合并排序C ++编译错误

Question

I'm trying to implement a merge sort function and I'm getting a compiler error that says "no match for 'operator=' " It occurs on my recursive assignment of left_list = mergesort(left_list) and the same error in the following line using right_list.

It compiles correctly if I take out the assignment and just have mergesort(left_list) but then it doesn't sort correctly. I thought that the way I have it now should merge sort correctly, but given that it doesn't, either the error has something to do with those lines or it's elsewhere in the mergesort() or merge() function.

Any help would be appreciated.

void mergesort(vector<int> &data) {

vector<int> left_list;
vector<int> right_list;

if ( data.size() <= 1 ) {
    return;
}

// creates a middle point to separate into 2 sub lists
int middle =  ( data.size() / 2 );

// create a list of elements to the left of middle
for ( int i = 0; i < middle; i++ ) {
    left_list.push_back(data[i]);
}

// create a list of elements to the right of middle
for ( unsigned int i = middle; i < data.size(); i++ ) {
    right_list.push_back(data[i]);
}

// break down the sub lists until they are of size 1
left_list = mergesort(left_list);
right_list = mergesort(right_list);

// merge the sublists in the correct order
merge(left_list, right_list);



}

vector<int> merge(vector<int> &left, vector<int> &right) {

vector<int> result;

unsigned left_it = 0;
unsigned right_it = 0;

while( left_it < left.size() && right_it < right.size() ) {


    // the smaller value is put into the result vector
    if( left[left_it] < right[right_it] ) {

        result.push_back(left[left_it]);
        left_it++;
    }
    else
    {
        result.push_back( right[right_it] );
        right_it++;
    }
}

// Put the rest of the data from both vectors onto result
while( left_it < left.size() ) {

    result.push_back( left[left_it] );
    left_it++;
}

while( right_it < right.size() ) {

    result.push_back( right[right_it] );
    right_it++;
}

return result;

}

Answer 1

You have code that is trying to accept a return value from the mergesort function. If that is what you want, my original answer addresses that. However, if mergesort is supposed to update the input parameter with the sorted result, then it need not return any value, and void is fine. Then, the assignment statements getting its return result is in error and should be changed.

mergesort(left_list);
mergesort(right_list);

However, the call to merge needs to assign the result to the input parameter.

data = merge(left_list, right_list);

My original answer follows:

You should change your mergesort function to return the same type that merge returns.

vector<int> mergesort(vector<int> &data) {

You then need to update the mergesort implementation so that the first return statement returns the input parameter:

return data;

The last statement should be changed to return the result:

return merge(left_list, right_list);

I haven't looked at the implementation of the algorithm itself.