Vectors, structs and std::find

Question

Again me with vectors. I hope I'm not too annoying. I have a struct like this :

struct monster 
{
    DWORD id;
    int x;
    int y;
    int distance;
    int HP;
};

So I created a vector :

std::vector<monster> monsters;

But now I don't know how to search through the vector. I want to find an ID of the monster inside the vector.

DWORD monster = 0xFFFAAA;
it = std::find(bot.monsters.begin(), bot.monsters.end(), currentMonster);

But obviously it doesn't work. I want to iterate only through the .id element of the struct, and I don't know how to do that. Help is greatly appreciated. Thanks !

Solution 1

std::find_if:

it = std::find_if(bot.monsters.begin(), bot.monsters.end(), 
        boost::bind(&monster::id, _1) == currentMonster);

Or write your own function object if you don't have boost. Would look like this

struct find_id : std::unary_function<monster, bool> {
    DWORD id;
    find_id(DWORD id):id(id) { }
    bool operator()(monster const& m) const {
        return m.id == id;
    }
};

it = std::find_if(bot.monsters.begin(), bot.monsters.end(), 
         find_id(currentMonster));

Solution 2

You need to write your own search predicate:

struct find_monster
{
    DWORD id;
    find_monster(DWORD id) : id(id) {}
    bool operator () ( const monster& m ) const
    {
        return m.id == id;
    }
};

it = std::find_if( monsters.begin(), monsters.end(), find_monster(monsterID));

Solution 3

Take a look at the std::find template, the third parameter especially:

template<class InputIterator, class EqualityComparable>
InputIterator find(InputIterator first, InputIterator last,
               const EqualityComparable& value);

What is this EqualityComparable? Again from the documentation:

A type is EqualityComparable if objects of that type can be 
compared for equality using operator==, and if operator== is 
an equivalence relation. 

Now, your type monster needs to define such an operator. If you don't the compiler generates one for you (as also the default ctor and the dtor) which does a memcmp sort of thing which doesn't work in your case. So, to use std::find first define a comparator function/functor that the algorithm can use to match your currentMonster i.e. something along the lines of:

 struct monster {
  // members
  bool operator==(const monster& l, const monster& r) const
  {
     return l.id == r.id;
  }
 };

Solution 4

or put the monsters in a map instead of a vector

or if they must be in a vector create an index map ie map of ID to vector index

View more solutions

Comments

[Martin Sherburn]

Nice answer, but there is a typo in the constructor. It should be ':' not ';'

[Christian Rau]

In addition to writing your own search predicate, you need to use std::find_if instead of std::find.

[Kyle Heuton]

Does this work? I did not have success because an operator definition inside a struct can only have 1 input

[H'H]

using boost, Perfect!

[Tomáš Zato]

I wonder, if I have {int, int}, could I cast it to long and use normal find then?

[Tomáš Zato]

Same problem as Snoozer. -1, please complete the answer.

[Semjon Mössinger]

What is the meaning of "bot" in bot.monsters.begin()?

[nathan]

What is the datatype if "it"??

[tomereli]

good thing you gave an example without boost - we never use boost in embedded systems, it is huge. I just didn't understand why you inherited the functor object from std::unary_function - looks like it would work either way, won't it? appreciate your answer.

[tomereli]

@Hiyper - The type would be std::vector<monster>::iterator. Or you can use auto it = std::find_if... if you are using c++11

[Johannes Schaub - litb]

@tomereli IIRC it wouldn't work without it in C++03, but I think it will in C++11, which can figure out the return type itself using decltype.

[Adam Erickson]

@SemjonMössinger This was not explicitly stated, but bot should be a struct containing the following vector of structs: std::vector<monster> monsters;

[Darkproduct]

unary_function is deprecated in C++11 and removed in C++17.

[Tzunghsing David Wong]

the concept is correct. this works for me: bool operator==(const monster& r) const { return id == r.id; }

[Mecanik]

How are you even compiling this? It fails on latest C++ and VS2019.