How to access c++ string by index for a integer number?

How do i edit this program for j to contain "1"?

Currently it shows 49 which is the ascii value i think.

#include <iostream>
using namespace std;

main()
{
  string i = "123";
  int j = i[0];
  cout << j;
}

You can do this as shown below:

int main()
{
  std::string i = "123";
  int j = i[0] - '0'; //this is the important statement
  std::cout << j;
}

Explanation

'0' is a character literal.

So when i wrote:

int j = i[0] - '0';

The fundamental reason why/how i[0] - '0' works is through promotion. In particular,

both i[0] and '0' will be promoted to int. And the final result that is used to initialize variable j on the left hand side will be the resultant of subtraction of those two promoted int values on the right hand side.

And the result is guaranteed by the Standard C++ to be the integer 1 since from C++ Standard (2.3 Character sets)

3. ...In both the source and execution basic character sets, the value of each character after 0 in the above list of decimal digits shall be one greater than the value of the previous.

So there is no need to use magic number like 48 etc.

1. Construct a new string from character.
2. Convert the substring to integer. Example:

#include <iostream>
using namespace std;

main() {
  string i = "123";

  // Method 1, use constructor
  string s1(1, i[0]);
  cout << s1 << endl;

  // Method 2, use convertor
  int j = atoi(s1.c_str());
  cout << j << endl;
}

The solution is simple , just cast j to char . Example:

#include <iostream>
using namespace std;

main()
{
  string i = "123";
  int j = i[0];
  cout << char(j);
}

You have to subtract ASCII '0' (48) from the character digit:

#include <iostream>
using namespace std;

int main()
{
  string i = "123";
  int j = i[0] - 48;  // ASCII for '0' is 48
  // or
  // int j = i[0] - '0';
  cout << j;
}