using a pointer to dynamically allocate storage for the numbers from file C++

Question

I am wanting to allocate storage for numbers from a file instead of storing them in a list and then finding the mean of the number. I have done this for a list below but i am struggling with the concept of doing this for a pointer. Any help is appreciated.

Answer 1

The main problem you need to overcome is to determine how much memory to allocate to hold all of your doubles. There a few ways you can do this:

The "easiest" way would be to do something naive, like reading the file once fully to get the number of doubles, then reading the file again to actually store the values in the the array:

std::ifstream in("numbers.txt");
unsigned int numElements = 0;

// Read the file once to determine how many elements it contains
double discard;
while (in >> discard) {
    numElements++;
}

// Go back to the beginning of file
in.clear() ;
in.seekg(0, std::ios::beg);

// Allocate array and read file contents into it
double* data = new double[numElements];
for ( int i = 0; i < numElements && is >> data[i]; i++ );

// Calculate the sum and average
double sum = 0;
for ( int i = 0; i < numElements; i++ ) {
    sum += data[i];
}

double average = sum / numElements;

// Free the dynanmically allocated memory
delete[] data;

It's pretty inefficient to read a file twice; and there are other ways to obtain the size beforehand. For example you could have a convention in your input file to include the number of elements as the first value before the data:

[infile.txt]
5 1.0 2.0 3.0 4.0 5.0

Now you can read the first value from the file, and you'll know that the rest of this file contains 5 doubles. Allocate your array with new (similar to above example), then continue to read the rest of the file into it.

Another alternative would be to do what std::vector does, which is begin by allocating enough memory in your array for 1 element. If you need to add another element, and the array is already full, re-allocate your array with twice as much memory and copy/move your old elements over to it. Repeat as necessary.

Answer 2

Using C++ containers you need not worry about memory allocation. List elements are stored on the HEAP. This is essentially what you would be trying to achieve by using a pointer.

See List and list elements, where are stored?, When vectors are allocated, do they use memory on the heap or the stack? for more info.

If you still want to use pointers I would recommend using shared pointers http://en.cppreference.com/w/cpp/memory/shared_ptr, or another std smart pointer.