如何在python中将sum用于列表中的特定值？

Question

I have a text file that looks like this; the values are tab separated:

diamond orange pear loc1  .  +    0.0  0.0  0.0  0.0  1.0 1.2  3.4 
diamond orange pear loc2  .  +    1.0  0.0  0.0  0.0  1.0 1.2  2.3
diamond orange pear loc3  .  +    2.0  0.0  3.0  0.0  0.0 0.0  1.4  
# ......

For each line in the file I want to make a ratio of the sum of the first 3 values divided by the sum of the last 4 values. The output would look like:

diamond orange pear loc1  .  +    0 
diamond orange pear loc2  .  +    0.22
diamond orange pear loc3  .  +    4.28 
 ......

I would like to do this in python.

with open('/path/to/file/') as inFile:
    inFile.next()
    for line in inFile:
        data = cols[6:]
        data = map(float,data)

        sum_3 = [sum[for x in x data[0:3]]
        sum_last = [sum[for x in x data[4:7]]
        average = sum_3/sum_last 

This doesn't work, and I was hoping if I could get some advice?

Answer 1

You don't show where cols comes from, but it appears you didn't actually split each line, in which case you are left with is a single string and you were trying to work with the characters of that string, without the first 6. Mapping individual characters to float values is not going to give you the data you need.

Next, sum() is a function, but you are using indexing syntax, sum[...] will throw an exception. You don't need to use a list comprehension for getting values out of a slice either, just sum(data[:3]) would do, provided the slice produces a sequence of float s.

You have a tab-delimited file, it is probably easiest to just use the csv module to do the splitting:

import csv

with open('/path/to/file/') as infile:
    reader = csv.reader(infile, delimiter='\t')
    next(reader)  # skip first row

    for row in reader:
        first3, last = row[-7:-4], row[-4:]
        try:
            average = sum(map(float, first3)) / sum(map(float, last))
        except ZeroDivisionError:
            # last four values are all zero; just set the average to zero.
            average = 0

I've made allowances for the last 4 values all being zero; at that point you'd be dividing by zero and you'd want to handle the exception that is thrown for that case.