STL Algorithm::Count()

转载自：http://www.dreamincode.net/forums/topic/52509-stl-algorithmcount/

What do I need to know before studying this tutorial?
You should have a knowledge of vectors and vector iterators.

What does the count() algorithm do?
It counts the # of a specified value within a vector

The syntax for the count() algorithm:
int count = count(<where to begin searching>, <where to stop searching>, <value to search for>);

What to include:

#include <iostream> // just to output what's going on
#include <algorithm> // algorithms!
#include <vector> // vector to use algorithms on

using namespace std;

Next, we need to begin our int main () function. I'm gonna assume you know how to do this, and move on.

We need to declare a vector and a vector iterator:

vector <int> myVec; // create a vector called myVec
vector <int> :: iterator it; // create a vector iterator called it

We now need a loop to fill our vector with some values. Let's use some random numbers:

for (int i = 0; i < 9; ++i)
{
    int random = rand () % 5; // generate a random number
    myVec.push_back(random); // fill the vector with some values
}

Let's print the contents:

cout << "Contents: ";
for (it = myVec.begin(); it != myVec.end(); ++it)
    cout << *it << " "; // print the contents of myVec

Next is the count function. Ready? (Take a look at the syntax at the beginning of this tutorial to compare them)

int count_value = count (myVec.begin(), myVec.end(), 4);

Now all we need to do is print our count variable and exit the program:

cout << endl << "Number of 4s stored in vector: " << count_value;

cin.get (); // pause
return EXIT_SUCCESS; // everything went OK

For those of you who learn better by example, here is the complete code:

#include <iostream> // just to output what's going on
#include <algorithm> // algorithms!
#include <vector> // vector to use algorithms on

using namespace std;

int main ()
{
    vector <int> myVec; // create a vector called myVec
    vector <int> :: iterator it; // create a vector iterator called it

    for (int i = 0; i < 9; ++i)
    {
        int random = rand () % 5; // generate a random number
        myVec.push_back(random); // fill the vector with some values
    }

    cout << "Contents: ";
    for (it = myVec.begin(); it != myVec.end(); ++it)
        cout << *it << " "; // print the contents of myVec

    int count_value = count (myVec.begin(), myVec.end(), 4);

    cout << endl << "Number of 4s stored in vector: " << count_value;

    cin.get (); // pause
    return EXIT_SUCCESS; // everything went OK
}