python - How to Properly Use Arithmetic Operators Inside Comprehensions? -

i dealing simple csv file contains 3 columns , 3 rows containing numeric data. csv data file looks following:

 col1,col2,col3  1,2,3  2,2,3  3,2,3  4,2,3 

i have hard time figuring out how let python program subtracts average value of first column "col1" each value in same column. illustration output should give following values 'col1':

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

here attempt gives me (typeerror: unsupported operand type(s) -: 'str' , 'float' ) @ last print statement containing comprehension.

import csv  # opening csv file file1 = csv.dictreader(open('columns.csv')) file2 = csv.dictreader(open('columns.csv'))  # calculations numofsamples = open('columns.csv').read().count('\n') sumdata = sum(float(row['col1']) row in file1) aver = sumdata/(numofsamples - 1) # compute average of data in 'col1'   # subtracting average each value in 'col1' data = [] row in file2:     data.append(row['col1'])  # print results print aver print [e-aver e in data] # trying use comprehension subtract average each value in list 'data'   

i not know how solve problem! idea how make comprehension working give supposed do?

the issue in code in case of data list (file2) , reading strings file , storing strings data list.

hence, when later on , try - [e-aver e in data] - errors out trying subtract float string.

you should convert float or int before storing data list. example -

data = [] row in file2:     data.append(float(row['col1']))