In-class notes for 03/19/2014 (CS 121B (CS1), Spring 2014)

In-class notes for 03/19/2014

CS 121B (CS1), Spring 2014

Homework assignment
Quiz

Submitted questions on assignments and technology

Upcoming

Homework 17
Motivation -- text analysis

Read first section of http://selkie.macalester.edu/csinparallel/modules/IntroWMR/build/html/index.html:
- Using Parallelism to Analyze Very Large Files: Google's Map Reduce Paradigm
Submit at least one reading question by 9am before the next class meeting

Summary of strings chapter

Files

open()
Traversing line-by-line with a for loop
Methods close(), read(), write()

Dictionaries

Exercises

Exercise: Write a function that reads lines from a file and returns a dictionary containing frequency counts for all the words in that file.

countWords

One argument: A string, representing a file name
Return: A dictionary whose keys are the words found in that file arg1, and whose values are the number of occurrences of those words in that file arg1.

Example calls:

    countWords("cathat.txt")) 
    --> {'The':1, 'cat':2, 'in':1, 'the':3, 'hat':3, 'wore':1, 'to':1, 'party':1}

Hints:

Use open() and close() to create a connection to the file near the beginning of your function and to disconnect near the end of your function.
The dictionary contains accumulators for all of the words found in that file.
Check to see if a word's count is already present in the dictionary. Example:
```
    words = { 'The': 1, 'cat': 1, 'in': 1 }
    'cat' in words --> True
    'hat' in words --> False
```
If a word isn't already a key, assign 1 to that key in the dictionary; otherwise, increment that key's prior value.

Exercise: Write a function that reads lines from a netflix data file and returns a dictionary containing frequency counts of ratings for each movie.

countRatings

One argument: A string, representing the name of a netflix data file.
Return: A dictionary whose keys are the movie ids found in that file arg1, and whose values are the number of ratings given by all reviewers to that movie in that file arg1. Return False if the data file fails to open.

Example calls:

    countRatings("netflix.dat")) 
    --> {1:3, 2:1, 3:7, 5:7, 6:6, 8:22}

Hints:

Netflix format

Submitted questions on readings

< >