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std::generate_n in C++

  • Difficulty Level : Expert
  • Last Updated : 21 Jul, 2017

std::generate is an STL algorithm, which is used to generate numbers based upon a generator function, and then, it assigns those values to the elements in the container in the range [first, last).

The generator function has to be defined by the user, and it is called successively for assigning the numbers.
Now, there can be a scenario, where we want to assign values only to the first n elements, for that we have another STL algorithm std::generate_n, which has the following syntax:
Template function:

OutputIterator generate_n (OutputIterator first, Size n, Generator gen);

first: Output iterator pointing to the beginning of the container.
n: No. of elements to be assigned a value, using generator function.
gen: A generator function for generating the values.

Returns: 
It doesnot have a void return type like std::generate, but, in fact, 
it returns an iterator pointing to the element that follows the last element 
whose value has been generated.




// C++ program to demonstrate the use of std::generate_n
#include <iostream>
#include <vector>
#include <algorithm>
  
// Defining the generator function
int gen()
{
    static int i = 0;
    return ++i;
}
  
using namespace std;
int main()
{
    int i;
  
    // Declaring a vector of size 10
    vector<int> v1(10);
  
    // using std::generate_n
    std::generate_n(v1.begin(), 10, gen);
  
    vector<int>::iterator i1;
    for (i1 = v1.begin(); i1 != v1.end(); ++i1) {
        cout << *i1 << " ";
    }
    return 0;
}

Output:

1 2 3 4 5 6 7 8 9 10

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