The essence of the program: get a map, where the values are char from the passed string, and the key is the number of these values in the string

using namespace std;

map<char, int> is_merge(const string& s) {
    map<char, int> sCount {};
    for (auto lp : s) {
        if (find_if(sCount.begin(), sCount.end(), lp) != sCount.end()) {
            sCount[lp]  = 1;
        } else {
            sCount.insert(make_pair(lp, 0));
        }
    }

    return sCount;
}

int main()
{
    string test = "aba";

    map <char, int> res = is_merge(test);

    for (auto lp : res) {
        cout << lp.first << ":" << lp.second << endl;
    }

    return 0;
}

But an error occurs in the console: /usr/include/c /12/bits/predefined_ops.h:318:30: error: expression cannot be used as a function 318 | { return bool(_M_pred(*__it)); } | ~~~~~~~^~~~~~~

CodePudding user response：

std::find_if takes a predicate not a value. Hence the error that lp is not a callable. To find a key in a map you should use std::map::find because it is O(logn) compared to O(n) for std::find/std::find_if (as a rule of thumb you can remember: If a container has a member function that does the same as a generic algorithm the member function is at least as effcient, often better).

However, there is not need to check if the key is present via find. The function can be this:

map<char, int> is_merge(const string& s) {
    map<char, int> sCount {};
    for (auto lp : s) {
           sCount[lp];
    }

    return sCount;
}

std::map::operator[] already does insert an element when none is found for the given key. You don't need to do that yourself.

PS: And if you do call insert then there is no need for std::make_pair : sCount.insert({lp, 0});. std::make_pair is for when you need to deduce the type of the pair from arguments to std::make_pair, but you don't need that here.

PPS: And if you do use std::find you need to consider that the element type of your map is std::pair<const char, int>, not char.