I have lots of questions about c++ that are really confusing me

Question

I started learning c++ about 3 weeks ago after 2 years of java. It seems so different but im getting there. my lecturer is a lovely guy but any time i ask a question as to why something is that way or this way. he just responds "because it is".

Theres lots of comments in the code below with a few random questions, but the main problem is im getting two build errors, one says the arraytotal has not been initialized (even though i found a value for it) and the other says an external reference in main.

would anyone mind reading the code and answering a few comments within, and maybe the overall problem im having?

#include<string>
#include<fstream>
#include<ostream>

using namespace std;

//double decimals[5] ={2,4,6,8,10};

const int arraySize = 5;
// does an arraySize have to be const always? is it so it doesnt channge after the array has been created?

//double decimals[arraySize];

/*
   this array is being created in the function averageN() but why?
   cant i just create it up top and reference it in?
 */

// why do you have to write the name of the function up here before you even create it?
double averageN();

int main()
{
    averageN();
    return 0;
}

// why does the array have to be created here?
double averageN(double decimals[arraySize])
{

    double average;
    double arrayTotal;
    for (int i = 0; i<5;i++)
    {
        // fills with random numbers from 0 - 10
        decimals[i] = (0+(rand()%10));
    }

    // find the total of all the elements in the array
    for (int i = 0; i < arraySize;i++)
    {
        double currentElement = decimals[i];
        arrayTotal = (currentElement+arrayTotal);
        //arrayTotal +=decimals[i]) ;
    }
    // return the average
    average = (arrayTotal/arraySize);
    return 0.0;
}

Answer 1

My quick answers without double checking (its been awhile since I've developed in C++) are:

1. arraytotal has not been initialized

   I suspect that your compiler flags this as an error to make sure you do it. If you don't, you can't be sure what it will be initialized to. Traditionally for debug builds, C/C++ initialized memory to some debug value to help identify uninitialized variables. Set arrayTotal = 0 when it is initialized and that should go away. (Best practice)

   e.g. double arrayTotal = 0;

2. external reference in main

   I suspect this is because your prototype for averageN does not match the method defined later. A prototype needs to include the types of the parameters as well as the return type. Change the prototype from double averageN(); to double averageN(double []); and I believe that will fix that problem.

3. does an arraySize have to be const always? is it so it doesnt channge after the array has been created?

   Since you are using it to define the size of the array passed into averageN, yes. Setting the size of the array like this requires a constant value.

4. This array is being created in the function averageN() but why? cant i just create it up top and reference it in?

   It is not being created in averageN. It is a formal parameter to averageN. The caller of averageN needs to provide the appropriate variable and pass it in. Then from within the method, you access it through decimals.

5. why do you have to write the name of the function up here before you even create it?

   This is the function prototype. It is necessary if the function is referenced in code before the function is defined. This could be resolved in other ways as well, such as moving the definition of averageN before all uses of it.