如何将数组中重复的值计数存储到 c++ 中的 map？

Question

I was trying to store count of words repeated in an array of string...

int countWords(string list[], int n)
{
    map <string, int> mp;

    for(auto val = list.begin(); val!=list.end(); val++){
        mp[*val]++;
    }
    int res = 0;
    for(auto val= mp.begin(); val!=mp.end(); val++){
        if(val->second == 2) res++;
    }
    return res;
}

but I was getting error like:

prog.cpp: In member function int Solution::countWords(std::__cxx11::string*, int):
prog.cpp:14:32: error: request for member begin in list, which is of pointer type std::__cxx11::string* {aka std::__cxx11::basic_string<char>*} (maybe you meant to use -> ?)
            for(auto val = list.begin(); val!=list.end(); val++){
                                ^
prog.cpp:14:51: error: request for member end in list, which is of pointer type std::__cxx11::stri.................

someone please look into this once.

Answer 1

The reason for the error is that list is an array, which does not have a begin method (or any other method).

This could be fixed by changing the function to take a std::vector instead of an array.

If you want to keep it as an array, the for loop should be changed to this, assuming n is the length of the array:

for(auto val = list; val != list + n; val++)

In C and C++, an array is somewhat equivalent to a pointer to the first element of the array; thus list gives the start pointer, and list + n gives a pointer to after the end of the array.

Answer 2

list is a pointer, it does not have begin or end members, nor is it a valid input to std::begin or std::end .

If you have n strings in an array, pointed to by list , then you can iterate them by constructing a std::span .

int countWords(std::string list[], int n)
{
    std::map<std::string, int> mp;

    for(auto & val : std::span(list, n)){
        mp[val]++;
    }
    int res = 0;
    for(auto & [key, value] : mp){
        if(value == 2) res++;
    }
    return res;
}