calculating standard deviation

[lcc_551]

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?

[woooee]

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>

[lcc_551]

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

[jrcagle]

As well he should :)

Here's what wooee means:

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:

def MyFunc():
    pass

print MyFunc

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

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