std::ranges::fold_left - cppreference.com (original) (raw)

Left-folds the elements of given range, that is, returns the result of evaluation of the chain expression:
f(f(f(f(init, x1), x2), ...), xn), where x1, x2, ..., xn are elements of the range.

Informally, ranges::fold_left behaves like std::accumulate's overload that accepts a binary predicate.

The behavior is undefined if [first, last) is not a valid range.

1. The range is [first, last). Equivalent toreturn ranges::fold_left_with_iter(std::move(first), last, std::move(init), f).value.

2. Same as (1), except that uses r as the range, as if by using ranges::begin(r) as first and ranges::end(r) as last.

3. Equivalent to:

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

Contents

• 1 Parameters
• 2 Return value
• 3 Possible implementations
• 4 Complexity
• 5 Notes
• 6 Example
• 7 References
• 8 See also

[edit] Parameters

[edit] Return value

An object of type U that contains the result of left-fold of the given range over f, where U is equivalent to std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t<I>>>.

If the range is empty, U(std::move(init)) is returned.

[edit] Possible implementations

struct fold_left_fn { template<std::input_iterator I, std::sentinel_for S, class T = std::iter_value_t, /* indirectly-binary-left-foldable */<T, I> F> constexpr auto operator()(I first, S last, T init, F f) const { using U = std::decay_t<std::invoke_result_t<F&, T, std::iter_reference_t>>; if (first == last) return U(std::move(init)); U accum = std::invoke(f, std::move(init), *first); for (++first; first != last; ++first) accum = std::invoke(f, std::move(accum), first); return std::move(accum); }   template<ranges::input_range R, class T = ranges::range_value_t, / indirectly-binary-left-foldable */<T, ranges::iterator_t> F> constexpr auto operator()(R&& r, T init, F f) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(init), std::ref(f)); } };   inline constexpr fold_left_fn fold_left;

[edit] Complexity

Exactly ranges::distance(first, last) applications of the function object f.

[edit] Notes

The following table compares all constrained folding algorithms:

[edit] Example

#include #include #include #include #include #include #include #include   int main() { namespace ranges = std::ranges;   std::vector v{1, 2, 3, 4, 5, 6, 7, 8};   int sum = ranges::fold_left(v.begin(), v.end(), 0, std::plus()); // (1) std::cout << "sum: " << sum << '\n';   int mul = ranges::fold_left(v, 1, std::multiplies()); // (2) std::cout << "mul: " << mul << '\n';   // get the product of the std::pair::second of all pairs in the vector: std::vector<std::pair<char, float>> data {{'A', 2.f}, {'B', 3.f}, {'C', 3.5f}}; float sec = ranges::fold_left ( data | ranges::views::values, 2.0f, std::multiplies<>() ); std::cout << "sec: " << sec << '\n';   // use a program defined function object (lambda-expression): std::string str = ranges::fold_left ( v, "A", s, int x) { return s + ':' + std::to_string(x); } ); std::cout << "str: " << str << '\n';   using CD = std::complex; std::vector nums{{1, 1}, {2, 0}, {3, 0}}; #ifdef __cpp_lib_algorithm_default_value_type auto res = ranges::fold_left(nums, {7, 0}, std::multiplies{}); // (2) #else auto res = ranges::fold_left(nums, CD{7, 0}, std::multiplies{}); // (2) #endif std::cout << "res: " << res << '\n'; }

Output:

sum: 36 mul: 40320 sec: 42 str: A:1:2:3:4:5:6:7:8 res: (42,42)

[edit] References

• C++23 standard (ISO/IEC 14882:2024):

• 27.6.18 Fold [alg.fold]

[edit] See also