Sorting a file in Python

One problem is that the initial for loop exhausts the data from the input file so that there is nothing left to read in the subsequent input_file.read(). Also read() will return a string, not a list. But in any case your insertion sort function is operating on an empty string, so it does nothing at all.

You could fix the first problem by seeking to the beginning of the file after the for loop. The second problem can be fixed by splitting the input by lines using splitlines():

header_line = next(input_file)
for line in input_file:
    print line

input_file.seek(0)
next(input-file)    # skip header again
list=input_file.read().splitlines()
print list

But it's probably better to just do this:

with open('input_file') as input_file:
    header_line = next(input_file).strip()
    numbers = [line.strip() for line in input_file]
    # if you really want to print them out first...
    for number in numbers:
        print number

    insertion_sort(numbers)

N.B. this code does not convert the data from the file into any numeric type (e.g. integer), because you said that the numbers are not integers... so what are they? Not converting to a numeric type means that your sort function will sort based on the ASCII collating sequence of the numeric strings, so '10' would sort before '2'.

If the numbers can be floats, you can do this when reading the file:

numbers = [float(line) for line in input_file]

now your sort function will sort numbers such as 1 or 1.0 as floats.

Sorry for the confusion with what your goal was. Here is the correct code:

input_file=open("C:\\Users\\Rachel\\Documents\\Natural Sciences MSci\\Year 2\\Env Sci\\Comp Modelling\\Pptn data.txt", "r")
header_line = input_file.readline()

list=input_file.read().split()


def insertion_sort(items):
    for i in range(1, len(items)):
        j = list[i]
        i = i - 1
        while i >= 0:
            if j < list[i]:
                list[i + 1] = list[i]
                list[i] = j
                i = i - 1
            else:
                break


insertion_sort(list)
print 'After sorting:', list

Your algorithm seems to work just fine. I tried the following on my computer. I created a file called numbers.txt and placed the numbers in the following manner:

23
23.4
4
5
6.7
1
0
6
34

And then ran the following code:

def insertion_sort(items):
    for i in range(1, len(items)):
        j = i
        while j > 0 and items[j] < items[j-1]:
            temp = items[j]
            items[j] = items[j - 1]
            items[j - 1] = temp
            j = j - 1

numbers = open("numbers.txt").read().split()
numbers = [float(number) for number in numbers]
print "Before sorting: ", numbers
insertion_sort(numbers)
print "After sorting: ", numbers

Which gave me the following output:

Before sorting:  [23.0, 23.4, 4.0, 5.0, 6.7, 1.0, 0.0, 6.0, 34.0]
After sorting:  [0.0, 1.0, 4.0, 5.0, 6.0, 6.7, 23.0, 23.4, 34.0]

I hope this helps.