Question

This is my second time using C++ and I’m trying to port some Java code I made but am having trouble understanding some behavior. I have a list of data and want to create another list of the same size but the values are default to zero(in Java I use Arrays.fill(list, 0); after creating it). When I try to do something similar in C++ I get variable-sized object 'list1' may not be initialized

Here’s a code to better example:

#include <iostream>
#include <boost/assign/std/vector.hpp> // for 'operator+=()'

using namespace std;
using namespace boost::assign;

int main()
{
    cout << "Hello World!" << endl;
    vector<short> data;
    data += -40, -30,-10, 20, 50;

    //int coeff [data.size()];
    cout << "data array size is " << data.size() << endl;
    short list1 [data.size()] = {0}; //does not work
    for (int i =0; i<data.size(); i++) {
        cout << "data is " << list1[i] << endl;
    }

    cout << "********** try 2 **************" << endl;
    //this works if I use a actual number to create the list but I want to declare it at runtime
    short list2 [5] = {0};
    for (int ii=0;ii<5;ii++) {
        cout << "data is " << list2[ii] << endl;
    }


    return 0;
}

Like I mentioned, I’m completely green when I comes to C++ (I have read a book and done a few tutorials) so I may be doing something completely wrong. In the event I cannot do this at runtime and only at compile time, is there something else I can use to get the same result?

Answer 1

c++ does not have dynamically sized arrays, so this is illegal:

short list1 [data.size()];

but you can use a vector:

std::vector<short> list1(data.size(),0); 

That creates a vector of the same length as data, full of zeroes.