Returning values from functions

This is a discussion on Returning values from functions within the C++ Programming forums, part of the General Programming Boards category; I'm having a lot of trouble figuring out how to pass values back and forth between functions. In the program ...

  1. #1
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    Returning values from functions

    I'm having a lot of trouble figuring out how to pass values back and forth between functions.

    In the program I'm working on now, I'm supposed to declare an integer array (n[15000]) and an integer (count) in main, then pass those to a function which opens a txt file and then starts a loop. The loop takes the values in the txt file and stores them in n[], while count keeps track of how many times the loop runs. At the end the function should pass n[] and count back to main, then main prints the results. That's where I'm having trouble.

    Here's a sample of my code:
    Code:
    #include <iostream>
    #include <fstream>
    using namespace std;
    
    int ReadFile(int n[], int count);
    
    int main()
    {
    	int n[15000];
    	int count = 0;
    
    	ReadFile(n, count);
    
    	cout << "There are " << count << " numbers in the file.\n";
    	cout << "The first number is " << n[0] << endl;
    
    	return 0;
    }
    
    int ReadFile(int n[], int count)
    {
    	ifstream input;
    	input.open("Numbers.txt");
    	if(!input)
    	{
    		cout << "Cannot open file" << endl;
    		return 0;
    	}
    	while (!input.eof())
    	{
    		input >> n[count];
    		++count;
    	}
    	input.close();
    
    	return n, count;
    }
    For some reason n[] makes it through and displays the correct number, but count always shows up as 0. Can anyone explain what I'm doing wrong?

  2. #2
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    You need to pass count as a reference,
    Code:
    int ReadFile(int n[], int &count);
    Since all C arrays are passed as "the address of", it will work fine for the array, but non-array parameters are passed by value, which means your function gets a copy of the variable, rather than the real value. And just like writing on a copy of a document, it doesn't change the original.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  3. #3
    and the hat of wrongness Salem's Avatar
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    > while (!input.eof())
    Also read the FAQ as to why this doesn't work as you expect.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  4. #4
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    Okay cool, everything seems to be working now.

    Thanks for the fast responses!

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