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= Lecture 3 =

Lets review! - lists - dictionaries - strings - comparison operators

== Command line arguments ==

The standard library has <code>sys</code> which give us access to the command line arguments executed.

<source lang="python">import sys

if '-h' in sys.argv or '--help' in sys.argv:
print('Sorry I can\'t help you :(')</source>
But <code>argparse</code> from the standard library is much easier if we want to use the commandline arguments.

== File opening ==

<source lang="python">f = open(filename)
# interact with the file
f.close()

# is the same as
with open(filename) as f:
# interact with the file</source>
here is an example of a script that opens a file, and print all lines in the file. The file name is specified as the first command line argument

<source lang="python">"""
Print all lines in file
"""
import argparse

def main():
parser = argparse.ArgumentParser()
parser.add_argument('file_name')
args = parser.parse_args()
text_file = open(args.file_name)
for line in text_file:
cleaned_line = line.strip() # Remove newline, i.e. '\n' or '\r\n'
print(cleaned_line)
text_file.close()

if __name__ == '__main__':
main()</source>
== TASK 1 ==

Print out a dictionary with words and the number of occurences of the word.

Example output:

{'Apple': 10, 'Bananan': 1}

=== How to count words in a list of words ===

<source lang="python">>>> words = ['hello', 'hi', 'hello']
>>> wc = {}
>>> for word in words:
... if word in wc:
... wc[word] = wc[word] + 1
... else:
... wc[word] = 1
...
...
...
>>> wc
{'hello': 2, 'hi': 1}
>>>
</source>
== Task 2 ==

Make a new csv file and save it, where the age become year born.

=== Solution ===

<source lang="python">"""
convert input csv to output csv
"""
import argparse
import csv

def convert_age_in_csv(data_in, data_out):
reader = csv.reader(data_in)
writer = csv.writer(data_out)
for row in reader:
name = row[0].strip()
age = int(row[1])
year_born = 2017 - age
writer.writerow([name, year_born])

def main():
parser = argparse.ArgumentParser()
parser.add_argument('data_in')
parser.add_argument('data_out')
args = parser.parse_args()

with open(args.data_in, 'r') as data_in:
with open(args.data_out, 'w') as data_out:
convert_age_in_csv(data_in, data_out)

if __name__ == '__main__':
main()
</source>
How to execute

<pre class="shell">python3 csvconverter.py input.csv output.csv</pre>

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