Python

calculating standard deviation

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Join Date: Feb 2009

Posts: 2

Reputation: lcc_551 is an unknown quantity at this point

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lcc_551

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calculating standard deviation

Feb 20th, 2009

Hi, I'm in first year computer science and could use some help on a program where I have to calculate the standard deviation from data on a txt file. Using some online help I've gotten somewhere but to be honest don't really know what's going on myself.

The program is meant to use data from an outside file so I can't hard code numbers or have the user input data.

data = open("datafile1.txt")
print data.read()

~stuff in between~

# a = numbers
# r = number of values in data
# m = mean (already calculated earlier in the programming)

def SD():
....b = []
....for n in range(r-1):
...........if r[n] > a:
.................b.append((r[n] - a)**2)
...........if r[n] < m:
.................b.append((a - r[n])**2)
...........SD = (float(b)/r)**0.5 #float because the data includes decimal values
....return SD
print "The standard deviation is", SD

Unfortunatly this is the result I get, including the number of values and mean which I had to calculate as well:

There are 4 records
The mean is 3.1422
The standard deviation is <function SD at 0x020EB630>
The standard deviation is <function SD at 0x058F6F30>
The standard deviation is <function SD at 0x020EB630>
The standard deviation is <function SD at 0x0552BA70>
The standard deviation is <function SD at 0x020EB630>

Could someone help me this?

Last edited by lcc_551; Feb 20th, 2009 at 12:08 am.

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Join Date: Dec 2006

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woooee

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Re: calculating standard deviation

Feb 20th, 2009

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The standard deviation is The standard deviation is <function SD at 0x020EB630>

Don't use the same name for the function and the variable returned, hence "<function SD" at 0x020EB630>

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Join Date: Feb 2009

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lcc_551

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Newbie Poster

Re: calculating standard deviation

Feb 21st, 2009

I'm very new to python so I don't quite understand the answer given, I've tried juggling around the names but still get the same result.

I would like to post all the code I have but my prof would probably peg me for it if I did :s

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Join Date: Jul 2006

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jrcagle

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Re: calculating standard deviation

Feb 22nd, 2009

As well he should

Here's what wooee means:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
def SD():
    b = []
    for n in range(r-1):
        if r[n] > a:
             b.append((r[n] - a)**2)
        if r[n] < m:
             b.append((a - r[n])**2)
    SD = (float(b)/r)**0.5 #float because the data includes decimal values
    return SD
print "The standard deviation is", SD
def SD():
    b = []
    for n in range(r-1):
        if r[n] > a:
             b.append((r[n] - a)**2)
        if r[n] < m:
             b.append((a - r[n])**2)
    SD = (float(b)/r)**0.5 #float because the data includes decimal values
    return SD
print "The standard deviation is", SD

Note that "SD" is a function. So when you print the function, you get, quite literally, the function: <function SD" at 0x020EB630>

Try this with a simpler example:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
def MyFunc():
    pass
 
print MyFunc
def MyFunc():
    pass

print MyFunc

What you actually want to print is the result of calling the function. So that needs this:

 Help with Code Tags 
 Python Syntax (Toggle Plain Text) 
def SD(...):
 ....
 
print SD()
def SD(...):
 ....

print SD()

The extra () mean "call the function and accept the result."

Mathematical aside: Your test 'if r[n] < a' is superfluous. For any values of r[n] and a, it will be true that

(r[n] - a) ** 2 == (a - r[n]) ** 2

which is why standard deviations are calculated using square residues in the first place (so that under- and over-values don't cancel each other out).

Jeff