what's wrong with this program?

[Zee Malik]

I am getting the error:

[Error] 'atoi' was not declared in this scope

#include <iostream>
	  #include <string.h>

	   using namespace std;
	   int main()
	   {
	       string str = "131.90";
        int n;
	       n = atoi(str);
	       cout << n;
	       return 0;
	   }

[Luuk]

Did it return an error?, or did it return unexpected results?

[Zee Malik]

[Error] 'atoi' was not declared in this scope

[Nepomuk]

Add

#include <stdlib.h>

to your imports. That's where atoi is defined.
Alternatively, if this is supposed to be C++, use the following imports:

#include <iostream>
#include <cstring>
#include <cstdlib>

[Zee Malik]

after added <cstdlib>

[Error] cannot convert 'std::string {aka std::basic_stri ng<char>}' to 'const char*' for argument '1' to 'int atoi(const char*)'

[Nepomuk]

And rightfully so - string is a type which has a char array but isn't one itself. Try using the string::c_str() function:

n = atoi(str.c_str());

[pundliknm]

See this below code. It's not a big deal. Again confusion?

#include <iostream>
    #include <string.h>
    #include <stdlib.h>
    
     using namespace std;
     int main()
     {
       char str[100] = "131.90";
       int n;
       n = atoi(str);
       cout << n;
       return 0;
     }

[Nepomuk]

That should work too, yes. But using the string class is a good idea, it's much easier to use in many cases than a char array or char pointer.

[pundliknm]

c_str() returns a const char*.

A common use of c_str() is precisely to convert a C++ std::string to a const char* C string.

Not only using the string class is a good idea but also char*.

Both works fine.

[Zee Malik]

thank you pundliknm
it is working.
i'm on initial stage to learn C++ language.
any other tip.

[pundliknm]

Zee Malik,
I am also not expert in C++. If I know something on it, definitely I will share with you.

[Banfa]

You should only really call a function like .c_str if you need to call a legacy C function. In this case atoi is a legacy C function but C++ has plenty of better options that you could use (actually the legacy C standard library has better options to use other than atoi). In a good C++ design you should be avoiding dropping down to C constructs like arrays if at all possible favouring vector<> or array<> instead.

This can be implemented entirely using the C++ library using a stringstream which has the advantage that you cen tell how much of the string has been converted.

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

using namespace std;

int main()
{
  string str = "131.90";
  string leftover;
  istringstream iss;
  int result;
  
  iss.str(str);
  
  iss >> result;
  iss >> leftover;
  
  cout << "Converted: " << result << " Leftover: " << leftover << endl;
}

Output: Converted: 131 Leftover: .90

By simply changing the type of result to double I can convert the whole string.

[pundliknm]

I can convert whole string in one line.

    #include <iostream>
    #include <string.h>
    
    using namespace std;
		 
     int main()
     {
       string str= "131.90";
       int n;
       n=stoi(str);
       cout<<n;
     }

stoi() converts string to integer.
Its better & easy to use compare to atoi() & c_str() & iss.str().

Don't forget to run like this : g++ -std=c++0x ............

[Banfa]

You are correct, I overlooked this because it is part of C++11 (and extensions on some compilers) and I am not altogether familiar with C++11 yet as my project is still using C++98