CompSci 101
Introduction to Computer Science
January 15, 2015
Prof. Rodger
1
Don’t forget the rule
• You can’t sit in the last two rows
• That’s you!
2
Announcements
• Want a permission number to add course?
– Fill out form on course web page (main page)
• Reading for next time, do RQ 3
• Assignment 1 due Tuesday, Jan 20
• APT 1 is out and due on Thursday, Jan 22
• Today – Problem Solving, Python practice,
solve an APT
3
Starting with Python
• Variable
– Name of a storage location – holds a value
= to assign a value to a variable
• Type
– Different types of data
– A variable can stores data of a certain type
– int, float, str
• operators in Python for numbers
– Arithmetic: +
-
*
/
%
**
• Built-in functions: pow, abs, round, int, float
example:
pow(2,3) + round (1.6) 4
Eclipse – Three ways to run
1. Write program and store in file
– Create a PyDev project – a folder for programs
– Create a PyDev module for each program (file)
– Run in console
2. Snarf an APT and run in Eclipse
3. Run interactively
– Open PyDev console
– Execute each line as typed
– Code not saved
5
Variables, Types, Expressions?
a=5
b=4
c = "fred"
print c
print a + b * 3
print (a + b) * 3
print a / b
print a / (b * 1.0)
6
Demo: Run interactively in
Eclipse PyDev Console
• If Console window is not showing then
– Click on Window, Show View, Console
• Then at the bottom of Eclipse, click here:
• Select PyDev Console, Python Console
7
Latanya Sweeney
I am a computer scientist with a long
history of weaving technology and policy
together to remove stakeholder barriers to
technology adoption. My focus is on
"computational policy" and I term myself a
"computer (cross) policy" scientist. I have
enjoyed success at creating technology that
weaves with policy to resolve real-world
technology-privacy clashes.
http://latanyasweeney.org/
Identify 87% of US population using
(dob,zip,gender). Director of Harvard
Data Privacy Lab, instrumental in HIPAA
because if de-identification work,
currently Chief Technologist FTC
8
Function – define vs call
• def functionName(parameters):
block
• def sum(a, b):
return a+b
• Call function
sum(7, 4)
sum(“a”, “cat”)
• Advantages
– Repeat code, call multiple times
– Flexible, call with different arguments
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Demo 2
• In Eclipse write a file with a function and
run
• Stuff.py
10
What is an APT? BMI APT
• Automated/Algorithmic Problem Testing
– Write one function, 2-30 lines, solve a problem
– Tested automagically in Eclipse or the browser
– Lots of test cases – test test test
• Start simple, build toward more complex
– What is a function? A function call?
– What is a parameter? Argument?
– How do you run/execute a program
11
Demo 3 on Solving APT BMI
• Write your code in Eclipse
• Snarf the apt testing materials
– Create the python file in apt folder
– Write your code
• Run in Eclipse or Run on the web
12
Skills and Practice for Game Playing
• http://www.youtube.com/watch?v=AEBbsZK39es
$193, $540, $820,
$700, $749. Are
these reasonable?
Why?
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I'm thinking of a number …
bit.ly/101S15-0113-03
• You guess. I'll tell you high, low, or correct
– Goal: guess quickly, minimal number of guesses
– Number between 1 and 100…
– Number between 1 and 1000…
• Can you describe an algorithm, instructions, that
would allow someone to use your instructions to
play this game correctly. Start with 1 and 100,
but ideally your instructions work with 1 and X
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Analyzing the binary search
algorithm
• Is the algorithm correct?
– Try it, again, and again and …
– Reason about it: logically, informally, …
• How efficient is the algorithm?
– How many guesses will it take (roughly,
exactly)
– Should we care about efficiency?
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Anderson
Applegate
Bethune
Brooks
Carter
Edwards
Foggle
Griffin
Holhouser
Jefferson
Klatchy
Morgan
Munson
Narten
Oliven
Parken
Rivers
Roberts
Stevenson
Thomas
Wilson
Woodrow
Yarbrow
Find Narten
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Anderson
Applegate
Bethune
Brooks
Carter
Edwards
Foggle
Griffin
Holhouser
Jefferson
Klatchy
Morgan
Munson
Narten
Oliven
Parken
Rivers
Roberts
Stevenson
Thomas
Wilson
Woodrow
Yarbrow
X
X
Find Narten
Found!
How many
words did we
look at?
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Searching for words
• If we had a million words in alphabetical
order, how many would we need to look at
worst case to find a word?
• 18
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Searching for words
• If we had a million words in alphabetical
order, how many would we need to look at
worst case to find a word?
1,000,000
976.56
500,000
488.
• 20
250,000
244
125,000
122
62,500
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If you are clever,
31,250
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Cut the number of numbers to look
15,620
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at in half over and over again,
7812.5
Then only 18 numbers to look
7.5
3906
at worst case
3.75
1953
1.875
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