How to fill a vector with non-trivial initial values?

Question

Hi all,

I know how to fill an std::vector with non-trivial initial values, e.g. sequence numbers:

void IndexArray( unsigned int length, std::vector<unsigned int>& v )
{
    v.resize(length);
    for ( unsigned int i = 0; i < length; ++i )
    {
        v[i] = i;
    }
}

But this is a for-loop. Is there an elegant way to do this with less lines of code using stl functionality (and not using Boost)?

Answer 1

You can use the generate algorithm, for a more general way of filling up containers:

#include <iostream>
#include <algorithm>
#include <vector>

struct c_unique {
   int current;
   c_unique() {current=0;}
   int operator()() {return ++current;}
} UniqueNumber;


int main () {
  vector<int> myvector (8);
  generate (myvector.begin(), myvector.end(), UniqueNumber);

  cout << "\nmyvector contains:";
  for (vector<int>::iterator it=myvector.begin(); it!=myvector.end(); ++it)
    cout << " " << *it;

  cout << endl;

  return 0;
}

This was shamelessly lifted and edited from cplusplusreference.

Answer 2

If you're using SGI STL (or a derivative, such as STLPort), you can use iota. :-)

void IndexArray(unsigned int length, vector<unsigned int>& v)
{
    vector<unsigned int>(length).swap(v);
    iota(v.begin(), v.end(), 0);
}

Answer 3

I usually go with std::generate plus a simple generator:

template <typename T>
struct gen {
    T x;
    gen(T seed) : x(seed) { }

    T operator ()() { return x++; }
};

generate(a.begin(), a.end(), gen<int>(0));

Answer 4

There is also a iota() function in adobe.ASL, (and a value_iterator as well). In boost, there is a counting_iterator, and I suspect a few other ways to do generate number sequences on the fly in boost.

Answer 5

If you have a C style array you can use std:copy, e.g.,

int c_array[] = {3,4,5};

const int* pbegin = &c_array[0];
const size_t c_array_size = sizeof(c_array) / sizeof(c_array[0]);
const int* pend  = pbegin + c_array_size;

std::vector<int> v;
v.reserve(c_array_size);
std::copy(pbegin, pend, std:back_inserter(v));