Python algorithm of counting occurrence of specific word in csv

Question

I've just started to learn python. I'm curious about what are the efficient ways to count the occurrence of a specific word in a CSV file, other than simply use for loop to go through line by line and read.

To be more specific, let's say I have a CSV file contain two columns, "Name" and "Grade", with millions of records.

How would one count the occurrence of "A" under "Grade"?

Python code samples would be greatly appreciated!

Answer 1

Basic example, with using csv and collections.Counter (Python 2.7+) from standard Python libraly:

import csv
import collections

grades = collections.Counter()
with open('file.csv') as input_file:
    for row in csv.reader(input_file, delimiter=';'):
        grades[row[1]] += 1

print 'Number of A grades: %s' % grades['A']
print grades.most_common()

Output (for small dataset):

Number of A grades: 2055
[('A', 2055), ('B', 2034), ('D', 1995), ('E', 1977), ('C', 1939)]

Answer 2

You should of course read all the grades, which in this case also means reading the entire file. You can use the csv module to easily read comma separated value files:

import csv
my_reader = csv.reader(open('my_file.csv'))
ctr = 0
for record in my_reader:
    if record[1] == 'A':
        ctr += 1
print(ctr)

This is pretty fast, and I couldn't do better with the Counter method:

from collections import Counter
grades = [rec[1] for rec in my_reader] # generator expression was actually slower
result = Counter(grades)
print(result)

Last but not least, lists have a count method:

from collections import Counter
grades = [rec[1] for rec in my_reader]
result = grades.count('A')
print(result)