индексы и итераторы

#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>
 
int main(){
    std::vector<int> vec;
    vec.push_back(1);
    vec.push_back(2);
    vec.push_back(3);
    vec.push_back(4);
    vec.push_back(5);
    vec.push_back(6);
    
    std::vector<int>::iterator it;
    if ( ( it = std::find(vec.begin(), vec.end(), 5) ) == vec.end() )
        std::cout << "No elements with value 5" << std::endl;
    else
        std::cout << "Value 5 contain element with index " << std::distance(vec.begin(), it) << std::endl;
    
    return 0;
}

#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>
 
int main(){
    std::vector<int> vec;
    vec.push_back(1);
    vec.push_back(3);
    vec.push_back(5);
    vec.push_back(7);
    vec.push_back(5);
    vec.push_back(5);
    
    int cnt;
    
    std::vector<int>::iterator it;
    for ( cnt = 0, it = std::find(vec.begin(), vec.end(), 5); it != vec.end(); 
        ++cnt, it = std::find(++it, vec.end(), 5) )
            std::cout << "ARRAY[" << std::distance(vec.begin(), it) << "]" << std::endl;
    if ( ! cnt )
        std::cout << "No elements with value 5 found in array." << std::endl;
    else
        std::cout << "Found " << cnt << " elements." << std::endl;
    
    return 0;
}