Search for a struct item in a vector by member data

Question

I'm very new to c++ and I'm trying to find a way to search a vector of structs for a struct with a certain member data.

I know this would work with simple types in the vector

std::find(vector.begin(), vector.end(), item) != vector.end()

But lets say I have a struct like this:

struct Friend
{
  string name;
  string number;
  string ID;
};

and a vector like this:

vector<Friend> friends;

Then the vector is filled with friends.

Let's say I want to search for a friend with a certain ID, and cout the details. Or delete the certain struct from the vector. Is there a simple way to do this?

Answer 1

This can be done with std::find_if and a search predicate, which can be expressed as a lambda function if you have C++11 (or C++0x) available:

auto pred = [](const Friend & item) {
    return item.ID == 42;
};
std::find_if(std::begin(friends), std::end(friends), pred) != std::end(friends);

To use an ID given as a variable, you have to capture it in the lambda expression (within the [...]):

auto pred = [id](const Friend & item) {
    return item.ID == id;
};
std::find_if(std::begin(friends), std::end(friends), pred) != std::end(friends);

If you don't have C++11 available, you have to define the predicate as a functor (function object). Remy Lebeau's answer uses this approach.

To remove elements matching the criteria as defined by the predicate, use remove_if instead of find_if (the rest of the syntax is the same).

For more algorithms, see the STL <algorithm> reference.

Answer 2

Use std::find_if(). @leemes and @AndyProwl showed you how to use it in a C++11 compiler. But if you are not using a C++11 compiler, then you can use it like this instead, which defines a functor comparing the ID of a given item with a previously specified ID in its constructor:

class MatchesID
{
    std::string _ID;

public:
    MatchesID(const std::string &ID) : _ID(ID) {}

    bool operator()(const Friend &item) const
    {
        return item.ID == _ID;
    }
};

std::find_if(vector.begin(), vector.end(), MatchesID("TheIDHere")) != vector.end();

If you have other classes in your project which use IDs, you can make this functor templated:

template<typename IDType>
class MatchesID
{
    IDType _ID;

public:
    MatchesID(const IDType &ID) : _ID(ID) {}

    template<class ItemType>
    bool operator()(const ItemType &item) const
    {
        return item.ID == _ID;
    }
};

std::find_if(vector.begin(), vector.end(), MatchesID<std::string>("TheIDHere")) != vector.end();

Answer 3

You can use std::find_if in combination with functors (if you are working with C++98) or lambdas (if you are using C++11, which I will assume):

using namespace std;
int ID = 3; // Let's say...
auto it = find_if(begin(vector), end(vector), [=] (Friend const& f) { 
    return (f.ID == ID); 
    });
bool found = (it != end(vector));

Answer 4

If you want to find an element in STL container, use std::find or std::find_if algorithms With C++03, you need to overload operator== for std::find

bool operator==(const Friend& lhs, const Friend& rhs)
{
  return lhs.ID == rhs.ID;
}

if (std::find(friends.begin(), friends.end(), item) != friends.end())
{
   // find your friend
}

OR C++11 with lambda:

std::find_if(friends.begin(), friends.end(),  [](Friend& f){ return f.ID == "1"; } );

If you want to remove a certain element, use std::remove_if

std::remove_if(friends.begin(), friends.end(), 
      [](Friend& f){ return f.ID == "1"; });