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how to compare a string to null...

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how to compare a string to null...

 
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  #1
May 10th, 2007
hello there, im new in c++...i just want to know how to compare a string to null like:

C++ Syntax (Toggle Plain Text) 
  1. string x;
  2. cin >> x;
  3.  
  4. if(x==NULL){
  5.  
  6. cout << "null";
  7.  
  8. }

im using this one, if(&x==0) ...but it does'nt work...
this is what my mentor told me to do...

help..thanks
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Re: how to compare a string to null...

 
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  #2
May 10th, 2007
string class has no overloaded "==" operator. You can do 

      string temp;
      cin >> temp;
 
      if(temp.empty())
            cout << "null";
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Re: how to compare a string to null...

 
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  #3
May 10th, 2007
uhmm, what about the;

C++ Syntax (Toggle Plain Text) 
  1.  
  2. char temp[100];

how would compare to null...

btw, thank you...
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Re: how to compare a string to null...

 
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  #4
May 10th, 2007
Given char temp[100]; I guess you must be talking of how to determine is a string is empty. How about something like if(strlen(temp) == 0) { }.
Last edited by ~s.o.s~; May 10th, 2007 at 11:08 pm.
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Re: how to compare a string to null...

 
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  #5
May 11th, 2007
Does it work in C++ like this?:

C++ Syntax (Toggle Plain Text) 
  1.  if(  *temp_ptr == 0 ) {}
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Re: how to compare a string to null...

 
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May 11th, 2007
its not working, i just press enter but it does not alert me that i placed nothing... T_T
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Re: how to compare a string to null...

 
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  #7
May 11th, 2007
>i just want to know how to compare a string to null
You don't. NULL is a null pointer and a string object isn't a pointer. Most likely you want to check for an empty string:
C++ Syntax (Toggle Plain Text) 
  1. if ( x == "" )
or
C++ Syntax (Toggle Plain Text) 
  1. if ( x.empty() )
And such.

>this is what my mentor told me to do...
Your mentor probably thought you were using C-style strings simulated with pointers. Your sample code isn't though, so that advice is useless.

>string class has no overloaded "==" operator.
Yes, it does. That's one of the selling points of the class.

>i just press enter but it does not alert me that i placed nothing...
Post your full program. We don't like playing 20 questions.
I'm here to prove you wrong.
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Re: how to compare a string to null...

 
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  #8
May 11th, 2007
> i just press enter but it does not alert me that i placed nothing.

Maybe the newline is getting the best of you...
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Re: how to compare a string to null...

 
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May 11th, 2007
In c++ you can compare a "null string" by going (x != npos) Cant you?
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Re: how to compare a string to null...

 
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  #10
May 11th, 2007
There is no 'null string' as such, only null pointers. A string instance can't be null because it has just been instantiated. And x != string::npos doesn't make sense either, one is a string object while the other is most probably an unsigned integer.
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