[Tutor] Statistic-Program Problems! Please Help Quickly!

[Steven D'Aprano]

On Fri, 15 Oct 2010 03:09:57 pm Colleen Glaeser wrote:
[...]
> So far, my program is as follows:
>
> Data = [[3,1],[4,3],[6, 4],[7, 6],[8, 8],[9, 8]]
>
> def X():
>     accX = 0
>     for item in Data:
>         [x,y] = item
>         accX = accX + x
>     print (accX)

I see four problems with this function. The first three are minor, 
stylistic problems. The fourth is the biggy -- it stops your function 
from working correctly.

(1) The name X doesn't mean anything. Functions should have a name that 
tells the reader what the function does. In this case, I 
suggest "sumx", since it sums the x values.

(2) The function gets its input from a global variable, Data. This isn't 
*wrong*, exactly, but it's usually inconvenient and leads to trouble 
later on. It is often better to pass the Data as an argument to the 
function, especially if you have more than one set of data to use.

Example: when you call the "len" function, to get the length of a list, 
you do this:

len(my_list)  # pass the list as an argument to the function

rather than this:

Data = my_list
len()  # only looks at the global variable Data


(3) We can simplify the for-loop a little bit. Instead of this:

for item in Data:
    [x,y] = item
    ...

you can do this:

for x,y in Data:
    ...

Since item is not used anywhere else, this saves time and code.

(4) This is a real problem. Your function doesn't return a result, it 
merely prints it to the screen. This means it's impossible to store the 
result for use later on, or to do calculations with it, or anything 
like that. This is why you get the error message:

TypeError: unsupported operand type(s) for *: 'NoneType' and 'NoneType'

The error is a bit cryptic, but it means that when you try multiplying 
Y() * Q() later on, neither function returns anything, so Python tries 
multiplying None and None, which is impossible.


Fixing these three minor issues and one major problem, I get:


def sumx(data):
    accumulator = 0
    for (x,y) in data:
        accumulator += x
    return accumulator

To use this, you might do this:

my_first_data_set = [ [1,2], [3,4], [5,6] ]
my_second_data_set = [ [3,4], [5,6], [100, 0] ]
a = sumx(my_first_data_set)
b = sumx(my_second_data_set)
print "Sum of x from first set:", a
print "Sum of x from second set:", b


which should print:

Sum of x from first set: 9
Sum of x from second set: 108


You should make similar adjustments to Y() and the other functions, and 
then see how the assignment goes.


-- 
Steven D'Aprano