std::count, std::count_if - cppreference.com (original) (raw)

Returns the number of elements in the range [first, last) satisfying specific criteria.

1. Counts the elements that are equal to value (using operator==).

2. Counts elements for which predicate p returns true.

2,4) Same as (1,3), but executed according to policy.

These overloads participate in overload resolution only if all following conditions are satisfied:

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Exceptions
• 5 Notes
• 6 Possible implementation
• 7 Example
• 8 Defect reports
• 9 See also

[edit] Parameters

[edit] Return value

The number of iterators it in the range [first, last) satisfying the following condition:

1,2) *it == value is true.

3,4) p(*it) != false is true.

[edit] Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons with value using operator==.

3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Notes

For the number of elements in the range [first, last) without any additional criteria, see std::distance.

[edit] Possible implementation

See also the implementations of count in libstdc++ and libc++.

See also the implementations of count_if in libstdc++ and libc++.

[edit] Example

#include #include #include #include #include #include   int main() { constexpr std::array v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10}; std::cout << "v: "; std::copy(v.cbegin(), v.cend(), std::ostream_iterator(std::cout, " ")); std::cout << '\n';   // Determine how many integers match a target value. for (const int target : {3, 4, 5}) { const int num_items = std::count(v.cbegin(), v.cend(), target); std::cout << "number: " << target << ", count: " << num_items << '\n'; }   // Use a lambda expression to count elements divisible by 4. int count_div4 = std::count_if(v.begin(), v.end(), [](int i) { return i % 4 == 0; }); std::cout << "numbers divisible by four: " << count_div4 << '\n';   // A simplified version of distance with O(N) complexity: auto distance = [](auto first, auto last) { return std::count_if(first, last, { return true; }); }; static_assert(distance(v.begin(), v.end()) == 10);   std::array<std::complex, 3> nums{{{4, 2}, {1, 3}, {4, 2}}}; #ifdef __cpp_lib_algorithm_default_value_type // T gets deduced making list-initialization possible auto c = std::count(nums.cbegin(), nums.cend(), {4, 2}); #else auto c = std::count(nums.cbegin(), nums.cend(), std::complex{4, 2}); #endif assert(c == 2); }

Output:

v: 1 2 3 4 4 3 7 8 9 10 number: 3, count: 2 number: 4, count: 2 number: 5, count: 0 numbers divisible by four: 3

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit] See also