1. Compsci 101, APTs and Sequences and Selection
Owen Astrachan
Kristin Stephens-Martinez
January 25, 2018
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

2. Compsci 101, Spring 2018, Sequences+Selection
D is for …
Debugging
A key skill in making your programs run
Data (Science)
Creating information from 0's and 1's
Dictionary
Ultimate Python Data Structure
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

3. Compsci 101, Spring 2018, Sequences+Selection
PFTD and toward PFTW
On Tuesday we saw:
Functions in Python
Decomposition, parameters, calling
Flow of Control
How does a program execute, how do function calls work?
Program Development and Execution
How do you run a program, test a program, debug a program
Remind students what we did last week, review functions, function calls. Differentiate. We’ll discuss more of that today
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

4. Compsci 101, Spring 2018, Sequences+Selection
Today we’ll see …
Quick review of functions
Foundation of writing complex programs in any language, grow a program, don't build it
Understanding and using sequences
Indexing and Slicing
Selective execution
Guarding statements depending on conditions
Functions are great. We need more tools. We need sequences, strings and lists and we need selective execution
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

5. Compsci 101, Spring 2018, Sequences+Selection
Organization Matters
This is an all time favorite of mine, may not be something you want to do? Apparently Susan still uses it too I think when Robert de Niro looks at Cybil Shepard in the clip it's clear that he's infatuated with her. It's a long time ago. We are still infatuated with teaching and coding and we hope they will be. We also want to be organizized with building programs.
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

6. Compsci 101, Spring 2018, Sequences+Selection
WOTO Redux
Sneak Preview: lists and tuples
Last time, do it and review the concepts
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

7. Compsci 101, Spring 2018, Sequences+Selection
Functions Summarized
Function call and Function definition related
Call must provide correct arguments
Names don’t matter, types are important: arguments match parameters
Functions help design, implement, organize
Without functions no APIs, no big programs
Opening a file would be 100’s of lines, instead it’s f = open(“hello.txt”)
Differentiate arguments and parameters
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

8. Lynn Conway See Wikipedia and lynnconway.com Joined Xerox Parc in 1979
Revolutionized VLSI design with Carver Mead
Joined U. Michigan 1985
Professor and Dean, retired '98
NAE '89, IEEE Pioneer '09
Helped invent dynamic scheduling early '60s IBM
Transgender, fired in '68

9. Compsci 101, Spring 2018, Sequences+Selection
Problem Interlude
We’re going to solve an APT together. Help with APT-0 due today and APT-1 due a week from today
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

10. APT Pancake: http://bit.ly/pancakes101
How do you solve this (or any) problem?
Run through examples, construct a method for solving – translate to algorithm for Python
If you can’t do this with paper/pencil? Tough!
Some APTs are hard problems to solve
Translating to code easy, it’s the problem
Some APTs have easy-to-see algorithms
Translating to code is hard
Read the problem. Look at an example and discuss why some are correct.
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

11. Solving (APT) Problems
Identify simple cases that are quickly solved
What are these in Pancake problem?
Don’t solve for N, solve for 5, then generalize
When there are two parameters? Fix one, vary the other one
Helps identify cases
Basic problem solving ideas.
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

12. Compsci 101, Spring 2018, Sequences+Selection
WOTO: Pancakes
Correctness counts, score > 1/3 == 100%
After discussion test student understanding
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

13. Three pancakes in a two-cake pan
Number of cakes in the system
First 5 minutes
Number of cakes in the system
Second 5 minutes
Walk through cases after going over examples. A' means A cooked on one side. A'' means cooked on both sides
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

14. Three pancakes in a two-cake pan
Number of cakes in the system
Third 5 minutes
How many minutes to cook all three pancakes?
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

15. Methodically by hand, small values
Pan has capacity 8, vary #pancakes
Can you cook 11 in 15 minutes? Why?
Can you cook 13 in 15 minutes? Why?
cakes 5 6 7 8 9 10 11 12 13 14 15 16 17 18
time ? 1 2 3 4 5 6 7 8 10 11 9 12
For 12, shown, we can cook in 15 minutes. Take 5-8 out and cook 1-4 all the way. Then finish them up. Can't do 13 though. Explain
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

16. Methodically by hand, small values
Pan has capacity 8, vary #pancakes, cook 12?
cakes 5 6 7 8 9 10 11 12 13 14 15 16 17 18
time 1 2 3 4 5 6 7 8 1 2
Filled in more from last slide, can we do 2? 3 11 5 6 7 8 4 10 9 12
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

17. Methodically by hand, small values
Pan has capacity 8, vary #pancakes, 17 cakes?
cakes 5 6 7 8 9 10 11 12 13 14 15 16 17 18
time 20 1 2 3 4 5 6 7 8 1 2
We can do in 20 minutes, what about 17? 3 11 5 6 7 8 4 10 9 12
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

18. Methodically by hand, small values
Pan has capacity 8, Generalize to algorithm?
cakes 5 6 7 8 9 10 11 12 13 14 15 16 17 18
time 20 25 1 2 3 4 5 6 7 8 1 2
Can we generalize? 3 11 5 6 7 8 4 10 9 12
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

19. Compsci 101, Spring 2018, Sequences+Selection
Pancake Algorithm
If you have pan of size 17 and 34 pancakes
If you have pan of size 17 and 43 pancakes
PAN fits 100 pancakes, but you have 452
PAN fits N pancakes, but you have P
if P <= N then time needed is …
X = P // N what does this mean for time?
Y = P % N what does this mean for time?
Summarize, review // and % with examples like N = 8 (PAN just did) and # pancakes = 12 and 13 (just did)
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

20. From Algorithm to Python
We need one more tool: selection
Execute different code depending on …
What is a vowel? When is a year a leap-year?
Is there a minimal age for US President?
If condition is true then do something
Condition: true or false
Something: any Python code
Motivate selection, example coming
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

21. Three versions of is_vowel
def is_vowel(ch): if ch =='e': return True if ch == 'a': if ch == 'i': if ch == 'o': if ch == 'u': return False
def is_vowel(ch): c = "aeiou".count(ch) if c > 0: return True else: return False
def is_vowel(ch):
return "aeiou".count(ch) > 0
Simple if statement, return. That’s all we need for APTs. Emphasize boolean part of if statement
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

22. Finishing the APT Pancake Problem
Use the algorithm developed AFTER verifying with pencil and paper aka by hand
Test first in our Eclipse (VM or Local)
Create main program block
Call minutesNeeded with several examples
Use APT testing system, submit, reflect
Can we have more than one return statement?
Develop from scratch with help from class. Get all green
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

23. How to teach pancake flipping
Is this computer science?
For longer, more complex robotic tasks
Do robots matter?
Do they dream?
Self-driving cars?
Machine learning?
Who developed a first upper bound on pancake sorting? Bill Gates
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

24. What problems can we solve?
We can create arithmetic expressions
+, -, *, /, %, **, // and more
Import math and math.sqrt(..), math.sin(..)
We can create and combine strings
Using + or * for example
We can write and call functions
That can use arithmetic expressions and strings 
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

25. Compsci 101, Spring 2018, Sequences+Selection
Selection Summarized
We can use selection: if statement
if boolean condition: block
What can change? Boolean and block
What is a boolean condition? True/False
See type(3 < 5)
Relational operators: <, <=, >, >=, ==, !=
Boolean operators: and, or, not
Won’t do this in detail, but a good summary. Show type(3 < 5) in pydev console
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

26. Additional Tools for Building
We need sequences to access and create data
Documents rather than words
Spread sheets rather than single data/formula
Operations on these sequences
Access all elements: loops
Access elements selectively: if statements
Motivate sequences. Collections of elements. Analogy as shown. We need operations on sequences. We'll show a few today, more next week
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

27. Compsci 101, Spring 2018, Sequences+Selection
Python Sequences
Types String and List share characteristics
Both are sequences, have a length
Both support indexing and slicing
Conversion functions help connect them
x="hello world" y=["hello”, "world"]
What is len(x)?
String is immutable, list is mutable
Two important examples. Preview with mutable, immutable, don't dwell
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

28. Indexing Python Sequences
x="hello world" y=["hello", "world"]
Indexing provides access to individual elements
Compare x[0]and y[0]
Start with 0, what is last valid positive index?
Compare x[-1] and y[-1]
What is negative index of second to last element?
Index –n is the same as index len(seq) - n
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

29. Slicing Python Sequences
x="hello world"
y=["my", "big", "beautiful", "world"]
Slicing provides sub-sequence (string or list)
Compare x[0:3]and y[0:3]
What is length of subsequence? seq[2:4]
Compare x[4:-1] and y[2:-1]
Is last index part of subsequence?
We can omit value, e.g., x[2:] or x[:4], good shortcut!
Show in pydev console
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

30. Compsci 101, Spring 2018, Sequences+Selection
One more APT
Let’s have Brunch with the Digerati
Read the problem statement carefully
Why can’t we create netizen in function? Combination of “internet” and “citizen”
How do we solve with slicing?
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

31. Anatomy of Python String
String is a sequence of characters
These are strings of length 1
Functions can be applied to strings: len
Operators can be applied: [n] and [m:n]
Methods/functions applied to strings
"HELLO".lower()
"the duke way".split(" ")
Review these, split needed for one APT
1/25/2018
Compsci 101, Spring 2018, Sequences+Selection

32. Bug and Debug software 'bug' Start small Judicious 'print'
Easier to cope
Judicious 'print'
Debugger too
Verify the approach being taken, test small, test frequently, add to working code
What does “working code” mean?