My professor wrote this median function and I don't understand it very well. Can someone please explain the part about i = len(list)/2 and median = avg() and the else statement?
def avg_list(numbers):
sum = 0
for num in numbers:
sum += num
avg = float(sum)/len(numbers)
print avg
def median(list):
list.sort()
if len(list)%2 == 0:
#have to take avg of middle two
i = len(list)/2
median = avg()
else:
#find the middle (remembering that lists start at 0)
i = len(list)/2
median = list
return median
To add from an example I saw, for even list length:
def median(s):
i = len(s)
if not i%2:
return (s[(i/2)-1]+s[i/2])/2.0
return s[i/2]
This works very well but I don't understand the last return s[i/2]?
For odd list length:
x = [1,2,5,2,3,763,234,23,1,234,21,3,2134,23,54]
median = sorted(x)[len(x)/2]
Since x has a list length of odd, wouldn't the [len(x)/2] be a floating number index? I'm not getting this all the way? Any explanation better than mine is much appreciated.
Answer
Why this is is very wrong, line by line:
def median(list): # 1
list.sort() # 2
if len(list)%2 == 0:
#have to take avg of middle two
i = len(list)/2 # 3
median = avg() # 4
else:
#find the middle (remembering that lists start at 0)
i = len(list)/2 # 5
median = list # 6
return median
#1: It's a bad idea to give your variables the same name as data types, namely list.
#2: list.sort() will modify the list that is being passed. One would expect a getter like median() not to do that.
#4 It calls a function avg() with no arguments, which is completely meaningless, even if such a function was defined.
#3 and #5 are calculated the same way regardless of the if branch taken. Regardless, i is never used.
#6 It sets median to the original list, which makes zero sense.
Here's how I would rewrite this (while maintaining clarity):
def median(alist):
srtd = sorted(alist) # returns a sorted copy
mid = len(alist)/2 # remember that integer division truncates
if len(alist) % 2 == 0: # take the avg of middle two
return (srtd[mid-1] + srtd[mid]) / 2.0
else:
return srtd[mid]
Also, the avg_list() function (which is not used nor could be used in median()) could be rewritten as:
def avg_list(numbers):
return float(sum(numbers))/len(numbers)
sum() is a function that returns the sum of all elements in an iterable.