Question

def merge(L, start_index, sublist_size):
    """
    Merge two sublists of a list L

    Parameters:
    L - the list
    start_index - the index of the first element to be merged
    sublist_size - the size of the chunks to be merged

    Left chunk: L[start_index] to L[start_index + sublist_size - 1]
    Right chunk: L[start_index + sublist_size] to L[start_index + 2 * sublist_size - 1]
    """

    index_left = start_index
    left_stop_index = start_index + sublist_size
    index_right = start_index + sublist_size
    right_stop_index = min(start_index + 2 * sublist_size,
                           len(L))

    print('Merging sublists:', L[index_left:left_stop_index], 'and',
          L[index_right:right_stop_index]);

    L_tmp = []

    while (index_left < left_stop_index and
           index_right < right_stop_index):
        if L[index_left] < L[index_right]:
           L_tmp.append(L[index_left])
           index_left += 1
        else:
           L_tmp.append(L[index_right])
           index_right += 1

    if index_left < left_stop_index:
           L_tmp.extend(L[index_left : left_stop_index])
    if index_right < right_stop_index:
           L_tmp.extend(L[index_right : right_stop_index])

    L[start_index : right_stop_index] = L_tmp
    print('Merged sublist:', L_tmp, '\n')

def merge_sort(L):
    """
    Sort a list L using the merge sort algorithm.

    (Starter code doesn't fully sort the list.)
    """
    left_start_index = 0
    chunksize = 1  # Start by dividing the list into N sub-lists of 1 element each

    while chunksize < len(L):`enter code here`
        print("\n*** Sorting sublists of size", chunksize)
        print(L)

        while left_start_index + chunksize < len(L):
            merge(L, left_start_index, chunksize)

            # Move to next pair of chunks
            left_start_index += 2 * chunksize

        chunksize= chunksize *2
        print('List is now',L)

嘿，伙计们在完成这段代码时遇到了很多困难。代码的def合并部分很好，我遇到的问题是def_merge排序部分。所以当排序大小的子列表为1时，代码工作正常，但我无法让代码在此之后继续合并排序。我觉得好像问题出在蓄能器上。

Answer 1

在merge_sort函数中，运行内部while循环后，left_start_index的值已更改为0，因此您的代码无法正确合并排序列表中的前两个元素。在内部while循环之后，在chunksize=chunksize*2之前，重置left_start_index=0应解决问题。

def merge_sort(L):
    ...
    #outer while loop
    while chunksize < len(L):
        ....
        #inner while loop
        while left_start_index + chunksize < len(L):
            merge(L, left_start_index, chunksize)

            # Move to next pair of chunks
            left_start_index += 2 * chunksize
        #Reset left_start_index to 0
        left_start_index = 0
        chunksize= chunksize *2
        print('List is now',L)

遇到MergeSort问题

1 个答案: