notes/OJ notes/pages/cpp_Range_based_for_loop.md

# cpp_Range_based_for_loop

#### 2022-06-11

---

##### Data structures:

#DS #array #vector #multiset

##### Difficulty:

#CS_analysis #difficulty_easy

##### Related problems:

##### Links:

- [cppreference](https://en.cppreference.com/w/cpp/language/range-for)

---

### What is cpp_Range_based_for_loop?

Executes a for loop over a range.

Used as a more readable equivalent to the traditional [for loop](https://en.cppreference.com/w/cpp/language/for "cpp/language/for") operating over a range of values, such as all elements in a container.

### Example

```cpp
#include <iostream>
#include <vector>

int main()
{
    std::vector<int> v = {0, 1, 2, 3, 4, 5};

    for (const int& i : v) // access by const reference
        std::cout << i << ' ';
    std::cout << '\n';

    for (auto i : v) // access by value, the type of i is int
        std::cout << i << ' ';
    std::cout << '\n';

    for (auto&& i : v) // access by forwarding reference, the type of i is int&
        std::cout << i << ' ';
    std::cout << '\n';

    const auto& cv = v;

    for (auto&& i : cv) // access by f-d reference, the type of i is const int&
        std::cout << i << ' ';
    std::cout << '\n';

    for (int n : {0, 1, 2, 3, 4, 5}) // the initializer may be a braced-init-list
        std::cout << n << ' ';
    std::cout << '\n';

    int a[] = {0, 1, 2, 3, 4, 5};
    for (int n : a) // the initializer may be an array
        std::cout << n << ' ';
    std::cout << '\n';

    for ([[maybe_unused]] int n : a)
        std::cout << 1 << ' '; // the loop variable need not be used
    std::cout << '\n';

    for (auto n = v.size(); auto i : v) // the init-statement (C++20)
        std::cout << --n + i << ' ';
    std::cout << '\n';

    for (typedef decltype(v)::value_type elem_t; elem_t i : v)
    // typedef declaration as init-statement (C++20)
        std::cout << i << ' ';
    std::cout << '\n';

#if __cplusplus > 23'00 + 200000
    for (using elem_t = decltype(v)::value_type; elem_t i : v)
    // alias declaration as init-statement, same as above (C++23)
        std::cout << i << ' ';
    std::cout << '\n';
#endif
}
```

Possible output:

```
0 1 2 3 4 5
0 1 2 3 4 5
0 1 2 3 4 5
0 1 2 3 4 5
0 1 2 3 4 5
0 1 2 3 4 5
1 1 1 1 1 1
5 5 5 5 5 5
0 1 2 3 4 5
0 1 2 3 4 5
```