C++ variable appears to have different values in different functions - c++

Question

This is a simplified and more informative version of a question that has now been deleted.
BACKGROUND
I am currently trying to familiarize myself with basic C++ programming and decided to make a game of hangman. However, I noticed that an integer variable called preset_count—which is meant to count how many letters have been pre-guessed by the game— always returns a value of 0 in the main() function whereas in other functions, it has a value of 2 (or anything greater than 0).
What I am trying to accomplish here is that the program will automatically fill in the vowels of the word the player is trying to guess. Depending on how many vowels are filled in, the score the player gets when guessing a character right increases.
For example, the player is trying to guess the word "eraser," so the hangman program will print out
"e _ a _ e _ " they will gain 334 points if they guess correctly.
In a word like "circle," though (which will be printed as "_ i _ _ _ e ") the player will gain 250 points per guess because of the fact that the player has to guess 4 characters instead of 3.
THE ISSUE AND CODE
In order to accomplish this, I added code meant to count how many vowels have been filled in by the game in a special function.
The code below is a simplified version of the one in the actual hangman program that re-creates the issue I have with the program. See, the game outputs a different value of the preset_count value in each function.
Essentially, the main() function has a
cout << "preset_count in main(): " << preset_count;
//This line of code prints out 0. However, the variant of this in find_preset():
cout << "preset_count in find_preset(): " << preset_count;
//Which, on the other hand, prints out 3.
Is there any reason behind these contradictory variable reports? Is there any way to solve this?
#include <iostream>
using namespace std;
void find_preset (int, const string, string); //Prototyping for find_preset()
int main() {
int preset_count = 0; //Amount of times a character in PRESET_LETTERS appears in the word variable.
const string PRESET_LETTERS = "AEIOUaeiou"; //The letters the program is looking out for.
string word = "eraser"; //The word being analyzed.
cout << "\nmain(): main() executed, variables declared, about to execute find_preset() function.";
find_preset(preset_count, PRESET_LETTERS, word); //find_preset() function; finds how many PRESET_LETTERS characters are in word.
cout << "\nmain(): find_preset() finished executing.\n"; //Announces that find_preset() function finished executing.
cout << "\n\nDEBUG word: " << word << endl; //Report on the set word value.
cout << "DEBUG preset_count in main(): " << preset_count << endl << endl; //Report on preset_count's value in main().
//This is where my issue takes place in. This reports a value whereas in find_preset(), the value of preset_count is 2.
return 0;
}
//find_preset() function; finds how many PRESET_LETTERS characters are in word.
void find_preset(int preset_count, const string PRESET_LETTERS, string word) {
int word_index = 0; //How many characters of word that find_preset() has gone through.
cout << "\nfind_preset() executed, now counting amount of instances of PRESET_LETTERS in word.";
//While word_index is less than the size of word. While the entire word variables hasn't been scanned yet.
while (word_index < word.size()) {
//If a PRESET_LETTERS character is found in word.
if(word.find(PRESET_LETTERS)) {
preset_count++; //preset_count increased by 1.
cout << "\nfind_preset(): preset_index and preset_count increased by 1."; //Reports preset_count++; has been executed.
}
word_index++; //Word index increased by 1.
cout << "\nfind_preset(): word_index increased by 1."; //Reports that word_index++; has been executed.
}
cout << "\nfind_preset(): while (word_index < word.size()) finished executing, now printing debug menu for find_preset().\n";
//Reports that the while loop has finished executing.
cout << "\n\nDEBUG: preset_count in find_preset(): " << preset_count; //Report on preset_count's value in find_preset().
//This is also where my issue takes place in. This reports that preset_count's value is 2 whereas in main, it reports 0.
cout << "\nDEBUG: word_index value: " << word_index << endl << endl; //Report on word_index's value.
}

Answer 1

The arguments in C++ are copies of what are passed by default. Therefore, modifications of arguments in callee functions won't affect what are passed in caller. You should add & to make the arguments to references if you want to have functions modify what are passed.
Both declaration and definition should be modified.
void find_preset (int&, const string, string); //Prototyping for find_preset()
void find_preset (int& preset_count, const string PRESET_LETTERS, string word) //find_preset() function; finds how many PRESET_LETTERS characters are in word.
{