Exercises (CSCI-UA.0002

Counting Letters

write a program that asks the user for a word
the program should output the total number of times each character in the word appears in the word
example output:

>hello
h 1
e 1
l 2
o 1

Counting Letters Version 1

word = input("Enter a word\n>")
for c in "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz":
	count = word.count(c)
	if count > 0:
		print(c, count)

Counting Letters Version 2

word = input("Enter a word\n>")
d = {}
for c in word:
	try:
		d[c] += 1
	except KeyError:
		d[c] = 1
for k,v in d.items():
	print(k, v)

Counting Letters Version 3

word = input("Enter a word\n>")
d = {}
for c in word:
	d[c] = d.get(c, 0) + 1
for k,v in d.items():
	print(k, v)

Counting Dice Rolls

roll two thee sided dice simultaneously 1000 times
count the frequency of the results… 2 through 6
pseudocode
- generate two random numbers
- do this 1000 times
- keep track of the number of times a two is rolled… a three… up through six
- use multiple assignment for initializing your counts!

An Implementation Using Dictionaries

import random
freq_dice_rolls = {}
for i in range(1000):
	roll = random.randint(1,3) + random.randint(1,3)
	freq_dice_rolls[roll] = freq_dice_rolls.get(roll, 0) + 1
print(freq_dice_rolls)

Dice Rolls Solution

Let's compare that to our previous solution without dictionaries!

import random
twos, threes, fours, fives, sixes = 0, 0, 0, 0, 0
for i in range(1000):
	roll = random.randint(1,3) + random.randint(1,3)
	if roll == 2:
		twos += 1
	if roll == 3:
		threes += 1
	if roll == 4:
		fours += 1
	if roll == 5:
		fives += 1
	if roll == 6:
		sixes += 1

print("twos: %s, threes: %s, fours: %s, fives: %s, sixes %s" % (twos, threes, fours, fives, sixes))

A List of Contacts

Create a text-based contact list that allows you to store first name, last name and room # →

It should support the following functionality:

(a)dd contact
(p)rint all contacts
(f)ind contact by first name
(q)uit

The primary interface will be a prompt that will continually ask the user for a letter (a, p, f or q)…

Add a Contact Example Interaction

adding will ask for further input…
first, last and room #

(a)dd contact, (p)rint all contacts, (f)ind contact, (q)uit 
>a
first name plz 
>tim
last name plz 
>test
room # plz 
>200

Find a Contact Example Interaction

Finding a contact:

asks for more input - the first name of the person to find
and prints out the contact

(a)dd contact, (p)rint all contacts, (f)ind contact, (q)uit 
>f
what's the firs name of the person you'd like to find? 
>tim
last name - test
first name - tim
room - 200

Print All Contacts Example Interaction

(a)dd contact, (p)rint all contacts, (f)ind contact, (q)uit 
>p
last name - test
first name - tabitha
room - 100

last name - test
first name - tim
room - 200

Before Diving In…

how do we want to represent the data?
can we break down the program into smaller chunks
diagram if necessary (flow charts, sequence diagrams, etc.)
write some pseudocode

Storage

How about using a list of dictionaries to store contacts?

Here's an example with one contact:

[{'first name': 'tabitha', 'last name': 'test', 'room': 100}]

Some Functions

We can break down the code into smaller chunks of functionality:

def contact_as_string(contact):
	s = ''
	for attribute, value in contact.items():
		s += '%s - %s\n' % (attribute, value)
	return s

def print_all_contacts(contact_list):
	for c in contact_list:
		print(contact_as_string(c))

And More Functions

def find_contact(contact_list, attribute, value):
	for c in contact_list:	
		if c[attribute] == value:
			return c
	return None

def find_contact_by_first(contact_list, first):
	return find_contact(contact_list, 'first name', first)

The Main Loop…

We can use the previous functions in a while loop that drives the interaction with the user:

contacts = [{'first name': 'tabitha', 'last name': 'test', 'room': 100}]
while True:
	command = input('(a)dd contact, (p)rint all contacts, (f)ind contact, (q)uit \n>')
	if command == 'a':
		first = input('first name plz \n>')
		last = input('last name plz \n>')
		room = input('room # plz \n>')
		contacts.append({'first name': first, 'last name': last, 'room': room})
	elif command == 'p':
		print_all_contacts(contacts)
	# continued in next slide...

Exercises