[Tutor] Please Help

[Amit Saha]

Hi Arijit,

On Thu, Mar 21, 2013 at 8:42 PM, Arijit Ukil <arijit.ukil at tcs.com> wrote:
>
> I am new to python. I like to calculate average of the numbers by reading
> the file 'digi_2.txt'. I have written the following code:
>
> def average(s): return sum(s) * 1.0 / len(s)
>
> f = open ("digi_2.txt", "r+")
>
> list_of_lists1 = f.readlines()
>
>
> for index in range(len(list_of_lists1)):
>
>
>     tt = list_of_lists1[index]
>
>     print 'Current value :', tt
>
> avg =average (tt)
>
>
> This gives an error:
>
> def average(s): return sum(s) * 1.0 / len(s)
> TypeError: unsupported operand type(s) for +: 'int' and 'str'
>
> I also attach the file i am reading.
>
>
>
> Please help to rectify.

The main issue here is that when you are reading from a file, to
Python, its all strings. And although, 'abc' + 'def' is valid, 'abc' +
5 isn't (for example). Hence, besides the fact that your average
calculation is not right, you will have to 'convert' the string to an
integer/float to do any arithmetic operation on them. (If you know C,
this is similar to typecasting). So, coming back to your program, I
will first demonstrate you a few things and then you can write the
program yourself.

If you were to break down this program into simple steps, they would be:

1. Read the lines from a file (Assume a generic case, where you have
more than one line in the file, and you have to calculate the average
for each such row)
2. Create a list of floating point numbers for each of those lines
3. And call your average function on each of these lists

You could of course do 2 & 3 together, so you create the list and call
the average function.

So, here is step 1:

with open('digi.txt','r') as f:
    lines = f.readlines()

Please refer to
http://docs.python.org/2/tutorial/inputoutput.html#methods-of-file-objects
for an explanation of the advantage of using 'with'.

Now, you have *all* the lines of the file in 'lines'. Now, you want to
perform step 2 for each line in this file. Here you go:

for line in lines:
    number_list = []
    for number in line.split(','):
        number_list.append(float(number))

 (To learn more about Python lists, see
http://effbot.org/zone/python-list.htm). It is certainly possible to
use the index of an element to access elements from a list, but this
is more Pythonic way of doing it. To understand this better, in the
variable 'line', you will have a list of numbers on a single line. For
example: 1350696461, 448.0, 538660.0, 1350696466, 448.0. Note how they
are separated by a ',' ? To get each element, we use the split( )
function, which returns a list of the individual numbers. (See:
http://docs.python.org/2/library/stdtypes.html#str.split). And then,
we use the .append() method to create the list. Now, you have a
number_list which is a list of floating point numbers for each line.

Now, step 2 & 3 combined:

for line in lines:
    number_list = []
    for number in line.split(','):
        number_list.append(float(number))
    print average(number_list)

Where average( ) is defined as:

def average(num_list):
    return sum(num_list)/len(num_list)



There may be a number of unknown things I may have talked about, but i
hope the links will help you learn more and write your program now.

Good Luck.
-Amit.


--
http://amitsaha.github.com/