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String Subscript out of Range

Zion93 (2)
Hey there. I'm fairly new to C++ and I've encountered an error that I'm not certain how to solve. The aim of the program is for the user to input a 4 digit number and for the computer to try and guess it.

The error states: "String Subscript out of Range"

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#include <iostream>
#include <random>
#include <string>

using std::cout;	using std::cin;
using std::endl;	using std::string;
using std::to_string;

std::default_random_engine generator;
std::uniform_int_distribution<int> distribution(0, 9);

string attempt(string ans, string prevGuess) {
	string a, b, c, d;
	if(prevGuess[0] != ans[0])
		string a = to_string(distribution(generator));
	else
		string a = to_string(prevGuess[0]);
		
	if(prevGuess[1] != ans[1])	
		string b = to_string(distribution(generator));
	else
		string b = to_string(prevGuess[1]);
		
	if(prevGuess[2] != ans[2])
		string c = to_string(distribution(generator));
	else
		string c = to_string(prevGuess[2]);
		
	if(prevGuess[3] != ans[3])
		string d = to_string(distribution(generator));
	else
		string d = to_string(prevGuess[3]);
		
	string atmp = a + b + c + d;
	return atmp;

}

int main() {
	cout << "Please enter a 4 digit number: ";
	int number;
	cin >> number;
	
	if(to_string(number).length() != 4) {
		cout << "Invalid Number!";
		return -1;
	}
	
	cout << "\n";
	string guess = attempt(to_string(number), to_string(1111));
	int counter = 1;
	while(guess != to_string(number)) {
		cout << "Guess " << to_string(counter) << ": " << guess;
		++counter;
		guess = attempt(to_string(number), guess);
	}

	cout << "The computer guessed " << to_string(number) << "in " << to_string(counter) << " guesses!\n";
        return 0;
}


Any help on this would be very much appreciated. Again, many apologies as I am still fairly new to C++. Thanks in advance.
keskiverto (10365)
How long is atmp?
If not 4, then how long are a, b, c, d?


Lines 17, 22, 27, and 32. You call to_string. Why?
Zion93 (2)
atmp should be 4 characters long and as for the to_string, I thought that the absence of a to_string may have caused the problem, so I added them.
keskiverto (10365)
No "should be". Make your program show the true length. That is one method of debugging.


prevGuess[x] is a char. to_string does not take a char. There must occur implicit conversion (to int?).

Now consider this:
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string attempt(string ans, string prevGuess) {
  if ( ans.size() == prevGuess.size() ) {
    for ( size_t pos = 0; pos < prevGuess.size(); ++pos ) {
      if ( prevGuess[pos] != ans[pos] ) {
        string guess { to_string(distribution(generator)) };
        if ( 1 == guess.size() ) prevGuess[pos] = guess[0];  // update mismatching position
        else cout << "size error (" << guess << ")\n";
      }
    }
  }
  return prevGuess;
}
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