CODEWITHSUNDEEP
c++
Alan Valejo
Alan Valejo

Reputation: 1315

Create fast a vector from in sequential values

How can I to create fast a vector from in sequential values

Eg.:

vector<int> vec (4, 100);
for (vector<int>::iterator it = vec.begin(); it != vec.end(); ++it) {
    cout << *it << endl;
}

Out:

# 100
# 100
# 100
# 100

I want

vector<int> vec (100, "0 to N");

I want to know the most efficient way to achieve this result. For example, without using the loop.

N it a runtime variable.

Upvotes: 5

Views: 6796

Answers (6)

Dawid
Dawid

Reputation: 643

I know this is old question, but I am currently playing with library to handle exactly this problem. It requires c++14.

#include "htl.hpp"

htl::Token _;

std::vector<int> vec = _[0, _, 100];
// or
for (auto const e: _[0, _, 100]) { ... }

Upvotes: 0

Dietmar K&#252;hl
Dietmar K&#252;hl

Reputation: 154005

Here is a version not using a visible loop and only the standard C++ library. It nicely demonstrates the use of a lambda as generator, too. The use of reserve() is optional and just intended to avoid more than one memory allocation.

std::vector<int> v;
v.reserve(100);
int n(0);
std::generate_n(std::back_inserter(v), 100, [n]()mutable { return n++; });

Upvotes: 17

Retired Ninja
Retired Ninja

Reputation: 4925

Here's another way...

int start = 27;
std::vector<int> v(100);
std::iota(v.begin(), v.end(), start);

Upvotes: 23

LarryPel
LarryPel

Reputation: 296

Using the generate algorithm : 

#include <iostream>     // std::cout
#include <algorithm>    // std::generate
#include <vector>       // std::vector
#include <iterator>

// function generator:
struct one_more { 
  int _count;
  one_more() : _count(0) {}
  int operator()() {
      return _count++;
  }
};

int main () {
  std::vector<int> myvector (100);
  std::generate (myvector.begin(), myvector.end(), one_more());

  std::copy(myvector.begin(), myvector.end(), std::ostream_iterator<int>(std::cout, " "));
  return 0;
}

Upvotes: 1

Michael
Michael

Reputation: 5939

const int num_seconds = 100;
vector<int> second( num_seconds );
for( int n = 0 ; n < num_seconds ; ++n ) {
    second[ n ] = n;
}

Upvotes: 0

Paul Evans
Paul Evans

Reputation: 27577

You want something like this:

std::vector<unsigned int> second(
    boost::counting_iterator<unsigned int>(0U),
    boost::counting_iterator<unsigned int>(99U));

Upvotes: 2

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