CODEWITHSUNDEEP
c++stl
Josh
Josh

Reputation: 6272

C++ std::find with a custom comparator

This is basically what I want to do:

bool special_compare(const string& s1, const string& s2)
{
    // match with wild card
}

std::vector<string> strings;

strings.push_back("Hello");
strings.push_back("World");

// I want this to find "Hello"
find(strings.begin(), strings.end(), "hell*", special_compare);

// And I want this to find "World"
find(strings.begin(), strings.end(), "**rld", special_compare);

But std::find doesn't work like that unfortunately. So using only the STL, how can I do something like this?

Upvotes: 34

Views: 46270

Answers (6)

Imtiaz Shakil Siddique
Imtiaz Shakil Siddique

Reputation: 4308

I wanted to have an example with custom class having custom find logic but didn't find any answer like that. So I wrote this answer which uses custom comparator function (C++11) to find an object.

class Student {

private:  
  long long m_id;
  // private fields

public:
  long long getId() { return m_id; };
  
};

Now suppose, I want to find the student object whose m_id matches with a given id. I can write std::find_if like this:

// studentList is a vector array

long long x_id = 3; // local variable
auto itr = std::find_if(studentList.begin(), studentList.end(),
                    [x_id](Student& std_val) 
                    { return std_val.getId() == x_id; }
                    );

if(itr == studentList.end())
  printf("nothing found");

Upvotes: 3

Olaf Dietsche
Olaf Dietsche

Reputation: 74028

Since nobody has mentioned std::bind yet, I'll propose this one

#include <functional>

bool special_compare(const std::string& s, const std::string& pattern)
{
    // match with wild card
}

std::vector<std::string> strings;
auto i = find_if(strings.begin(), strings.end(), std::bind(special_compare, std::placeholders::_1, "hell*"));

Upvotes: 7

Angew is no longer proud of SO
Angew is no longer proud of SO

Reputation: 171127

Based on your comments, you're probably looking for this:

struct special_compare : public std::unary_function<std::string, bool>
{
  explicit special_compare(const std::string &baseline) : baseline(baseline) {}
  bool operator() (const std::string &arg)
  { return somehow_compare(arg, baseline); }
  std::string baseline;
}

std::find_if(strings.begin(), strings.end(), special_compare("hell*"));

Upvotes: 37

Nikos C.
Nikos C.

Reputation: 51840

You'll need std::find_if(), which is awkward to use, unless you're on a C++11 compiler. Because then, you don't need to hardcode the value to search for in some comparator function or implement a functor object, but can do it in a lambda expression:

vector<string> strings;

strings.push_back("Hello");
strings.push_back("World");

find_if(strings.begin(), strings.end(), [](const string& s) {
    return matches_wildcard(s, "hell*");
});

Then you write a matches_wildcard() somewhere.

Upvotes: 12

cooky451
cooky451

Reputation: 3510

With C++11 lambdas:

auto found = find_if(strings.begin(), strings.end(), [] (const std::string& s) { 
    return /* you can use "hell*" here! */;
});

If you can't use C++11 lambdas, you can just make a function object yourself. Make a type and overload operator ().

Upvotes: 8

Sarfaraz Nawaz
Sarfaraz Nawaz

Reputation: 361472

The function you need to use is this : std::find_if, because std::find doesn't take compare function.

But then std::find_if doesn't take value. You're trying to pass value and compare both, which is confusing me. Anyway, look at the documentation. See the difference of the usage:

auto it1 = std::find(strings.begin(), strings.end(), "hell*");
auto it2 = std::find_if(strings.begin(), strings.end(), special_compare);

Hope that helps.

Upvotes: 12

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