Convert string to int in C++

Question

I have a string xxxxxxxxxxxxxxxxxxx

I am reading the string into a structure of smaller strings, and using substr to parse it. I need to convert one of those string types to integer.

atoi is not working for me,. any ideas? it says cannot convert std::string to const char*

Thanks

#include<iostream>

#include<string>

using namespace std;

void main();

{
    string s="453"

        int y=atoi(S);
}

Solution 1

std::atoi() requires const char * to pass in.

Change it to:

int y = atoi(s.c_str());

or use std::stoi() which you can pass a string directly:

int y = stoi(s);

---

You program has several other errors. Workable code could be something like:

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

int main()
{
    string s = "453";
    int y = atoi(s.c_str());
    // int y = stoi(s); // another method
}

Solution 2

#include <sstream>

int toInt(std::string str)
{
    int num;
    std::stringstream ss(str);
    ss >> num;
    return num;
}

Solution 3

In c++ the case matters, if you declare your string as s you need to use s not S when calling it, you are also missing a semicolon to mark the end of the instruction, on top of that, the atoi takes char * as parameter not a string, so you need to pass in an array of char or a pointer to a char array :

function signature: int atoi (const char * str);

string s="453"; // missing ;

int y=atoi(s.c_str());  // need to use s not S

UPDATE:

#include<cstdlib>
#include<iostream>
#include<string>

using namespace std;

void main()    // get rid of semicolomn here 
{
    string s="453"; // missing ;
    int y=atoi(s.c_str());  // need to use s not S
    cout << "y =\n";
    cout << y;
    char e;     // this and the below line is just to hold the program and avoid the window/program to close until you press one key.
    cin  >> e;  
}

Comments

[Rogue]

atoi is not a very good function to use if you need to validate that it successfully converted, something better to use would be strtol: Documentation

[Admin]

when i write your code this erroe apear error C2447: '{' : missing function header (old-style formal list?)

[Mehdi Karamosly]

you need to include whatever header your atoi function belongs to, and you need to make sure you include header of any function you us. take a look at the updated code.

[Admin]

thanks alot for your help