Counting matches in vector of structs - c++

Question

I have a problem that requires me to count the number of instances within this array that uses either std::count() or std::find(). I'm aware of how to do this using a standard data (see bottom code) type but not with the NameContainer that I'm using.
//Type
struct NameContainer{
char name [32];
}
//An array of containers
NameContainer *_storedNames = new NameContainer[_numberOfNames];
//An example of what I'm trying to do with a string rather than the NameContainer
std::vector<string> v(_storedNames, _storedNames + _numberOfNames);
//returns an numeric value
return std::count(v.begin(), v.end(), nameToSearch))

Answer 1

You can use a functor
struct names_equal {
string comp_to;
names_equal(string a) : comp_to(a) {}
bool operator()(NameContainer& p) {
return p.name == comp_to;
}
};
And count like
cout << std::count_if(v.begin(), v.end(), names_equal(nameToSearch));
This way nameToSearch doesn't have to be hard coded.
EDIT
If you can not use count_if, and has to be count then modify NameContainer and overload == for it.
struct NameContainer{
string name;
bool operator==(string str) {
return name == str;
}
};
Then count like this
cout << std::count(v.begin(), v.end(), nameToSearch);

Answer 2

you can use count_if and you provide a predicate (Unary function that accepts an element in the range as argument, and returns a value convertible to bool)
for example
bool myPred(NameContainer n){
return (strcmp(n.name, "name") == 0); }
std::vector<NameContainer> v(_storedNames, _storedNames + _numberOfNames);
int i=std::count_if(v.begin(), v.end(), myPred))
you can use strcmp() to compare character arrays.
if using only std::count or std::find:
both count and find takes the same type argument to compare as the type of conatainer, in your case NameContainer. std::count will execute following to compare searched values:
if (*first == val)
what means you have to overload operator== taking your class as arguments.
inline bool operator == (const NameContainer &first,const NameContainer &second){
return (strcmp(first.name,second.name)==0);
}
and then call std::count(v.begin(), v.end(), myObjectPredicate))
with myObjectPredicate being your NameContainer class object with name to be searched in vector.
so here is working solution. you might improve it in details:
struct NameContainer{
char name [32];
};
inline bool operator== (const NameContainer &first,const NameContainer &second){
return (strcmp(first.name,second.name)==0);
}
int main(int argc, char** argv) {
NameContainer* _storedNames = new NameContainer[1];
std::vector<NameContainer> vn(_storedNames, _storedNames + 1);
const char* cc="piotr";
NameContainer nc;
memcpy(nc.name,cc,strlen(cc)+1);
vn.push_back(nc);
NameContainer myObjectPredicate;
memcpy(myObjectPredicate.name,cc,strlen(cc)+1);
int count=std::count(vn.begin(), vn.end(), myObjectPredicate);
std::cout<<count;
return 2400;
}
output:
1

Answer 3

Read the docs on std::count, you'll see that it uses operator== for it's comparisons. Therefore, if you want to use std::count, the thing you want to compare MUST have an operator== defined for it. In your case, you could add one to your NameContainer pretty easily.