Can you pass an additional parameter to a predicate?

Question

I'm trying to filter a vector so it contains only a specific value.

e.g. Make sure the vector only contains elements of the value "abc."

Right now, I'm trying to achieve this with remove_copy_if.

Is there any way to pass an additional parameter to a predicate when using one of std's algorithms?

std::vector<std::string> first, second;
first.push_back("abc");
first.push_back("abc");
first.push_back("def");
first.push_back("abd");
first.push_back("cde");
first.push_back("def");

std::remove_copy_if(first.begin(), first.end(), second.begin(), is_invalid);

I'm hoping to pass the following function as a predicate but it seems more likely that this would just end up comparing the current value being examined by remove_copy_if and the next.

bool is_invalid(const std::string &str, const std::string &wanted)
{
   return str.compare(wanted) != 0;
}

I have a feeling I'm probably approaching this wrong so any suggestions would be appreciated!

Thanks

Just wondering - what do you use a vector with only identical elements for? — Zane, Nov 23 '12 at 10:15
I tried to make my example as simple as possible so it made sense :) What I did with this was to remove elements from a vector that fit a certain pattern (with regex). — noko, Nov 23 '12 at 10:36

score 28 · Accepted Answer · edited May 23 '17 at 12:14

Define a functor instead:

struct is_invalid
{
    is_invalid(const std::string& a_wanted) : wanted(a_wanted) {}
    std::string wanted;
    bool operator()(const std::string& str)
    {
        return str.compare(wanted) != 0;
    }
};

std::remove_copy_if(first.begin(),
                    first.end(),
                    second.begin(),
                    is_invalid("abc"));

or if C++11 use a lambda:

std::string wanted("abc");
std::remove_copy_if(first.begin(), first.end(), second.begin(), 
    [&wanted](const std::string& str)
    {
        return str.compare(wanted) != 0;
    });

Note that the output vector, second, must have elements before the call to remove_copy_if():

// Create 'second' after population of 'first'.
//
std::vector<std::string> second(first.size());

std::string wanted = "abc";
int copied_items    = 0;
std::remove_copy_if( first.begin(), first.end(), second.begin(),
    [&wanted, &copied_items](const std::string& str) -> bool
    {
        if (str.compare(wanted) != 0) return true;
        copied_items++;
        return false;
    });
second.resize(copied_items);

As functor predicates are copied more effort is required to retain the copied_items information. See Pass std algos predicates by reference in C++ for suggested solutions.

I seem to get an error about an "undefined reference" when I try to use this solution, any ideas on what might be happening? — noko, Nov 23 '12 at 09:49
@noko, can you post you code to ideone or similar? Note you compiler must support C++11 lambdas (in g++ you need to use `-std=c++0x` compiler switch). — hmjd, Nov 23 '12 at 09:50
never mind, I forgot to add a ClassName:: before the function :( — noko, Nov 23 '12 at 09:58

score 10 · Answer 2 · answered Nov 23 '12 at 08:33

Make functor, or use std/boost::bind.

struct is_invalid
{
public:
   is_invalid(const std::string& w):wanted(w) { }
   bool operator () (const std::string& str)
   {
       return str.compare(wanted) != 0;
   }
private:
   std::string wanted;
};

std::remove_copy_if(first.begin(), first.end(), second.begin(), is_invalid("abc"));

Example with bind

bool is_invalid(const std::string &str, const std::string &wanted)
{
   return str.compare(wanted) != 0;
}

std::remove_copy_if(first.begin(), first.end(), second.begin(),
boost::bind(is_invalid, _1, "abc"));

Can you pass an additional parameter to a predicate?

2 Answers2

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