How to Properly Use Arithmetic Operators Inside Comprehensions?

Question

I am dealing with a simple csv file that contains three columns and three rows containing numeric data. The csv data file looks like the following:

 Col1,Col2,Col3
 1,2,3
 2,2,3
 3,2,3
 4,2,3

I have hard time figuring out how to let my python program subtracts the average value of the first column "Col1" from each value in the same column. For illustration the output should give the following values for 'Col1':

1 - 2.5 = -1.5
2 - 2.5 = -0.5
3 - 2.5 =  0.5
4 - 2.5 =  1.5

Here is my attempt that gives me (TypeError: unsupported operand type(s) for -: 'str' and 'float' ) at the last print statement which containing the comprehension.

import csv

# Opening the csv file
file1 = csv.DictReader(open('columns.csv'))
file2 = csv.DictReader(open('columns.csv'))

# Do some calculations
NumOfSamples = open('columns.csv').read().count('\n')
SumData = sum(float(row['Col1']) for row in file1)
Aver = SumData/(NumOfSamples - 1) # compute the average of the data in 'Col1'


# Subtracting the average from each value in 'Col1'
data = []
for row in file2:
    data.append(row['Col1'])

# Print the results
print Aver
print [e-Aver for e in data] # trying to use comprehension to subtract the average from each value in the list 'data'

I do not know how to solve this problem! Any idea how to make the comprehension working to give what is supposed to do?

The error says that you can't do a string minus a float. The problematic line has `e-Aver`. Therefore, `e` is a string and `Aver` is a float. Thus, you have to convert `e` to a float. — TigerhawkT3, Oct 10 '15 at 04:04

score 2 · Accepted Answer · answered Oct 10 '15 at 04:02

2

The issue in your code is that in case of data list (file2) , you are reading strings from the file and storing strings into the data list.

Hence, when later on , you try to do - [e-Aver for e in data] - it errors out as you are trying to subtract float from string.

You should convert to float or int before storing into data list. Example -

data = []
for row in file2:
    data.append(float(row['Col1']))

answered Oct 10 '15 at 04:02

Anand S Kumar

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Thanks!I followed your instruction and it makes sense. I am still getting an error when I convert data to float type. It says: `ValueError: could not convert string to float: ` Is there a way to overcome this error? – MEhsan Oct 10 '15 at 04:14
That means you have a row that does not have `float` in the `Col1` . – Anand S Kumar Oct 10 '15 at 04:14
Are you getting that error from the `SumData` line? – Anand S Kumar Oct 10 '15 at 04:15
:D Yahhhh it is working! The columns.csv file has extra line with spaces. – MEhsan Oct 10 '15 at 04:16

score 2 · Answer 2 · answered Oct 10 '15 at 04:02

You're not converting the Col1 value from a string to an float(). You can do the conversion either on read (as below) or in the list comprehension.

data = []
for row in file2:
    data.append(float(row['Col1']))

# Print the results
print Aver
print [e - Aver for e in data]

How to Properly Use Arithmetic Operators Inside Comprehensions?

2 Answers2