Question

Can someone please help me understand this?

I have created a map of char strings as key and int as a value from a two dimensional char array. I see duplicate entries being inserted in the map!

To test further, I added two char strings of the same value to a map (also in the code, commented), and only one of them was added.

void countstr(char words[][NUM_OF_STR])
{
    map<char*, int> mwords;

    cout<<"ORIG"<<endl;
    for(int i = 0; i < NUM_OF_STR; i ++)
    {
        cout<<words[i]<<endl;
        mwords.insert(pair<char*, int>(words[i], 0));
        cout<<mwords.size()<<endl;
    }

    map<char*, int>::iterator itr;

    cout<<endl<<"MAP"<<endl;
    for(auto i = mwords.begin(); i != mwords.end(); i ++)
    {
        cout<<i->first<<"\t"<<i->second<<endl;
    }

    return;
}

int main()
{
    char words[NUM_OF_STR][5] = { "abc", "pqr", "xyz", "abc", "pqr" };

    /*map<char*, int> mwords;
    mwords.insert(pair<char*, int>("abc", 1));
    cout<<mwords.size()<<endl;
    mwords.insert(pair<char*, int>("abc", 2));
    cout<<mwords.size()<<endl;*/

    countstr(words);
    return 0;
}

Output:

ORIG
abc
1
pqr
2
xyz
3
abc
4
pqr
5

MAP
abc     0
pqr     0
xyz     0
abc     0
pqr     0

Answer 1

如果比较comentario和-verbose，您会发现它们并不相同。那是因为它们是指针，所以运算符class ABC: def __init__(self,debugFlag): self.debugFlag = False def some_function(): if self.debugFlag is True: print "Some debug logs" def set_flag(): self.debugFlag = True，words[0]等将查看您C字符串的地址，而不是内容。

由于它们的内容相同，所以it is possible — but not guaranteed — that their addresses will be the same too（这是字符串文字的事实）。在您的情况下，它们是不相同的，因此键在逻辑上是不同的，并且两个元素都被接受。

指针不是容器。

您可以通过提供自定义比较器来make the map use strcmp instead，也可以只将words[3]（或==）用作密钥。：）

Map allowing duplicates?

1 个答案: