Convert string to int in C++ [duplicate]

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. How can I do it? It says:

cannot convert std::string to const char*

Code

#include <iostream>
#include <string>

using namespace std;

void main();
{
    string s = "453"

    int y = atoi(S);
}

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 — Rogue
– Rogue, Commented May 9, 2014 at 15:18

herohuyongtao · Accepted Answer · 2014-05-09 16:45:48Z

6

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
}

edited May 9, 2014 at 16:45

answered May 9, 2014 at 15:05

herohuyongtao

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Comments

Peter Mortensen · Accepted Answer · 2022-10-07 20:11:40Z

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

Full code:

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

using namespace std;

void main()    // Get rid of the semicolon 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;
}

edited Oct 7, 2022 at 20:11

Peter Mortensen

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answered May 9, 2014 at 15:09

Mehdi Karamosly

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2 Comments

user3472947 Over a year ago

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

Mehdi Karamosly Over a year ago

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.

user3623304 · Accepted Answer · 2014-05-10 12:36:43Z

2

#include <sstream>

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

answered May 10, 2014 at 12:36

user3623304

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