Dynamic Arrays

Question

I'm just starting to learn C++ so excuse me for this simple question. What I'm doing is reading in numbers from a file and then trying to add them to an array. My problem is how do you increase the size of the array? For example I thought might be able to just do:

#include <iostream>
using namespace std;

int main() {
    double *x;
    x = new double[1];
    x[0]=5;
    x = new double[1];
    x[1]=6;
    cout << x[0] << "," << x[1] << endl;
    return 0;
}

But this obviously just overwrites the value, 5, that I initially set to x[0] and so outputs 0,6. How would I make it so that it would output 5,6?

Please realize that for the example I've included I didn't want to clutter it up with the code reading from a file or code to get numbers from a user. In the actual application I won't know how big of an array I need at compile time so please don't tell me to just make an array with two elements and set them equal to 5 and 6 respectively.

Thanks for your help.

Answer 1

You don't want to work with arrays directly. Consider using a vector, instead. Then, you can call the push_back function to add things to the end, and it will automatically resize the vector for you.

#include <iostream>
#include <vector>

int
main() {
    double value;
    std::vector<double> values;

    // Read in values
    while (std::cin >> value) {
        values.push_back(value);
    }

    // Print them back out
    for (std::size_t i(0), len(values.size()); i != len; ++i) {
        std::cout << values[i];
    }
}

Answer 2

You should use a collection class to do this for you rather than managing it yourself. Have a look at the "vector" class. It's essentially a dynamic array that resizes automatically as required.

In your situation you would use "vector" with the "double" type. You may also need to read up on templates in C++.

http://www.cplusplus.com/reference/stl/vector/

Answer 3

Or, if you don't want to use STL or another dynamic thing, you can just create the array with the correct size from the beginning: x = new double[2];

Of course the problem there is how big to make it. If you don't know, then you'll need to just create it "big enough" (like a hundred, or a thousand)... which, at some point, won't be big enough and it will fail in some random looking way. So then you'll need to resize it. And once you get to that point, you'll wish you'd used the STL from the start, like the other answers are telling you to do.

#include <iostream>
using namespace std;
int main() {
    double *x = new double[2];
    x[0]=5;
    x[1]=6;
    cout << x[0] << "," << x[1] << endl;
    return 0;
}