One of the basic and most useful STL algorithms is algorithm which can be used to count number of elements within selected container according to specified criteria.
In order to do that we can use std::count or std::count_if algorithm.
For better understanding let's take a look on balow example: Output of this example is: In point I we are defining vector of int elements which we will examine. We are using C++11 std::initializer_list for it.
In point II we are examining all myVector elements and getting how many elements has value 6. There are 4 such elements.
Point III presents how we can examing one part region of vector. Here we are examining only first 5 elements of vector and finding how many of its have value 6. Of course there is only one such element within first 5 elements of myVector.
Point IV presents how to use std::count_if value with predefined UnaryPredicateFunction. We are defining our UnaryPredicateFunction in point IVa as isEven() function. This predicate function will examine whether its argument is even int number. We are using our isEven function within std::count_if algorithm and getting that we have 6 even numbers within myVector.
Source code of above example can be downloaded from our github account here: https://github.com/xmementoit/CppAdventureExamples/tree/master/stl/countAndCountIf
- std::count(firstElementIterator, lastElementIterator, elementForSearch) - is function which will go through container using firstElementIterator and lastElementIterator and return number of container elements which value is equal elementForSearch
- std::count_if(firstElementIterator, lastElementIterator, UnaryPredicateFunction) - is function which examine range from firstElementIterator to lastElementIterator and return number of container elements which fulfill UnaryPredicateFunction criteria. UnaryPredicateFunction is function having following signature: bool functionName(const Type& a). So, count_if returns number of elements where UnaryPredicateFunction returns true for.
For better understanding let's take a look on balow example: Output of this example is: In point I we are defining vector of int elements which we will examine. We are using C++11 std::initializer_list for it.
In point II we are examining all myVector elements and getting how many elements has value 6. There are 4 such elements.
Point III presents how we can examing one part region of vector. Here we are examining only first 5 elements of vector and finding how many of its have value 6. Of course there is only one such element within first 5 elements of myVector.
Point IV presents how to use std::count_if value with predefined UnaryPredicateFunction. We are defining our UnaryPredicateFunction in point IVa as isEven() function. This predicate function will examine whether its argument is even int number. We are using our isEven function within std::count_if algorithm and getting that we have 6 even numbers within myVector.
Source code of above example can be downloaded from our github account here: https://github.com/xmementoit/CppAdventureExamples/tree/master/stl/countAndCountIf
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