ; ++loopCount; cout << ch << '\n'; }

Why that instead of this? Seems like more for less.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
  while ( is.get( ch ) ) {
    ++loopCount;
    cout << ch << '\n';
  }
  while ( is.get( ch ) ) {
    ++loopCount;
    cout << ch << '\n';
  }

This will break the loop on errors too.

Last edited by Dave Sinkula; Feb 4th, 2007 at 3:43 pm.

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iamthwee

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Re: help with an assignement please

Feb 4th, 2007

•

Yes, I agree, feof( ) and eof( ) are not that great when compared to checking the stream state while reading files. But for small fixes I normally use:

As always, in these circumstances go for the solution that works every time. Sure you can build functionality in eof() so that it works correctly.

Hell you can even use gets() safely if you go outta your way to implement the requisite error checks.

But that's extra work for very little benefit.

See you don't learn this at the google groups.:lol:

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Re: help with an assignement please

#10

Feb 4th, 2007

•

Originally Posted by iamthwee

As always, in these circumstances go for the solution that works every time. Sure you can build functionality in eof() so that it works correctly.

Hell you can even use gets() safely if you go outta your way to implement the requisite error checks.

But that's extra work for very little benefit.

The code snippet / my post was meant to illustrate that using eof( ) is not completely wrong or taboo if used the right way. Just ignoring a part of language just because using it in the wrong way ensues chaos is foolishness.

An use of eof( ) would be:

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
 char buf[BUFSIZE];
 do {
   in.read( buf, BUFSIZE );
   std::streamsize n = in.gcount();
   out.write( buf, n );
 } while( in.good() );
 if( in.bad() || !in.eof() ) {
   // fatal error occurred
 }
 in.close();
 char buf[BUFSIZE];
 do {
   in.read( buf, BUFSIZE );
   std::streamsize n = in.gcount();
   out.write( buf, n );
 } while( in.good() );
 if( in.bad() || !in.eof() ) {
   // fatal error occurred
 }
 in.close();