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Join Date: Jul 2004

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lara_

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Newbie Poster

infinite loop...

Sep 27th, 2004

i dunno whether this question has been ask before but i've try to search but didn't found. so i ask it here...

very simple... if i declare

int Z;
cin >> Z;

if I enter integer, it works fine but i enter character it's looping.

how to prevent it from looping when character was entered?

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Join Date: Feb 2002

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cscgal

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The Queen of DaniWeb

Re: infinite loop...

Sep 27th, 2004

It looks as if you are automatically assuming that Z is an integer, and then doing some math calculations with it. When it's not an integer, but it's treated as one, you could end up in some sorta loop further down in your program. It looks as if your best option here is to determine whether Z is an integer, and if it isn't, prompt the user again to re-enter a number. You might also try doing

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
z=(int)(z);
z=(int)(z);

to force z to be treated as an integer.

Regardless ... there might be a built-in function (I'm not exactly sure) that determines whether a value isInt() but if not, you will need to write one yourself. You would look at the ASCII equivalent of the value and see if it is an integer range or a character range. I'm not exactly positive how this would be done or if there is a simpler way (basically because I don't have an ASCII chart handy right now.) Perhaps someone else can offer more assistance than I.

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Join Date: Apr 2004

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: infinite loop...

Sep 28th, 2004

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Originally Posted by lara_

i dunno whether this question has been ask before but i've try to search but didn't found. so i ask it here...

very simple... if i declare

int Z;
cin >> Z;

if I enter integer, it works fine but i enter character it's looping.

If you enter characters that are not part of an int, then the input will fail. But the offending character(s) will be left in the input stream.

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Originally Posted by lara_

how to prevent it from looping when character was entered?

Read user input as a string. Then attempt to convert the test to an integer. If it fails, try again.

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand

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Join Date: Jun 2004

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Chainsaw

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Unprevaricator

Re: infinite loop...

Sep 28th, 2004

As a side note, if you think your program has an infinite loop, use the compiler's optimizer switches. That way the infinite loop will run 2-4 times faster!

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Narue

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Code Goddess

Re: infinite loop...

Sep 29th, 2004

>It looks as if you are automatically assuming that Z is an integer
That's a reasonable assumption seeing as how Z was declared as int.

>When it's not an integer, but it's treated as one, you could end up in some sorta loop further down in your program.
The problem is with cin. The >> operator of cin will figure out what type the object is and convert the data from the standard input stream to that type. If there's no conversion then cin will leave the unconverted data in the stream and enter a failure state. This is a common problem with loops like this:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
int number;
 
while ( cin>> number )
  cout<< number <<endl;
int number;

while ( cin>> number )
  cout<< number <<endl;

If a letter is entered, this will be an infinite loop because cin will continue to fail on the invalid data in the stream. The solution is to remove the offending input and clear the stream or read all data as a string and parse it for error handling. The latter is easy and the former can be done (rather naively) like this:

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#include <iostream>
 
using namespace std;
 
bool get_number ( int& number )
{
  while ( !( cin>> number ) ) {
    if ( cin.eof() )
      return false;
    else {
      char ch;
 
      cin.clear();
 
      cout<<"Invalid input, please try again: ";
      while ( cin.get ( ch ) && ch != '\n' )
        ;
    }
  }
 
  return true;
}
 
int main()
{
  int number;
 
  while ( get_number ( number ) )
    cout<< number <<endl;
}
#include <iostream>

using namespace std;

bool get_number ( int& number )
{
  while ( !( cin>> number ) ) {
    if ( cin.eof() )
      return false;
    else {
      char ch;

      cin.clear();

      cout<<"Invalid input, please try again: ";
      while ( cin.get ( ch ) && ch != '\n' )
        ;
    }
  }

  return true;
}

int main()
{
  int number;

  while ( get_number ( number ) )
    cout<< number <<endl;
}

>to force z to be treated as an integer.
It's too late at that point. The issue isn't with Z being treated as an integer, but with cin's >> operator returning a failure status.

>there might be a built-in function (I'm not exactly sure) that determines whether a value isInt()
Provided the value is a string, you can try to convert it to an integer with atoi (yuck) or strtol (better). If the conversion succeeds then it's an integer, otherwise not. But this is a moot point if you're trying to read and convert input with the same function call, such as scanf or cin's >> operator.

>You would look at the ASCII equivalent of the value and see if it is an integer range or a character range.
Or just use

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
if ( Z >= '0' && Z <= '9' )
if ( Z >= '0' && Z <= '9' )

Since the standard requires digits to have adjacent values regardless of the character set. Or better yet, include <cctype> and say

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
if ( std::isdigit ( Z ) )
if ( std::isdigit ( Z ) )

But that only handles one character. For numbers longer than a single digit you need a loop. Once again, the point is moot because cin will try to convert the input to an integer before you can get your hands on it. By then the damage has presumably already been done.

>That way the infinite loop will run 2-4 times faster!
Kind of like trying to optimize your idle system process, no?