Operating on functions and arrays - c++

Question

The purpose of this program is for the user to declare the size of their array, then have various functions operate on it. My problem is I realize nothing valid is declared in FunctionTwo. How can I get information from my main function to FunctionTwo and so on for my other functions as well?
int main()
{
srand(time(NULL));
int arraySize = 0;
cout << "How large would you like your array to be?" << endl;
cin >> arraySize;
int theArray [arraySize];
int selection = 0;
cout << "What would you like to do with your array? " << endl << endl;
cout << "1. Pass in an integer location and return the value. " << endl;
cout << "2. Initialize an array of all 0's. " << endl;
cout << "3. Initialize an array of random numbers between 1 and your specification. " << endl;
cout << "4. Populate an array one at a time. " << endl;
cout << "5. Select a position in the array and set that value to your specification. " << endl;
cout << "6. Print the entire array. " << endl;
cout << "7. Find the average of each value in the array. " << endl;
cout << "8. Find the largest element of the array. " << endl;
cout << "9. Find the smallest element of the array. " << endl;
cout << "12. Print all numbers in the array larger than your input. " << endl;
cout << "13. Tell if the array is empty. " << endl;
cout << "15. Return the difference between the largest and smallest value in the array. " << endl;
cin >> selection;
}
int FunctionTwo()
{
int theArray [arraySize] = {0};
return theArray;
}

Answer 1

int theArray [arraySize]; is not valid C++ if arraySize is only known at run-time.
The correct solution for your problem in C++:
std::vector<int> theArray(arraySize);

Answer 2

In C++ the size of an array shall be a constant expression known at compile time. This code
int arraySize = 0;
cout << "How large would you like your array to be?" << endl;
cin >> arraySize;
int theArray [arraySize];
is not C++ compliant because variable arraySize is not a constant expression.
So either you will set a fixed size of the array yourself without asking the user to specify the size as for example
const int arraySize = 20;
int theArray [arraySize];
or you must to allocate dynamically the array as for example
int arraySize = 0;
cout << "How large would you like your array to be?" << endl;
cin >> arraySize;
int *theArray = new int[arraySize];
In this case do not forget to delete the array before exiting the program:
delete []theArray;
You could use standard container std::vector<int> but I am sure that your assignment requires to use an array.
As for the function call of FunctionTwo then the function has to have two parameters: pointer to the first element of the array and the number of elements in the array. There is no sense to declare its return type as int, It would be better to declare it as void Here an example
void FunctionTwo( int theArray[], int arraySize )
{
for ( int i = 0; i < arraySize; i++ ) theArray[i] = 0;
}
Or you may use standard algorithm std::fill. For example
#include <algorithm>
//...
void FunctionTwo( int theArray[], int arraySize )
{
std::fill( theArray, theArray + arraySize, 0 );
}
Here is an example of so-called FunctionThree that has to initialize the array with random numbers in the range [1, n]
void FunctionThree( int theArray[], int arraySize, int n )
{
for ( int i = 0; i < arraySize; i++ ) theArray[i] = std::rand() % n + 1;
}
Or the same but using standard algorithm std:::generate
#include <algorithm>
//...
void FunctionThree( int theArray[], int arraySize, int n )
{
std::generate( theArray, theArray + arraySize, [&] { return ( std::rand() % n + 1 ); } );
}
Take into account that functions shall be declared before their usage.
As for the menu that it should be enclosed in a loop. For example
do
{
cout << "What would you like to do with your array? " << endl << endl;
cout << "1. Pass in an integer location and return the value. " << endl;
cout << "2. Initialize an array of all 0's. " << endl;
cout << "3. Initialize an array of random numbers between 1 and your specification. " << endl;
cout << "4. Populate an array one at a time. " << endl;
cout << "5. Select a position in the array and set that value to your specification. " << endl;
cout << "6. Print the entire array. " << endl;
cout << "7. Find the average of each value in the array. " << endl;
cout << "8. Find the largest element of the array. " << endl;
cout << "9. Find the smallest element of the array. " << endl;
cout << "12. Print all numbers in the array larger than your input. " << endl;
cout << "13. Tell if the array is empty. " << endl;
cout << "15. Return the difference between the largest and smallest value in the array. " << endl;
cout << "\n0. Exit from the program" << endl;
cin >> selection;
//...some other code
} while ( selection != 0 );

Answer 3

How can I get information from my main function to FunctionTwo and so on for my other functions as well?
You can provide function arguments. For example:
int FunctionTwo(std::vector<int>& the_vector)
Then in main():
switch (selection)
{
case 2: FunctionTwo(the_vector); break;
...other cases...
}

Answer 4

int FunctionTwo(int[] theArray) {}
This would allow you to use theArray inside the function. Hopefully this is what you meant. I was somewhat unclear on exactly what you wanted to do.
Now you can call the function as so:
FunctionTwo(anArray);