istringstream question

[smiler]

Hello,

I have a question :)

istringstream line("Hello ");
string s;
while(line.good())
{
line >> s;
cout << "s is " << s << endl;
}

Why does it print two times the word Hello? The >> operator should stop at the frist space but then at next iteration it's like it starts again from the beginning. Can you help me please?

Thank you very much

[Bazzy]

After you read the 1^st from 'line', is is still good so try moving the condition after getting input:

while(true)
{
    line >> s;
    if (!line.good()) break;
    cout << "s is " << s << endl;
}

[smiler]

That works but if line is "Hello message", then message is never shown because after reading the next word line is not good.

[Gumbercules]

it's still good while reading to the end, it goes bad after trying to read after the end

[smiler]

Thank tou for your answer but if I try to run the following program:

#include <stdio.h>
#include <iostream>
#include <sstream>

using namespace std;

int main(int argc, char *argv[])
{
        string s;
        istringstream cmd_line("Hello message");
        while(true) {
                cmd_line >> s;
                if(!cmd_line.good()) break;
                cout << "s is " << s << endl;
        }
        return 0;
}

output is:
s is Hello

[Duthomhas]

It is because you should be checking on the fail() condition.
Try this to see what is happening:

#include <iomanip>
#include <iostream>
#include <sstream>

using namespace std;

int main()
  {
  string s;
  istringstream ss( "one two three four" );

  cout << "message-------- good()- bad()-- eof()-- fail()-\n"
       << left;

  do
    {
    s  = "(failed)";
    ss >> s;
    cout << " "
         << setw( 16 ) << s
         << setw(  8 ) << ss.good()
         << setw(  8 ) << ss.bad()
         << setw(  8 ) << ss.eof()
         << setw(  8 ) << ss.fail()
         << endl;
    }
  while (ss);

  return 0;
  }

Try changing things here and there to see how it affects the flags. For example, you can try commenting-out line 17, or putting spaces at the end of the string on line 10.

Hope this helps.

[z05DSL3A]

One of the issuses with Bazzy's code,

while(true)
{
    line >> s;
    if (!line.good()) break;
    cout << "s is " << s << endl;
}

is that when line becomes not good, it beaks out of the loop and thus does not go on to the cout.


You cold swap line 3 and 4

while(true)
{
    if (!line.good()) break;    
    line >> s;   
    cout << "s is " << s << endl;
}

but you would still have a problem with whitspaces (if any) at the end of the stream.

You could do somthing like:

    istringstream line(" Hello \t message \n hello message ");
 
    if(line.good())
    {
        string str;
        int index =0;
        do
        {
            str.clear();
            line >> str;
            if(str.empty()) continue;

            cout << ++index << " str: " << str << endl;

        }while(line.good());
    }

[smiler]

Thank you! That helped me. I check fail() after >> operator ;)