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c++list
Lukas
Lukas

Reputation: 2470

C++: How can I push to elements of an array into a list of arrays?

I get two variables a and b and want to add them directly to a list of arrays. How can I avoid to define another array, which will be pushed into the list afterwards?

I am searching for a construction similar to the line beginning with //.

Minimal example:

#include <list>
#include <boost/array.hpp>

using namespace std;

int main()
{
    cout << endl;

    /* given values */
    int a = 1;
    int b = 2;

    /* arrayList contains arrays of integers. */
    list<boost::array<int, 2> > arrayList;

    /* item to add values */
    boost::array<int, 2> item;

    item[0] = a;
    item[1] = b;
    arrayList.push_back(item);

//  arrayList.push_back({{a, b}});

    cout << arrayList.front()[0] << ", " << arrayList.front()[1] << endl;

    return 0;

}

My g++ version is the following: gcc-Version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5).

Compiling (within Eclipse) throws this warning listed as an error: Warnung: erweiterte Initialisierungsliste nur mit -std=c++0x oder -std=gnu++0x verfügbar [standardmäßig aktiviert] main.cpp /testArrayListAndFind line 24 C/C++ Problem which means: Extended initilization list only with ... or ... available.

Upvotes: 1

Views: 2830

Answers (3)

Jesse Good
Jesse Good

Reputation: 52365

You could use boost::assign::list_of if you don't have C++11 support:

arrayList.push_back(boost::assign::list_of(1)(2));

Upvotes: 2

ozox
ozox

Reputation: 472

You can resize the list to the size you want and mutate the array directly in the list, e.g.,

arrayList.resize(10);

for(list::iterator iter = arrayList.begin(); iter != arrayList.end(); ++iter) {
  (*iter)[0] = <value_a>;
  (*iter)[1] = <value_b>;
}

Note that resize() actually call the constructor of boost::array in your example, so the performance saving would be minimal.

Upvotes: 0

Attila
Attila

Reputation: 28762

The array that is pushed to the list will have to be created, the question is what do you consider acceptable work/typing on your part.

For example, you could create a helper function:

boost::array<int, 2> make_array(int a, int b)
{
  boost::array<int, 2> item;
  item[0] = a;
  item[1] = b;
  return item;
}

then you can do

arrayList.push_back(make_array(a, b));

This assumes you will only need 2 elements in the array. If this is the case, you could just as well use std::pair<int, int> instead of boost::array<>. Otherwise, you could create a few overloads of make_array()

Upvotes: 2

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