Getting help with my code

[jollyholly11]

I have a homework assignment on a compiler called zybooks. It has an automatic grader. I get the right output however for two of my functions is outputs the error.
0 / 2
Test that ReplaceExclamation() works correctly with "We'll continue our quest in space. There will be more shuttle flights and more shuttle crews and, yes, more volunteers, more civilians, more teachers in space. Nothing ends here; our hopes and our journeys continue!"
Your output
Edited text: We'll continue our quest in space. There will be more shuttle flights and more shuttle crews and, yes, more volunteers, more civilians, more teachers in space. Nothing ends here; our hopes and our journeys continue.
Test feedback
ReplaceExclamation() incorrectly edits the string.
Result: We'll continue our quest in space. There will be more shuttle flights and more shuttle crews and, yes, more volunteers, more civilians, more teachers in space. Nothing ends here; our hopes and our journeys continue!

What am I doing wrong?

  #include <iostream>
#include <cmath>
#include <string>
#include <cstdlib>
#include <iomanip>
#include <ctime>
#include <math.h>
#include <stdio.h>

using namespace std;


void ReplaceExclamation(string userInput) {
	string newText = userInput;
	int i = 0;
	int len = userInput.size();
	for (i = 0; i < len; i++) {
		if (userInput[i] == '!') {
			newText[i] = '.';
		}
		userInput = newText;

	}
	cout << "Edited text: " << userInput << endl;
}
int FindText(string userInput, string phrase) {
	int count = 0;
	size_t offset = userInput.find(phrase);
	if (phrase.size() == 0) {
		return 0;
	}

	while (userInput.find(phrase, offset) != string::npos)
	{

		offset = userInput.find(phrase, offset + phrase.size());
		count++;
	}


	return count;
}
void ShortenSpace(string userInput) {

	int i, j,c  = 0, n;
	n = userInput.size();

	while (i < n)
	{
		if (i == ' ' && (i + 1) == ' ' || (i - 1) == ' ')
		{
		for (j = i; j < n; j++) {
			j = j + 1;
			n--;
			}
		}
		else {
			i++;
		}
	
	}
	

	cout << "Edited text: " << userInput << endl;

}

int GetNumOfWords(const string userInput)
{
	int words = 0;
	int len = userInput.size();
	int i = 0;
	for (i = 0; i<len;)
	{
		if (isspace(userInput[i]))
		{
			while (isspace(userInput[i]))
				i++;
			words++;

		}
		else
		{
			i++;
		}
	}
	words = words + 1;
	cout << "Number of words: " << words << endl;
	return words;
}

int GetNumOfNonWSCharacters(const string userInput) {

	int i = 0;
	int numOfCharacters = 0;
	int length = userInput.size();
	for (i = 0; i < length; ++i) {
		if (!isspace(userInput[i])) {
			numOfCharacters++;
		}
	}

	cout << "Number of non-whitespace characters: " << numOfCharacters << endl << endl;
	return numOfCharacters;
}


void printMenu(string userInput) {
	cout << "MENU" << endl;
	cout << "c - Number of non-whitespace characters" << endl;
	cout << "w - Number of words" << endl;
	cout << "f - Find text" << endl;
	cout << "r - Replace all !'s" << endl;
	cout << "s - Shorten spaces" << endl;
	cout << "q - Quit" << endl << endl;
	cout << "Choose an option: " << endl;
}

int main() {

	string phrase;
	string userInput;
	string numOfCharacters;
	string userLetter;

	cout << "Enter a sample text: " << endl;
	getline(cin, userInput);
	cout << "You entered: " << endl;
	cout << userInput << endl;

	do {
		printMenu(userLetter);
		cin >> userLetter;
		if (userLetter == "w") {
			GetNumOfWords(userInput);
		}
		else if (userLetter == "f") {
		
			cout << "Enter a word or phrase to be found: " << endl;
			cin.ignore();
			getline(cin, phrase);
			cout << "\" " << phrase << "\" " <<  "instances: " << FindText(userInput, phrase) << endl;
		}
		else if (userLetter == "r") {
			ReplaceExclamation(userInput);

		}
		else if (userLetter == "s") {
			
			ShortenSpace(userInput);

		}
		else if (userLetter == "c") {

			GetNumOfNonWSCharacters(userInput);
		}
		else if (userLetter == "q") {

		}

		else {
			cout << "Please select a valid character." << endl;
		}

	} while (userLetter != "q");

	return 0;
}

[JLBorges]

In void ReplaceExclamation(string userInput) {, the string userInput is passed by value.
The function gets a copy of the string, which it modifies; but the changes to the copy do not affect the original.

Either pass by reference: void ReplaceExclamation( string& userInput) {

Or return the modified string from the function, and assign the result to the original string in main()

[jollyholly11]

Thank you! That really helped me!