• This is a module in Python. 
    csv
  • To open the file for reading, we can do: 
    with open('10_02_us.csv', 'r') as f:
       # We take that file object and pass it to a new csv reader object
       # The reader object is not a list
       reader = csv.reader(f)
       for row in reader:
       		 print(row)
  • The output of the above example would print out all of the rows in the CSV file.
    • This would work for a comma delimited value.
  • If you run type(reader), you will see the output as: 
    _csv.reader
  • For a tab separated value we would do: 
    with open('10_02_us.csv', 'r') as f:
       reader = csv.reader(f, delimiter='\t')
       for row in reader:
       		 print(row)
  • The output would be that all of the values are split up.
  • If you want to skip the header, you do: 
    with open('10_02_us.csv', 'r') as f:
       reader = csv.reader(f, delimiter='\t')
       # Add this to skip the header (uses the `next` function)
       next(reader)
       for row in reader:
       		 print(row)
  • It is possible to call next(reader) multiple times to skip over the row.
  • The alternative is to convert it to a list: 
    with open('10_02_us.csv', 'r') as f:
       reader = list(csv.reader(f, delimiter='\t'))
       for row in reader[1:]:
       		 print(row)
  • The above configuration will also skip over the header as well.
  • If you want to use the header data, using the dict reader is a good fit: 
    with open('10_02_us.csv', 'r') as f:
       reader = list(csv.DictReader(f, delimiter='\t'))
       for row in reader:
       		 print(row)
  • The header is used as the keys in each dictionary in the list.
  • To convert it to a list object, we can do: 
    with open('10_02_us.csv', 'r') as f:
       data = list(csv.DictReader(f, delimiter='\t'))
  • To find postal codes that are only divisible by 1 and themselves (prime), we have: ``` with open(‘10_02_us.csv’, ‘r’) as f: data = list(csv.DictReader(f, delimiter=’\t’))

This code block gets all of the prime numbers between 2 and 99999

primes = [] for number in range(2, 99999): for factor in range(2, int(number**0.5)): if number % factor == 0: break else: primes.append(number)

Next we filter these for prime locations. Limiting the search in the below example:

data = [row for row in data if int(row[‘postal code’]) in primes and row[‘state code’] == ‘MA’]

* If we run the above and then run `len(data)`, the total output would be `91`.
* To write this back to a CSV file:

with open(‘10_02_us.csv’, ‘r’) as f: data = list(csv.DictReader(f, delimiter=’\t’))

This code block gets all of the prime numbers between 2 and 99999

primes = [] for number in range(2, 99999): for factor in range(2, int(number**0.5)): if number % factor == 0: break else: primes.append(number)

Next we filter these for prime locations. Limiting the search in the below example:

data = [row for row in data if int(row[‘postal code’]) in primes and row[‘state code’] == ‘MA’]

with open(‘10_02_ma_prime.csv’, ‘w’) as f: # Can make the delimiter tabs, by passing a keyword to the writer argument below. By default a comma is used writer = csv.writer(f) for row in data: # Have to pass in the row as a list writer.writerow([‘place name’], row[‘county’]) ```

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