Functions in C++ - The 13 Sone game - c++

Question

I am trying to write a program that requires input validation through functions. The idea behind it is much like the 21 stones only it is with 13 and the computer always wins. The game starts with 13 stones and the computer will always choose 1 on the first turn creating a multiple of 4 scenario. This means if the user takes 3 computer takes 1, user takes 2 computer takes 2 and so on until no stones remain. My problem is I am having a hard time getting my head around functions and how data is called from the parameters within so any help with this would be greatly appreciated!
This is what I have sofar.
#include <iostream>
using namespace std;
//function prototypes
bool validPick(int numStones);
int computerPick(int stones_in_pile, int player2taken);
int playerPick(int stones_in_pile);
int main()
{
int stones_left = 13, P1Taken, P2Taken;
cout << "You have shosen to play the game 13 stones against me, the MIGHTY "
<< "COMPUTER!\nThe object of the game is to take 1, 2 or 3 stones from"
<< " the pile on your turn.\nThe player that removes the last stone "
<< "or stones from the pile wins the game.\nGood Luck... You will need"
<< " it! I NEVER LOOSE!!"
<< endl << endl;
computerPick(stones_left, P2Taken);
playerPick(P1Taken);
validPick(stones_left);
//game logic here -- This is far from done.
stones_left -= P1Taken;
stones_left -= P2Taken;
return 0;
}
/******************************************************************************\
* Validate the picked number 1-3 are only valid numbers to choose from. *
\******************************************************************************/
bool validPick(int numStones)
{
if((numStones < 1) || (numStones >3))
cout << "Invalid Selection. 1-3 is all you can have!";
else
return numStones;
}
/******************************************************************************\
* Computer's function calls. Should start with 1. We always want the computer *
* to win the game. *
\******************************************************************************/
int computerPick(int stones_in_pile, int player2taken)
{
if(player2taken == 0)
stones_in_pile -= 1;
else
{
if(player2taken == 1)
stones_in_pile -= 3;
else
if(player2taken == 2)
stones_in_pile -= 2;
else
stones_in_pile -=1;
}
return stones_in_pile;
}
/******************************************************************************\
* Player's Pick function call goes here. The player goes second *
\******************************************************************************/
int playerPick(int stones_in_pile)
{
cout << "Please choose the ammount of stones. 1-3 only! : ";
cin >> stones_in_pile;
return stones_in_pile;
}

Answer 1

Despite the fact that you should better read a beginners book than trying to understand C++ by asking such questions, I will try to explain what is wrong in your code by an example:
bool validPick(int numStones) {
if((numStones < 1) || (numStones >3))
cout << "Invalid Selection. 1-3 is all you can have!";
else
return numStones;
}
This function is declared to return a bool value. However, if the condition in the if-clause turns out to be true, the function does not return anything, that is a mistake. Second, numStones is an int, so when you return it as a bool it will get converted (from int to bool) which is probably not what you want. Honestly, I didnt even try to understand the logic of your program but a valid version of this function could look like this:
bool validPick(int numStones) {
if((numStones < 1) || (numStones >3)) {
cout << "Invalid Selection. 1-3 is all you can have!";
return false;
}
return true;
}

Answer 2

There are many philosophies with functions that produce values and how those values are passed back.
Functions can pass values back to the caller by either modifying the parameter or by returning a value.
The playerPick function can either modify the passed value (by passing by reference):
void playerPick(int& stones_in_pile)
{
cout << "Please choose the ammount of stones. 1-3 only! : ";
cin >> stones_in_pile;
}
Or by returning a value:
int playerPick(void)
{
// Local variable to temporarily hold a value.
int stones_in_pile = - 1;
cout << "Please choose the ammount of stones. 1-3 only! : ";
cin >> stones_in_pile;
return stones_in_pile;
}
Note that the latter version uses a local, temporary, variable and the compiler will return a copy of the value upon end of the function.
I'm using void in the parameter for emphasis here.