Solved

# sstream basics

Posted on 2004-04-01
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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
0
Question by:jhshukla
• 7
• 4
• 2

LVL 10

Assisted Solution

Sys_Prog earned 25 total points
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
0

LVL 17

Expert Comment

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<--------
0

LVL 17

Expert Comment

[Sorry Amit got there first!]
0

LVL 10

Expert Comment

No probs  rstaveley, happens quite a lot at EE

Amit
0

LVL 9

Author Comment

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
:::::::::::::::::
0

LVL 17

Expert Comment

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");
0

LVL 17

Expert Comment

NB: If there's any danger of emptying the stringstream after an extraction, you should call sstr.clear() to clear the flags before inserting.
0

LVL 17

Expert Comment

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.
0

LVL 9

Author Comment

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 <]]]?

0

LVL 17

Expert Comment

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
0

LVL 9

Author Comment

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?
0

LVL 17

Accepted Solution

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

if (!(str >>d))
throw YourException();
0

LVL 9

Author Comment

Amit, plz don't be pissed off. I gave more points to rstaveley 'coz of his/her continued response.
thx both of u.
0

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