Convert string element to integer (C++11)

Question

I am trying to convert string element to integer using stoi function in C++11 and using it as parameter to pow function, like this:

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

using namespace std;

int main()
{
    string s = "1 2 3 4 5";

    //Print the number's square
    for(int i = 0; i < s.length(); i += 2)
    {
        cout << pow(stoi(s[i])) << endl;
    }
}

But, i got an error like this:

error: no matching function for call to 
'stoi(__gnu_cxx::__alloc_traits<std::allocator<char> >::value_type&)'
cout << pow(stoi(s[i])) << endl;

Anyone know what is the problem with my code?

Answer 1

The problem is that stoi() will not work with char. Alternatively you can use std::istringstream to do this. Also std::pow() takes two arguments the first being the base and the second being the exponent. Your comment says the number's square so...

#include <sstream>

string s = "1 2 3 4 5 9 10 121";

//Print the number's square
istringstream iss(s);
string num;
while (iss >> num) // tokenized by spaces in s
{
    cout << pow(stoi(num), 2) << endl;
}

Edited to account for numbers larger than single digit in the original string s, since the for loop approach breaks for numbers larger than 9.

Answer 2

stoi() works fine if you use std::string. So,

string a = "12345";
int b = 1;
cout << stoi(a) + b << "\n";

Would output:

12346

Since, here you are passing a char you can use the following line of code in place of the one you are using in the for loop:

std::cout << std::pow(s[i]-'0', 2) << "\n";