Week 6 Notes - 132.md

Week 6 Lab

CSSSKL 132 - 11/1/24

🔑 Key Points 🔑

1. Utilizing functions
2. Pass by value vs pass by reference
3. Functions that return values
4. Separating code into a header file and main.cpp file

📦 Moving code into functions

• Let's look back to the in-class code from week 4. It was about loops and we made a user-generated todo list.

• Consider what blocks of code would be good in a function.

❓🤔 How do you know if you should make a function?

Start with the first problem (taking in user todo items)

void getTodoItem(vector<string> todo_list) {
  bool theres_room_left = true;
	int max_items = 4;
	int count = 0;

	cout << "What is your todo item #" << count + 1 << "? Type stop to finish" << endl;
	string todo_item;
	getline(cin, todo_item);
	cin.clear();

	while (theres_room_left && todo_item != "stop") { 
		count++;
		todo_list.push_back(todo_item);
		cout << "What is your todo item #" << count + 1 << "? Type stop to finish" << endl;
		getline(cin, todo_item);
		cin.clear();

    theres_room_left = (count == max_items) ? false : true;
	}
}

• I told you that the function needed to be placed above the main function. But there's a better way to do that!

📝 Add a function declaration. This way you can put the function definition (block of code) after main.

Problem: Nothing went into the vector. How do you suppose we fix that?

📩 Pass by value vs pass by reference

• This code uses pass by value meaning we just passed the value of the vector, a copy, into the function. When a value was added, it was added to the copy that is only reachable from within the function.

• Solution: You might think, "What if I just return a vector to main, that way it has all the information?" Sure, you could do that. But that wastes memory and will slow things down.

• The real solution: Pass a reference of the vector to the function. Any changes made to the vector in the function will be made on the original vector. There's only one original vector in play here.

• A reference provides direct access to the memory location where the element is stored, allowing you to access and modify the original element directly..

✨ This concept is tricky for many students and it's going to get a bit more challenging! Make sure you understand this concept this week.

✔️ Tip: if you can explain pass by value and pass by reference to a friend, that's a good way you know you understand. Even better: explain it to your friend who doesn't know programming, or even a child! That means you really know.

📝 Fix the function header so that it passes in a reference to a vector.

📝 Also update the function declaration.

✔️ Now complete the rest of the functions.

🚚 Functions that return values

• On lab 1, we went over functions that were void (didn't return anything). It would be helpful if we could use functions that do return something. Maybe they calculate a value, or generate a string from user input.

📝 Make a function that returns the most recently added item to the list.

string getLastItem(vector<string>& todo_list) {
  // Returns the last element of the list
  if (todo_list.empty()) {
    return "The list is empty!";
  }
  
  return todo_list.back();
}

✨ Make sure you test it for both full and empty lists!

🗂️ Working with multiple files

• Now that we've separated the code into functions, we can organize it even further.

Start with the Header file

• A file that contains

1. Guards - so the contents of the header file doesn't get accidentally reused and cause the "multiple definition" error

• Example: There are multiple files in the project that #include that header file

✨ Include these three guards

#ifndef HEADER_H // All caps, use _, replace HEADER with the name of your header
#define HEADER_H

// function declarations

#endif

2. Your function declarations

If you wrote them above main, copy paste them over

Moving on to main.cpp

#include "header.h"
#include <iostream>
#include <vector>

using namespace std;

int main() {
  
	vector<string> todo_list;
	
	getTodoItem(todo_list);
	forLoopPrint(todo_list);
	printListBackwards(todo_list);
	string last = getLastItem(todo_list);
	
	cout << "the last item is: " << last;
	
	return 0;
}

🏔️ Bonus: Constants

• Your code could have elements that are declared once and never changed. In fact, they probably aren't ever supposed to change. An example of that would be int max_items = 6;. We can declare it as a constant to ensure it can never be changed during the life of the program.

const int MAX_ITEMS = 6;

✨ It is conventional to use this style of all caps and underscores.

• I'm leaving it in functions.cpp but if you need it to be used in other files you could put it in the header.