问题
I have made a merge sort program in Python and it is running perfectly but I have modified it to count the number of inversions involved and now it is giving me an error :
Here's my code:
def merge_list(left,right,c):
result=[]
i,j=0,0
while i < len(left) and j < len(right):
if left[i] < right[j]:
result.append(left[i])
print "Left result",result
i=i+1
elif left[i] > right[j]:
result.append(right[j])
print "Right result",result
j=j+1
if right[j] < left[i] and i<j:
c=c+1
result=result+left[i:]
result=result+right[j:]
print "Inversions: ",c
return result,c
def sort_and_count(lis,count):
if len(lis)<2:
return lis
middle=len(lis) / 2
left,c1=sort_and_count(lis[:middle],count)
print "left",left
right,c2=sort_and_count(lis[middle:],count)
print "right",right
m,c=merge_list(left,right,count)
c=c+c1+c2
return m,c
if __name__=="__main__":
print "Enter 6 elements: "
i=0;lis=[];merge_lis=[];inv=0
while i<=5:
x=int(raw_input())
lis.append(x)
i=i+1
count=0
merge_lis,inv=sort_and_count(lis,count)
print "Sorted list is: ",merge_lis,inv
And my traceback:
Traceback (most recent call last):
File "Sort_and_count.py", line 53, in <module>
merge_lis,inv=sort_and_count(lis,count)
File "Sort_and_count.py", line 31, in sort_and_count
left,c1=sort_and_count(lis[:middle],count)
File "Sort_and_count.py", line 31, in sort_and_count
left,c1=sort_and_count(lis[:middle],count)
ValueError: need more than 1 value to unpack
Where am I going wrong with this approach?
回答1:
This line:
return lis
This is a problem, because you are expecting sort_and_count to return a tuple containing two values, so when it returns only one value you have a problem with the tuple unpacking in lines like left,c1=sort_and_count(lis[:middle],count). This line should return two values, like the last line of that method:
return m,c
回答2:
Actually, you've implemented wrong algorithm to count the number of inversions.
1) In function merge_list instead of:
elif left[i] > right[j]:
result.append(right[j])
print "Right result",result
j=j+1
if right[j] < left[i] and i<j:
c=c+1
you should use this code:
elif right[j] < left[i]:
result.append(right[j])
j += 1
inv_count += (len(left)-i)
2) Function merge_list doesn't need variable c as an input.
3) Function sort_and_count doesn't need variable count as an input.
Try this code:
def merge_list(left,right):
result = list()
i,j = 0,0
inv_count = 0
while i < len(left) and j < len(right):
if left[i] < right[j]:
result.append(left[i])
i += 1
elif right[j] < left[i]:
result.append(right[j])
j += 1
inv_count += (len(left)-i)
result += left[i:]
result += right[j:]
return result,inv_count
def sort_and_count(array):
if len(array) < 2:
return array, 0
middle = len(array) / 2
left,inv_left = sort_and_count(array[:middle])
right,inv_right = sort_and_count(array[middle:])
merged, count = merge_list(left,right)
count += (inv_left + inv_right)
return merged, count
if __name__=="__main__":
array = [2,3,1,4,5]
merge_array,inversions = sort_and_count(array)
print 'Start with array: %s;\nSorted array: %s;\nInversions: %s.'%(array,merge_array,inversions)
回答3:
The error message is telling you that sort_and_count is only returning a single value. There are only two returns in the function, so the culprit is this one:
if len(lis)<2:
return lis
回答4:
Instead of
return lis
do
return lis, count
回答5:
Well, you're returning a single value where he's expecting two.
Look at
def sort_and_count(lis,count):
if len(lis) < 2:
return lis
middle = len(lis) / 2
left, c1 = sort_and_count(lis[:middle],count)
# etc
If you call sort_and_count([1], count), the len(lis) will be < 2 and it will return the single-element list, but will not return a count, which is expected in the call below.
Just return a value for c1 like
return lis, count # do your things with count
来源：https://stackoverflow.com/questions/14733119/counting-inversions-using-merge-sort