In this next part of the big STL algorithm tutorial, we will discover the 4 modifying sequence algorithms that fill in or generate data:

• fill
• fill_n
• generate
• generate_n

Let’s get started!

## fill

This is a fairly simple algorithm that takes two iterators defining a range and value that it will assign to each and every element in the range.

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

int main() {
std::vector<int> numbers(8); // a vector of 8 elements zero initialized
std::cout << "numbers after the initialization of the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}
std::cout << '\n';

std::fill(numbers.begin(), numbers.begin()+4, 42);
std::fill(numbers.begin()+4, numbers.end(), 51);

std::cout << "numbers after filling up the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}

return 0;
}


When can you use it? If you want to initialize a vector with the same items, do use it, you can pass the value in the vector’s constructor like this:

std::vector<int> numbers(8, 42); // a vector of 8 elements initialized to 42


Otherwise, in case you need to create a vector with sequences of the same item as we did in the first example of the section, it comes quite handy.

## fill_n

fill_n is pretty similar to fill, the only difference is that while fill takes two iterators defining a range, fill_n takes one iterator pointing at the beginning of the range and instead of the second iterator, it takes a number indicating how many elements have to be filled.

Here is the example used for fill with the necessary changes:

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

int main() {
std::vector<int> numbers(8); // a vector of 8 elements initialized to 42
std::cout << "numbers after the initialization of the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}
std::cout << '\n';

std::fill_n(numbers.begin(), 4, 42);
std::fill_n(numbers.begin()+4, 4, 51);

std::cout << "numbers after filling up the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}

return 0;
}


What you really have to pay attention to is that pass in a valid number as a second parameter. If you don’t, it’s undefined behaviour. That means you cannot really know what would happen, but it’s better not to play with production code.

There might be no visible consequences. For example, when I changed the second fill command to update 5 items (the 9th is already out of the vector), I still get the expected output. But when I pass 8, so half of those are out of the vector’s bounds, I got a core dump when the vector’s memory is deallocated.

Just pay attention to pass in the good values.

## generate

How generate works, is similar to fill. It also takes two iterators defining a range that has to be updated. The difference is that while fill takes a value as a third parameter, generate takes a - drumbeat, please - generator, that’s right!

But what is a generator?

It is any function that is called without any arguments and that returns a value convertible to those pointed by the iterators.

As it’s the simplest example, it can be just a function always returning the same value. It’s not very useful, especially not compared to fill, but let’s use it just to show how this algorithm works. As usual, the generator doesn’t have to be a function, it can be a function object or a lambda just as well.

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

int main() {
std::vector<int> numbers(8); // a vector of 8 elements initialized to 0
for (const auto& number : numbers) {
std::cout << ' ' << number;
}
std::cout << '\n';

auto staticGenerator = [](){ return 42; };

std::generate(numbers.begin(), numbers.end(), staticGenerator);

std::cout << "numbers after filling up the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}

return 0;
}


It’s as simple as that.

To get random numbers, you have to use a random generator. How random generation works is outside the scope of this article.

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

int main() {
std::vector<int> numbers(8); // a vector of 8 elements initialized to 0
for (const auto& number : numbers) {
std::cout << ' ' << number;
}
std::cout << '\n';

// Random generator beginning
std::random_device rd;
std::mt19937 mt(rd());
std::uniform_real_distribution<double> distribution(1.0, 10.0);

auto randomGenerator = [&distribution, &mt](){ return distribution(mt); };
// Random generator end

std::generate(numbers.begin(), numbers.end(), randomGenerator);

std::cout << "numbers after filling up the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}

return 0;
}


## generate_n

If you read the last three sections with care, this one will not give you any surprise at all.

It works like fill_n in terms of passing in the values to be updated - a start iterator and a number of items -  and like generate in terms of generating the values to be assigned - a function not taking any parameter but returning a value that can be converted into to the target type.

Which one to use, generate or generate_n? It should depend on your use-case to see which provides better readability. If you focus on a range, then use generate, but if the number of items to be filled/generated is more important, use the _n version.

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

int main() {
std::vector<int> numbers(8); // a vector of 8 elements initialized to 0
for (const auto& number : numbers) {
std::cout << ' ' << number;
}
std::cout << '\n';

// Random generator beginning
std::random_device rd;
std::mt19937 mt(rd());
std::uniform_real_distribution<double> distribution(1.0, 10.0);

auto randomGenerator = [&distribution, &mt](){ return distribution(mt); };
// Random generator end

std::generate_n(numbers.begin(), 8, randomGenerator);

std::cout << "numbers after filling up the vector: ";
for (const auto& number : numbers) {
std::cout << ' ' << number;
}

return 0;
}


## Conclusion

Today, we learned about 4 algorithms filling up values in a container. fill and fill_n put static values in a container, while generate and generate_n dynamically creates the values populating the target.

Their usage should depend on your use-case, whether you need a fixed number of generated values or a containerful of items.

Next time we’ll learn about the remove algorithms. Stay tuned!