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How do you find the median of a list?

P: 2
I have a little bit of trouble finding the median in a list. I am trying to ask the user what his/her marks are and i am trying to find the median. I have a little bit of trouble because the user could be inputting either an odd or an even number of marks but i don't know how to ask him/her. it would be great if someone could help me. This is what i have so far:
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  1. mark = 0
  2. count = 0
  3. yourmarks =[]
  4. while True:
  5.     marks = input("Enter student marks here[-1 to stop]")
  6.     yourmarks.append(marks)
  7.     count += 1
  8.     if marks == -1:
  9.         break
  10. print "Your highest mark is",max(yourmarks)
  11. del yourmarks[-1]
  12. print "Your lowest mark is", min(yourmarks)
  13.  
Nov 9 '08 #1
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8 Replies


boxfish
Expert 100+
P: 469
A number is even if dividing it by two leaves no remainder. To get the remainder from a division, you use the modulus operator, %. So you would use
len(yourmarks) % 2
if that is equal to zero, then there are an even number of items in youmarks.
By the way, it would be helpful if you used code tags around your code. Put [CODE] before the code and [/CODE] after it, so it shows up in a code box and the indentation isn't wrecked. Thanks.
I hope this is helpful.
Nov 9 '08 #2

P: 2
thanks for the help and advice but i still dont seem to understand what you are talking about.. could you show me exactly how you would do it?
Nov 9 '08 #3

boxfish
Expert 100+
P: 469
You know what len does, right?
8 % 4 is equal to zero, because 8 is divisible by 4 with no remainder. But 11 % 4 is 3, because you get a remainder of 3 when you divide 11 by 4.
Does the following example help?
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  1. if len(yourmarks) % 2 == 0:
  2.     print "Division by two leaves no remainder,"
  3.     print "so list length is even."
  4. else:
  5.     print "There was a remainder,"
  6.     print "So the list contains an odd number of items."
  7.  
Nov 10 '08 #4

P: 15
You want to find the median of a list?

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  1. x=[1,2,5,2,3,763,234,23,1,234,21,3,2134,23,54]
  2. median=sorted(x)[len(x)/2]
  3.  
Nov 13 '08 #5

bvdet
Expert Mod 2.5K+
P: 2,851
You want to find the median of a list?

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  1. x=[1,2,5,2,3,763,234,23,1,234,21,3,2134,23,54]
  2. median=sorted(x)[len(x)/2]
  3.  
That works great when the list length is an odd number. When it is an even number, you take the average of the two middle numbers in a sorted list.
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  1. def median(s):
  2.     i = len(s)
  3.     if not i%2:
  4.         return (s[(i/2)-1]+s[i/2])/2.0
  5.     return s[i/2]
Nov 13 '08 #6

P: 1
This post is a year old so I don't know if I'll get any replies, but my median function doesn't work.... It yields <function median at 0x01D36F30> this is wut i used, just like what is posted
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  1. def median (y):
  2.     z = len(y)
  3.     if not z%2:
  4.         return (y[(z/2)-1] + y[z/2]) / 2
  5.     return y[z/2]
what am i doing wrong?
Nov 6 '09 #7

bvdet
Expert Mod 2.5K+
P: 2,851
You are not calling the function with an argument. You have another problem. If you have a list of integers and the length of the list is an even number, the function calculates the average of the two middle numbers. Dividing by the integer 2 results in integer division. In the example below, (8+9)/2 = 8 but the answer is 8.5.
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  1. >>> def median(y):
  2. ...     z = len(y)
  3. ...     if not z%2:
  4. ...         return (y[(z/2)-1] + y[z/2]) / 2
  5. ...     else:
  6. ...         return y[z/2]
  7. ...        
  8. >>> median
  9. <function median at 0x0109CEF0>
  10. >>> y=[6,3,9,1,2,8,9,12,34,12]
  11. >>> y.sort()
  12. >>> median(y)
  13. 8
  14. >>> y
  15. [1, 2, 3, 6, 8, 9, 9, 12, 12, 34]
  16. >>> 
Nov 6 '09 #8

bferguson94
P: 11
def main():
myList=[]
choice = 0
while choice !='4':
choice= raw_input('1.add number to list\n2.get mean \n3.get median \n4.quit')
if choice == '1':
addNum(myList)
if choice == '2':
getMean(myList)
if choice == '3':
getMedian(myList)
if int(choice) < 0:
print "invalid input"
if int(choice) > 4:
print "invalid input"
##break
##input error messages
def addNum(myList):
while True:
num = raw_input("input new number:")
try:
num = int(num)
if num < 0:
raise ValueError
break
except:
print"invalid input"


myList.append(num)

def getMean(myList):
total = 0.0
for value in myList:
total = total + value
mean = total/len(myList)
print "the mean is:" , mean

def getMedian (myList):
index= 2
if len(myList)%2 == 0:
myMed = (myList[index] + myList[myMed-1])/2
else:
myList[len(myList)/2]
print myMed
Feb 26 '10 #9

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