How to fill a container?

Posted by chunyang on May 10, 2020
TAGS: #cpp

Most of the time we initialize an empty instance of STL containers and keep adding elements to or removing elements from it. For example, we create a vector and push_back elements. But what if we want to fill a couple of initial elements?

Sequence container

  • vector
  • deque
  • string

Use initializer list

Almost all the container types support this kind of initialization.

vector<int> a{1,2,3};
deque<int> b{4,5,6};

Fill the same number

vector<int> a(10);
fill(begin(a), end(a), 0);

deque<int> b(10);
fill(begin(b), end(b), 0);

Fill custom numbers

vector<int> a(10);
int i = 0;
auto x = [&]() {return i++;};
generate(begin(a), end(a), x);

deque<int> b(10);
generate(begin(b), end(b), x);

Fill without predefined size

vector<int> a;
fill_n(back_inserter(a), 10, 1);

deque<int> b;
fill_n(back_inserter(b), 10, 1);

vector<int> c;
int i = 0;
auto x = [&]() {return i++;};
int n = 10;
generate_n(back_inserter(c), n, x);

deque<int> d;
generate_n(back_inserter(d), n, x);

Associative container

  • set, unordered_set
  • map, unordered_map

Initializer list

set<int> a{1,2,3};
map<int, int> b{ {1,2}, {2,3} };

Fill different number

set<int> a;
map<int, int> b;

int i = 0;
int n = 10;
generate_n(inserter(a, a.end()), n, [&]() {return i++;})
generate_n(inserter(b, b.end()), n, [&](){return make_pair(i++, 0);});

The iterators dereferenced from set or map is const which means we cannot directly modify it.

set<int> a{3};
*a.begin() = 3;

Due to previous reasons, we cannot use fill or fill_n to fill set or map.

Other possible methods

  • swap: swap two containers
  • assign: clean the elements and replace them
    • vector, deque, string
  • constructors
vector<int> a{1,2,3};
vector<int> b;

vector<int> c;
c.assign(b.begin(), b.end());

vector<int> d(c);
vector<int> e(c.begin(), c.end());