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Homework Help: Trouble with Python sum function

  1. Nov 12, 2011 #1
    I don't know why we have to write a sum function for python when there already is one but
    this is supposed to sum a list with varible values and I don't understand why it doesn't work, can someone help explain there error, I believe that I am not giving proper values to the line that adds but I don't realize why<

    3. The attempt at a solution

    Code (Text):
    def sum_list (a_list):
        length = len(a_list)
        counter = 0
        total= 0
        while(counter < length):
            (a_list[counter] +total)          
            counter = counter + 1
        total = total + counter    
        return total      

    #testing the functions
    my_list = [3,3,3]

    print sum_list(my_list)
  2. jcsd
  3. Nov 12, 2011 #2


    Staff: Mentor

    Your while loop isn't right. Python evaluates a_list[counter] + total for each element in the array, but you aren't storing the value anywhere.

    You need something like this:
    total = total + a_list[counter]

    Then get rid of the line that says total = total + counter
  4. Nov 13, 2011 #3
    Thanks that's fixed.

    To understand my mistake and clarify; is it fair to say that every time the loop ran it was adding 3 to zero then chucking that value into nothing as opposed to saving it, that's why the print was 3?

    Again thankyou, I learn more here with a few of my mistakes and all your replies then my instructors teach.
  5. Nov 13, 2011 #4


    Staff: Mentor

    Yes, that's exactly what was happening.
    You're welcome! We enjoy being able to help out.
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