Programming Forums > Application Development > C++

HackeZ · Dec 9th, 2005, 12:32 AM

Alright Ive Used google for like 5 hours now. I know to convert to char * is c_str() but how would I go about changing this type to a int? Any Help Shown appreciated.

grumpy · Dec 9th, 2005, 12:58 AM

I assume you want to convert a string like "42" into the value 42.

In vanilla C, this would be done by;

(Toggle Plain Text)

#include <stdlib.h>

int main()
{
    const char v[] = "42";
    int x;
    x = atoi(v);
}

That is deprecated in C++ (i.e. it is allowed, but discouraged and may be removed from a future C++ standard). The C++ equivalent would be;

(Toggle Plain Text)

#include <string>
#include <cstdlib>

int main()
{
    std::string v("42");
    int x;
    x = std::atoi(v.c_str());
}

If you want to do more complex things then you have a few choices, such as;

sscanf() is a C function that reads data from a C-style string

in C++, look up the ostringstream class. An example of it's usage follows;

(Toggle Plain Text)

#include <string>
#include <sstream>
#include <ostream>

int main()
{
    std::string v("42");
    std::ostringstream str(v);
    int x;
    str >> x;   // will get value 42
}

E&OE: I've written the above examples quickly, so may have introduced minor errors. You can also remove the need for "std::" prefixes by judicious use of "using namespace std;", but I prefer code that is guaranteed not to be ambiguous.

Klipt · Dec 9th, 2005, 9:48 AM

These string streams look quite interesting. Turns out you want an istringstream though, since that acts as a 'input' from a string, i.e. allows you to read from it.

This runs:

(Toggle Plain Text)

#include <string>
#include <sstream>
//#include <ostream>
#include <iostream>

int main()
{
    std::string v("42");
    std::istringstream str(v);
    int x;
    str >> x;   // will get value 42
    
    std::cout << x << std::endl;
    std::cin.sync();
    std::cin.get();
}

grumpy · Dec 9th, 2005, 6:30 PM

Ah yes, indeed. That's what happens when I rattle off an answer quickly.

HackeZ · Dec 14th, 2005, 7:52 PM

Thanks for the support guys. Appreciated.

Dec 9th, 2005, 12:32 AM	#1
HackeZ Programmer Join Date: Dec 2004 Location: Ontario, Canada. Posts: 38 Rep Power: 0	Conversion problem. string type to type int. Alright Ive Used google for like 5 hours now. I know to convert to char * is c_str() but how would I go about changing this type to a int? Any Help Shown appreciated.

Dec 9th, 2005, 12:58 AM	#2
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	I assume you want to convert a string like "42" into the value 42. In vanilla C, this would be done by; (Toggle Plain Text) #include <stdlib.h> int main() { const char v[] = "42"; int x; x = atoi(v); } #include <stdlib.h> int main() { const char v[] = "42"; int x; x = atoi(v); } That is deprecated in C++ (i.e. it is allowed, but discouraged and may be removed from a future C++ standard). The C++ equivalent would be; (Toggle Plain Text) #include <string> #include <cstdlib> int main() { std::string v("42"); int x; x = std::atoi(v.c_str()); } #include <string> #include <cstdlib> int main() { std::string v("42"); int x; x = std::atoi(v.c_str()); } If you want to do more complex things then you have a few choices, such as; sscanf() is a C function that reads data from a C-style string in C++, look up the ostringstream class. An example of it's usage follows; (Toggle Plain Text) #include <string> #include <sstream> #include <ostream> int main() { std::string v("42"); std::ostringstream str(v); int x; str >> x; // will get value 42 } #include <string> #include <sstream> #include <ostream> int main() { std::string v("42"); std::ostringstream str(v); int x; str >> x; // will get value 42 } E&OE: I've written the above examples quickly, so may have introduced minor errors. You can also remove the need for "std::" prefixes by judicious use of "using namespace std;", but I prefer code that is guaranteed not to be ambiguous.

Dec 9th, 2005, 9:48 AM	#3
Klipt Hobbyist Programmer Join Date: Dec 2005 Posts: 118 Rep Power: 0	These string streams look quite interesting. Turns out you want an istringstream though, since that acts as a 'input' from a string, i.e. allows you to read from it. This runs: (Toggle Plain Text) #include <string> #include <sstream> //#include <ostream> #include <iostream> int main() { std::string v("42"); std::istringstream str(v); int x; str >> x; // will get value 42 std::cout << x << std::endl; std::cin.sync(); std::cin.get(); } #include <string> #include <sstream> //#include <ostream> #include <iostream> int main() { std::string v("42"); std::istringstream str(v); int x; str >> x; // will get value 42 std::cout << x << std::endl; std::cin.sync(); std::cin.get(); }

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Dec 9th, 2005, 6:30 PM	#4
grumpy Programming Guru Join Date: Jun 2005 Location: Adelaide, South Australia Posts: 1,260 Rep Power: 5	Ah yes, indeed. That's what happens when I rattle off an answer quickly.

Dec 14th, 2005, 7:52 PM	#5
HackeZ Programmer Join Date: Dec 2004 Location: Ontario, Canada. Posts: 38 Rep Power: 0	Thanks for the support guys. Appreciated.

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Programming Forums > Application Development > C++

Conversion problem. string type to type int.