HELP!!! Error: string out of range

[Spikes6669]

So I had a whole bunch of runtime errors that I posted last night for my program sue this morning(eek!) but not it complies and I was starting to get the program to work properly and complete its task (translate sentences into Pig Latin) but now it gives me the "Debug Assertion Failed!" and is is because error string out of range, but only when I choose option 1. And when I choose option 2, it asks for the sentence, but never lets me input it and skips directly to "Press any key to continue..." Can you help???

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

//Globally declare variables.
string sentence;

//Functions.
int getMenuOption();
string translateLine (string);
string translateWord (string);
string getNextWord(string, int);
bool isVowel (char);
bool isPunctuation (char);
void splitWord (string, int, string &, string &);

int main()
{

	int option;
	ifstream infile;
	ofstream outfile;

	//Get user menu option.
	option = getMenuOption();

	//Determine if will read from a file or from user input.
	if (option == 1)
	{
		//Get sentence.
		infile.open("Program5_Input.txt");
		getline(infile, sentence);
		infile.close();

		//Translate file.
		cout << "Your file is processing...\n";
		sentence = translateLine(sentence);

		//Place translation into outfile.
		outfile.open("Program5_Output.txt");
		outfile << sentence;
		cout << "Your file has been processed.\n" << endl;
		outfile.close();
	}
	else
	{
		//Get sentence.
		cout << "Please enter your sentence:\n";
		getline(cin, sentence);
		cout << endl;

		//Translate sentence(s).
		string translateLine(string sentence);

		//Display resuts.
		cout << sentence << endl << endl;
	}

	system("pause");
	return 0;

}


//Display Menu.
int getMenuOption()
{

	int option;

	//Display options and validate.
	do
	{
		cout << "              Menu:\n";
		cout << "================================\n";
		cout << "[1] to process a file\n";
		cout << "[2] to process a single sentence\n";
		cout << "================================\n";
		cout << "Your choice: ";
		cin >> option;
	} while (!(option == 1 || option == 2));

	return option;

}


//Translate sentence.
string translateLine(string sentence)
{

	int i = 0, length = 0;
	string pigLatin, word;

	while (i <= sentence.length())
	{

		//Get next word.
		word = getNextWord(sentence, i);

		//Find out how many characters are in i and increment i.
		length = word.length();
		i = i + length + 1;

		//Translate word and add it to new sentence.
		pigLatin += translateWord(word) + ' ';

	}

	return pigLatin;

}
	
//Get the next word.
string getNextWord (string sentence, int i)
{

	char letter = 'a';
	string word;

	//Find end of word.
	while (sentence[i] != ' ')
	{
		letter = sentence[i];
		word += letter;
		i++;
	}

	return word;

}

//Translate word.
string translateWord(string word)
{

	string holdPunct, front, end;
	char letter;
	int length, j = 0, k = 1;
	bool ansVowel, ansPunct; 

	//Get word length.
	length = word.length();

	//Get first letter and check to see if it is a vowel.
	while (j == 0)
	{
		letter = word[j];
		ansVowel = isVowel(letter);
	}

	//Get check for punctuation.
	while (j <= length)
	{
		letter = word[j];
		ansPunct = isPunctuation(letter);
		if (ansPunct == true)
			holdPunct += letter;
		j++;
	}

	//If vowel, finish translating word; else find first vowel.
	if (ansVowel == true)
	{
		word += "way";
		word += holdPunct;
	}
	else
	{
		ansVowel = false;

		if (k == 'u')
		{
			letter = word[k];
			void splitWord(string word, int k, string & end, string & front);
		}
		else
		{
			while (ansVowel == false)
			{
				letter = word[k];
				ansVowel = isVowel(letter);
				k++;
			}

			void splitWord(string word, int k, string & end, string & front);
		}

		word = front + end + "ay";
		word += holdPunct;

	}

	return word;

}

//Check for vowel.
bool isVowel(char letter)
{

	letter = tolower(letter);
	if (letter == 'a' || letter == 'e' || letter == 'i' || letter == 'o' || letter == 'u')
		return true;
	else
		return false;

}

//Check for punctuation.
bool isPunctuation (char letter)
{

	if (letter == '.' || letter == '?' || letter == '!' || letter == ',')
		return true;
	else
		return false;

}

//Split word to add consonants to the end.
void splitWord(string word, int k, string & end, string & front)
{

	int length = 0, i;

	length = word.length();

	for (i = 0; i <= k; i++)
		end += word[i];
	while (i <= length)
		front = word[i];

}