C++ Algorithm Generate: std::fill, std::generate & std::iota Guide

Introduction

STL generation algorithms fill containers or produce values from a generator: fill, generate, iota, and related helpers keep initialization code short and clear.

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1. std::fill

Basic use

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

int main() {
    std::vector<int> v(10);
    
    std::fill(v.begin(), v.end(), 42);
    
    for (int x : v) {
        std::cout << x << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

std::fill_n

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

int main() {
    std::vector<int> v(10, 0);
    
    std::fill_n(v.begin(), 5, 1);
    
    for (int x : v) {
        std::cout << x << " ";
    }
    std::cout << std::endl;
    
    return 0;
}

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2. std::generate / generate_n

Use a generator callable; generate_n with back_inserter appends n generated values.

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3. std::iota

Fills [first, last) starting from value, using ++ for each successive element. Include <numeric>.

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4. Practical examples

Examples in the Korean article cover random numbers (<random>), functor counters, struct initialization, and ID generators — same code; prefer <random> over rand().

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5. Comparison

Algorithm	Header	Role	Time
fill	<algorithm>	Single value	O(N)
fill_n	<algorithm>	First n with one value	O(N)
generate	<algorithm>	Callable per element	O(N)
generate_n	<algorithm>	n elements from callable	O(N)
iota	<numeric>	Sequential values	O(N)

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6. Common pitfalls

• Zero size: fill/generate on empty range does nothing — resize first or use generate_n + back_inserter.
• Capture: use [&counter] not [=] when the lambda must update outer state.
• Random: use C++11 <random>, not rand().
• Performance: fill is often faster than generate returning a constant; use fill for constants.

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7. Test data generator

The TestDataGenerator example (random, sequence, constant, pattern, custom) is unchanged from the source article.

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Summary

1. fill — one value, fast
2. fill_n — first n with one value
3. generate / generate_n — flexible generation
4. iota — sequential values in <numeric>

Next steps

• C++ Algorithm Reverse
• C++ Algorithm Count

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Related posts

• C++ Algorithm Copy

Keywords

std::fill, std::generate, std::iota, fill_n, generate_n, STL, C++