如何编写用户定义的std :: sort（）比较

Question

I have a two dimensional vector array vector<vector<int> > and I need to sort the vectors inside it in order. So... I thought I would write a function which compare two vectors and then use sort()

However I couldn't understand and then was not able to find anywhere the proper syntax for this. Here is my compare function:

bool vectorcompare (vector<int> v, vector<int> w){ 
    for(int i=0; i<v.size(); i++){
        if(v[i]<w[i]) return 1;
        if(v[i]>w[i]) return 0;
    }
    return 1;
}

And then I call sort function, which doesn't work:

sort(vector.begin(),vector.end(), vectorcompare());

Anything I am doing wrong?

Answer 1

First, your compile error:

sort(vector.begin(),vector.end(), vectorcompare());

vectorcompare is a function, not a type . That is, it's not an object that you can construct. So you don't do () on it; you just pass the function as is:

sort(vector.begin(),vector.end(), vectorcompare);

Now, for a worse problem:

bool vectorcompare (vector<int> v, vector<int> w)

Take your parameters by const& , not by value. std::vector s are big; you shouldn't needlessly copy them.

And an even worse problem:

for(int i=0; i<v.size(); i++){
    if(v[i]<w[i]) return 1;
    if(v[i]>w[i]) return 0;
}
return 1;

This does not constitute a strict-weak ordering . This provides less than or equals to , which is not a strict-weak ordering. std::sort requires sorting based on a strict-weak ordering.

Answer 2

You may not need to write your own comparison function. std::vector already has an operator< defined that performs the equivalent of std::lexicographical_compare . Have you tried simply calling std::sort on your vector<vector> ?

Note: added my comment as this answer because the user found it helpful.

Answer 3

Please look at this site, it has a nice example .

In your case, you would call std::sort like this:

sort(vector.begin(),vector.end(), vectorcompare);