Thread: converting from a string to an int

hey everyone im having this little problem. Im trying to convert from a string to an int. ive tried to use atoi but that didnt work. that only works if it were char string[1] = "1"; int x; x = atoi(string);
however in my situation, i have something delcared as a string like this:

string x = "1";
int y;

y = atoi(x) // Will Not Work

my friend told me something like s.at() would work, but i dont know how to use that, so someone please help because this program is due tomorrow. thanx

Greetings,

That is because 'atoi' is expecting a C string.
There are at least a couple of ways you could do this, first by using the 'c_str()' member of std::string and pass that to 'atoi' like 'y = atoi(x.c_str());', this should work, although i preferer to use stringstreams because the 'atoX' family isn't part of the standart, i think. Besides, stringstreams are much sexyer!
Stringstreams version:

Code:

#include <string>
#include <sstream>

using namespace std;

int to_int(const string& val)
{
    int number;
    ostringstream os(val);
    os >> number;
    return number;
}

// Example of how to use:
int i = to_int("123");
or 
string x = "123";
int i = to_int(x);

atoi() is ANSI C compatible, but not as sexy
Also, you need to use istringstream, since you are getting input from the supplied string and putting it into an int.

Just for fun, I traced to the execution alway down to a strtol(), which is also ANSI C compatible.

gg

You are absolutely right about the 'istringstream', my bad!
And I was wrong, again, about 'atoi', it is 'itoa' that is not part of the standart.

i couldnt get either to work

what im tryin to do in my program is that i have a binary tree, and when there is a number in the binary tree, i want to take that number, and convert it into an int and store it in a array and all the values are of string type.

so lets say your thing is nodePtr->value, well see i tried coding to_int and havin it go x = to_int(nodePtr->value), but tha twouldnt work, and i tried going strtol, strtol(nodePtr->value), but that wouldnt work either. please help, thanx

What type is the value member?

gg

Well, there was an error with my 'to_int' function, as Codeplug pointed out. Try this:

Code:

int to_int(const string& val)
{
    int number;
    istringstream is(val);
    is >> number;
    return number;
}

This should work now.

still doesnt work, i still get 2 errors, here they are

C:\CECS328\bintree.cpp(652) : error C2079: 'is' uses undefined class 'basic_istringstream<char,struct std::char_traits<char>,class std::allocator<char> >'

this one is reffering to is>>number


C:\CECS328\bintree.cpp(652) : error C2440: 'initializing' : cannot convert from 'const class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >' to 'int'

and this one is referring to istringstream is(val) heres what i did


else
{

x = to_int(nodePtr->value);
return newvalues[ x-1 ];

}


i dunno whats wrong with it, x is an int, and nodePtr->value is a string value. its not char string, its simply string. thanx.

Do this:

Code:

#include<stdlib.h>
...
strtol(nodePtr->value.c_str())

Or to get to_int() to work, add this:

Code:

#include <string>
#include <sstream>
using namespace std;

gg