CPSC120A - Day 26 Notes

Can change a value by assigning

a_dict = {'butyraceous': 'buttery', 'inutile': 'no use'}
a_dict['butyraceous'] = 'like butter'
print(a_dict['butyraceous'])

Can create new entries by assigning to keys that are not already in the dictionary

a_dict = {'butyraceous': 'buttery', 'inutile': 'no use'}
a_dict['Pickwickian'] = 'odd or unusual'
print(a_dict['Pickwickian'])

Can remove from a dictionary by using del

a_dict = {'butyraceous': 'buttery', 'inutile': 'no use'}
del a_dict['butyraceous']
print(a_dict['butyraceous']) # an error

A histogram is way of visualizing the distribution of data
A histogram is a bar chart where the bar sizes coorespond to the the number of values in a range
For example:

In addition to visualizing distribution, a histogram can be used to break a substitution cipher
Recall the ceasar cipher you coded a couple of weeks ago, where characters are shifted some amount in the alphabet
Breaking a message encoded this way is simple, just try all shifts until you can read the message.
Recall the substitution cipher, each letter is mapped to a different letter
Breaking this by brute force would require 26!, or 403291461126605635584000000, attempts
If the message is long enough, we can take advantage of the language it is written in to break it more quickly
In English, the most frequently occuring letter is ‘e’
As an aside, this is why there are a lot of E tiles that aren’t worth very much in Scrabble.
So, we find the most frequently occuring letter in the encoded message and replace it with ‘e’
The second most frequently occuring letter is ‘t’, replace the second most frequently occuring with that
Repeat until the message starts to make sense
Finding the number of occurances of each letter is a histogram

CPSC120A
Fundamentals of Computer Science