Q and A for Sections 2.6 – 2.8
CS 106
Victor Norman
Pure Functions
Q: Why are there pure functions? Why not just
normal functions?
A: You may call them “normal” functions, if you
want. They really are just normal functions:
they take parameters and compute and return a
value.
Q: What makes a function “pure”?
A: It only thinks clean thoughts…
Pure Functions
Q: If the syntax for a method call is
object.method(parameters), then what is the
syntax for a pure function call?
A: function(parameters)
Q: What is a non-pure function called? An
“impure” function?
A: It is called a bound function or member
function, because it belongs to an object.
Pure function?
Q: What is this? A pure function or a member
function?
res = math.sin(0.25)
A: It is a pure function, in the math module.
Sometimes the "thing" before the . is an object,
sometimes it is a module. Sometimes it is really
hard to tell the difference.
Q: If sin() were a bound function, how would we
write it?
A: 0.25.sin(), I guess. Which is really weird...
More examples of pure functions
• look at page 62
Q: Is the converting letters to numbers function
(ord()) commonly useful?
A: No.
min() and max()
Q: Can you call min() or max() of a list that isn’t
integers or floats?
A: Yes! As long as the items in the list can be
compared to each other, there is a min() and a
max() value.
imports
Q: Suppose we want to use the value for pi in
our code. The book says there are 3 ways to
import code from a library. Here are 2 ways.
import math
from math import pi
What is the 3rd?
A: from
math import *
Qualified names
Q: Which of the 3 ways of importing a library
requires you to use qualified names?
A: The first. E.g.,
import math
(BTW, this is the preferred way, IMO. Why? So
we don't "pollute" the namespace.)
What’s in a library?
Q: How does a young enterprising student find
out what libraries are available and what's in
these libraries?
A: google
Huh?
Student writes, ‘that eerie sense that “I’ve
experienced this before.” Cues from the current
situation may unconsciously trigger retrieval of
an earlier experience’
Precedence
Q: I understand the concept of precedence but
the evaluation trees make it a little confusing.
Will we need to be able to draw evaluation
trees?
A: No.
Boolean
• In python, the type is bool. Two possible values:
True and False.
• We don't often have variables of type bool, but
we can.
found = False
for female in listOfFemales:
if female.hairColor == “brown”:
found = True
possibleWife = female
if found:
askOnDate(possibleWife)
Figure 2.12
Q: Do we have to memorize Figure 2.12?
A: No. In your gut you know this table anyway.
Practice with boolean expressions
• How do you decide which class to sign up for?
How do you decide which section to sign up for?
• How do you decide whether or not to have
another slice of pizza?
• How do you decide what to eat in the dining hall?
• How do you decide when to go to sleep?
• How do you decide when to ask someone on a
date?
Inclusive vs. exclusive or
Q: What is the difference between inclusive 'or'
and exclusive 'or'?
A: Exclusive or means one is True or the other is
True, but not both.
It is almost never used. You can use != .
Practice
• You have variables x and y which are the
location of your ball on the screen that is size
600 by 400 (600 across, 400 down). Write
code to set inCorner to True if the ball is
within 20 pixels of the right side and 20 pixels
of the bottom (i.e., in the lower-right corner).
inCorner = (x >= 580 and y >= 380)
• Now, make a variable inMiddle that is the
opposite of inCorner.
More
Write code to set nearEdge to be True if the ball
is 20 pixels within the bottom edge or the right
edge.
nearEdge = (x >= 580 or y >= 380)
Now fix it to be True if it is within 20 of any
edge.
nearEdge = (x < 20 or x >= 580 or y < 20 or
y >= 380)
Even More
Write a line to set variable is5 to True if the value
in num is 5.
Write a line to set variable is56or7 to True if the
value in num is 5 or 6 or 7.
Write a line to set variable isMultOf2Or3 to True if
the value in num is a multiple of 2 or a multiple of
3.
Write a line to set variable iNotMultOf2Or3
Extra old slides
Multiple Functions on 1 line
Q: You can call multiple functions on the same
line of code? How does that work?
A: It is just the same as having values there, but
the functions have to be run ("evaluated") to get
the value. They are run in the order in which
the values are needed, according to precedence
runs.
Using common pure functions
Q: Suppose you are given a list, and you want to
iterate through the list using an index. I.e., you
want to get a list of integers 0, 1, 2, 3, ..., len-1,
where len is the number of items in the list. Use
range() and len() to do this, on list majors.
A: range(len(majors))
(Note to student: memorize this. You'll see it a lot.)
Compound expressions
Q: Rewrite this code as a single compound
expression, on one line:
tmp1 = fehr - 32
multiplier = 5.0 / 9.0
cels = multiplier * tmp1
A: cels = (5.0 / 9.0) * (fehr - 32)
Assignment details
Q: Why does code like this work?
num_seen = num_seen + 1
A: It works because = is an operator and the
rules say that you evaluate each side and then
apply = (assignment) last. So, num_seen + 1 is
evaluated to some value, and then num_seen
is set to refer to that value.
Evaluating boolean expressions
Q: What does this evaluate to, True or False?:
is_cute = True
has_money = False
has_a_car = True
(is_cute and not has_money and has_a_car)
or has_money
A: True: (T and T and T) or F
Is True true?
Q: Shorten this code:
if is_cheap == True:
buy_it()
A: if is_cheap:
buy_it()
Eval Boolean Expressions (2)
Q: What does the last line below evaluate to?
is_dutch = True
is_much = <this value hidden from view>
is_dutch or is_much
A: True
Eval Boolean Expressions (3)
Q: Are these equivalent?:
not (is_tall or is_employable)
not is_tall or not is_employable
A: No: if the second one were
not is_tall and not is_employable
then they would be equivalent
(this is something called DeMorgan's Law)
Eval Boolean Expressions (4)
Q: What does this do?:
if len(names_list) > 20 and \
names_list[-1].startsWith(’Glaza'):
do_something()
A: do_something() is called if there are more
than 20 names in the list and the last name in
the list starts with the string ’Glaza'
Hint for upcoming assignment…
Q: Picking off a word from within a line is a
common task. Rewrite this code to use one
line:
words = line.split()
third_word = words[2]
A: third_word = line.split()[2]