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sstream basics

Posted on 2004-04-01
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Last Modified: 2009-07-29
I want to convert a string of digits (may include +,-,.) to a double using sstream. plz give a good general example. so far I have found that I can convert the string to a buffer (I don't know how I can do that) and read from that buffer into a double.

thx Xperts
J
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Question by:jhshukla
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Assisted Solution

by:Sys_Prog
Sys_Prog earned 25 total points
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Have a look at this code

#include <iostream>
#include <string>
#include <sstream>

using namespace std ;

int main(int argc, char* argv[])
{
      string test ( "123.543  162.124 542 123.134" ) ;
      stringstream str ( test ) ;
      double d1, d2, d3 ;
      int i, j ;
      
      str >> d1 ;
      cout << "D1 : " << d1 << endl ;
      str >> d2 ;
      cout << "D2 : " << d2 << endl ;
      str >> i ;
      cout << "i : " << i << endl ;      

      str >> j ;
      cout << "j : " << j << endl ;      
      str >> d3 ;
      cout << "D3 : " << d3 << endl ;
       
       
      system("pause");
      return 0;
}


Amit
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Expert Comment

by:rstaveley
Comment Utility
This should give you a kick start...
--------8<--------
#include <iostream>
#include <sstream>

int main()
{
        std::string str = "-39.76 8.76e-99";
        std::istringstream istr(str);
        double d;
        while (istr >> d)
                std::cout << "Here's a double " << d << '\n';
}
--------8<--------
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Expert Comment

by:rstaveley
Comment Utility
[Sorry Amit got there first!]
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Expert Comment

by:Sys_Prog
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No probs  rstaveley, happens quite a lot at EE

Amit
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Author Comment

by:jhshukla
Comment Utility
perfect! now you guys can clear a few doubts --
is it possible to add some more text to the stringstream after starting to extract from it? if yes where will the text be placed - at index 0 or at the end?

is there a way to extract the whole buffer NOT using [[[> s_stream >> some_string_variable <]]]?

plz also look at two peices of code below and corresponding output on VC++6.0

thx
J

EXAMPLE 1
=======
#include <sstream>
#include <iostream>
#include <string>

using namespace std;

int main(){
      string s = "345.89 23.9 3e-6 4";
      stringstream str(s);
      double d; int i;
      for(i=0; i<4; i++){
            str >> d;
            cout << d << endl;
      }
      return 0;
}
OUTPUT 1
------------
345.89
23.9
3e-006
4

EXAMPLE 2
=======
#include <sstream>
#include <iostream>
#include <string>

using namespace std;

int main(){
      string s = "345.89 23.9 3e-6 4";
      stringstream str(s);
      str << " 56.7";
      double d; int i;
      for(i=0; i<5; i++){
            str >> d;
            cout << d << endl;
      }
      return 0;
}
OUTPUT 2
------------
56.79
23.9
3e-006
4
4
:::::::::::::::::
 end of output
:::::::::::::::::
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Expert Comment

by:rstaveley
Comment Utility
You can get it to work bidirectionally, provided you don't use the std::string constructor.

i.e. This works:
--------8<--------
#include <iostream>
#include <sstream>
#include <string>

int main()
{
      std::stringstream sstr;
      sstr << "the rapist Mr physio";
            // Don't use constructor for this
            // otherwise you get an istringstream

      std::string str;
      for (int i = 0;i < 2;i++) {
            sstr >> str;
            sstr << str;
      }

      while (sstr >> str)
            std::cout << ' ' << str;
      std::cout << '\n';
}
--------8<--------

This doesn't work, because it constructs an istringstream:

      std::stringstream sstr("the rapist Mr physio");
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Expert Comment

by:rstaveley
Comment Utility
NB: If there's any danger of emptying the stringstream after an extraction, you should call sstr.clear() to clear the flags before inserting.
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Expert Comment

by:rstaveley
Comment Utility
You fix the error in EXAMPLE 2, by doing the following:

--------8<--------
    for(i=0; i<5; i++){
          if (str >> d)
              cout << d << endl;
     }
--------8<--------

There are only four tokens, but you attempt to extract 5. The fifth token fails, because you are EOF, but you are not testing the result of the extraction. You therefore see the 4th token twice.
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Author Comment

by:jhshukla
Comment Utility
thanks, i got that. but one part of the question is still unanswered.
 )= is there a way to extract the entire buffer NOT using [[[> s_stream >> some_string_variable <]]]?

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Expert Comment

by:rstaveley
Comment Utility
Yes, use sstr.str(). It rturns the contents of the buffer in a std::string.

See http://www.cplusplus.com/ref/iostream/stringstream/str.html
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Author Comment

by:jhshukla
Comment Utility
how do I get errorflags or exception when I extract a number from sstream and it fails. eg

stringstream str("b456");
double d;
str >>d; // error! how do I catch it?
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Accepted Solution

by:
rstaveley earned 50 total points
Comment Utility
Sorry about the late response, I've just got back from holiday.

if (!(str >>d))
   throw YourException();
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Author Comment

by:jhshukla
Comment Utility
Amit, plz don't be pissed off. I gave more points to rstaveley 'coz of his/her continued response.
thx both of u.
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