Extract from string put into char

[weaboo]

I am working on a program which input large numbers and break them up into elements of 4 to put them into a char array, which will then be converted into an int array using atoi (because for some reason I can't just use atoi with string directly :/). Long story, but I'm avoiding the use of sstream or at(). I can get the string and char array to copy each other by putting them through a loop one by one, but will not work using substr(0,4).
This isn't the actual program but just some exploring that'll explain what I'm trying to say.

#include <iostream>
#include <string>

using namespace std;

int main()
{
	string a = "123443216542";
	char b[50] = {0};

// Here they will not cooperate.
	for (int i = 0; i < a.length()/2; i++)
		b[i] = a.substr(0, 2);

// Here they go through fine and copy each other.
	for (int i = 0; i < a.length(); i++)
		b[i] = a[i];

// This is to see output.
	for (int i = 0; i < a.length(); i++)
		cout << b[i];
	
}

What do you guys think? Am I missing something here? Are there any alternatives for going from string to char? And are there any alternatives to sstream and at() for converting from string to int besides atoi?

[Bazzy]

You can pass a.c_str() to atoi.
Any reason you don't want to use stringstreams?
C library provides some other functions besides atoi:
atol, atof, atof, sscanf