Accessing element of an array of struct. - c++

Question

So I have an struct
struct car{
string ownerName;
float price;
int year;
};
and I declared an array of these structs
car *cars = new car[1000]
Each car has an index, for example, the car with index 0 has name John Smith.
So, my question is knowing the name of the owner how do I access the index of the car. I know that the other way i would write
cars[0].name,
to get the name, but how would i do it backwards?

Answer 1

Two possible ways come to my mind.
One is writing a function that finds index by name.
#include <string>
using namespace std;
car *find_by_name(car* cars, const string& name, int from, int to) {
car* car_ptr = NULL;
for(int i = from; i < to; i++) {
if (cars[i].ownerName == name) {
car_ptr = cars+i;
break;
}
}
return car_ptr;
}
As you may notice, this function is very expensive (O(n)).
The other one and the easiest one, in my opinion, is using Map or HashMap to do so.
#include <map>
#include <string>
#include <iostream>
using namespace std;
car set_new_val(map<string, car*>& my_map, const string& name, const float price, const int year) {
car* car_heap = new car();
car_heap->ownerName = name;
car_heap->price = price;
car_hep->year = year;
my_map.insert(pair<string, car*>(name, car_heap));
}
car* find_car_by_name(const map<string, car*>& my_map, const string& name) {
map<string, car*>::iterator it;
if ((it = my_map.find(name)) == my_map.end())
return NULL;
return it->second;
}
int main(int argc, char* argv[]) {
map<string, car*> my_cars_data;
set_new_val(my_cars_data, "James", 2233000.5, 2013);
set_new_val(my_cars_data, "Dave", 1222000.5, 2011);
set_new_val(my_cars_data, "Aaron", 1222000.75, 2012);
car* james_car = find_car_by_name(my_cars_data, "James");
cout << "Year " << james_car->year << endl;
car* null_car = find_car_by_name(my_cars_data, "Santa");
if (null_car == NULL)
cout << "No owner with the name Santa is recorded" << endl;
...
...
free_map(my_map);
return 0;
According to C++11, lookup for a key using Map takes O(lgn) (HashMap is O(1)), for more details read here . That's a big pro, if you handle mass of data (not to mention that it is easier to maintain).

Answer 2

If you use a sequential container (array, vector...) you have to search for the name. In an unsorted array a linear search is required.
// linear search
string name = "joe";
auto it = find_if(begin(cars), end(cars),
[&name](const car& c) { return c.ownerName == name; });
auto index = it - begin(cars);
If you have performance problems with this approach you could sort the array and use a binary search (preferable if your array of cars does not change) or use a associative container which gives you fast access to an element by key (map, multi_map, unordered_map...)
// binary search
struct {
bool operator()(const car& lh, const car& rh) { return lh.ownerName < rh.ownerName; };
bool operator()(const car& lh, const std::string& rh) { return lh.ownerName < rh; };
bool operator()(const std::string& lh, const car& rh) { return lh < rh.ownerName; };
} byName;
sort(begin(cars), end(cars), byName);
auto it2 = lower_bound(begin(cars), end(cars), name, byName);
if (it != end(cars) && it->ownerName == name)
{
auto index2 = it - begin(cars);
}