Transcript lect04 - Duke Computer Science

Today’s topics
Notes from Zachary Dodds’ CS 5 course at Harvey Mudd
Upcoming
 More Python
Reading
How to Think, 7.1-7.7 & 9.1-9.12
CompSci 001
4.1
Data and lists?
‘Many years later, as he faced the firing squad,
Colonel Aureliano Buendia was to remember that
distant afternoon when his father took him to
discover ice.’
{ 2, 3, 5, 7, 11 }
Sets
Can all information be
represented using lists ?
Sounds/Speech
Networks
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Text
Images/Video
4.2
Ideas?
Inside the machine…
What's happening in python:
x = 41
y = x + 1
What is happening behind the scenes:
"variables as containers"
42
41
name: y
type: int
LOC: 304
name: x
type: int
LOC: 300
memory location 300
Computation
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memory location 304
Data Storage
id, 4.3del
Python Data Types
Numeric
Name
Example
float
3.14
values with a
fractional part
long
10**100
integers > 2147483647
int
bool
42
True
False
What is it?
integers <= 2147483647
the results from a comparison:
==, !=, <, >, <=, >=
"Boolean value"
CompSci 001
4.4
Datatypes as genes…
Dominant
What will these results be?
float
1.0 / 5
long
10**100 - 10**100
int
1 / 5
bool
41 + True
Recessive
CompSci 001
4.5
Precedence
( )
Caution Level
Highest
**
-
*
/
+
%
Python
Operators
set equal to
=
divide
/
remainder
%
power
**
is equal to
==
*
-
+
>
<
==
>
as usual
=
Lowest
<
-
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( 4.6
)
% the "mod" operator
x%y returns the remainder when x is divided by y
7 % 3
8 % 3
9 % 3
16 % 7
x%2 == 0
For what values of x are
these True?
x%2 == 1
x%4 == 0
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What happens on these years?
4.7
Computer Memory
Random Access Memory (RAM)
is a long list of memory locations
on or off
bit = 1 "bucket" of charge
byte = 8 bits
word = 4 bytes = 32 bits
42
is it really a coincidence
that this looks like the
string theory picture??
name: x
type: int
LOC: 300
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4 bytes for an int
4.8
string functions
str
len
+
*
str(42) returns '42'
converts input to a string
len('42') returns 2
returns the string’s length
'XL' + 'II' returns 'XLII'
concatenates strings
'VI'*7 returns 'VIVIVIVIVIVIVI'
Given these strings
What are
repeats strings
s1 = "ha"
s2 = "t"
s1 + s2
2*s1 + s2 + 2*(s1+s2)
CompSci 001
4.9
String surgery
s = ’duke university baby'
0 1 2 3 4 5 6 7 8 9
s[ ]
10
11
12
13
14
15
16
18
17
19
indexes into the string, returning a one-character string
index
s[0] returns ’d'
Read "s-of-zero" or "s-zero"
s[6] returns
s[ ] returns 'e'
Which index returns 'e'?
s[len(s)] returns
CompSci 001
4.10
python != English
More on strings

Negative indices count from the back
s[-1]
returns ‘y'
s[-11] returns
s[-0]
 s[ : ]

returns
s[ : : ]
slices the string, returning a substring
skip-slices, returning a subsequence
the third index is the "stride"
length
it defaults
to 1
CompSci 001
4.11
Lists ~ Strings of anything
Commas
separate
elements.
L = [ 3.14, [2,40], 'third', 42 ]
Square brackets tell python you want a list.
len(L)
L[0]
Indexing: could return a different type
Slicing: always returns the same type
L[0:1]
How could you
extract from L
CompSci 001
'hi'
4.12
Raising and razing lists
Name(s):
pi = [3,1,4,1,5,9]
"Quiz"
Q = [ 'pi', "isn't", [4,2] ]
message = 'You need parentheses for chemistry !'
Part 1
What are
Part 2
len(pi)
len(Q)
len(Q[1])
What are
What slice of pi is [3,1,4]
What slice of pi is [3,4,5]
Q[0]
Q[0:1]
Q[0][1]
Q[1][0]
What is message[9:15]
What are
pi[0] * (pi[1] + pi[2])
and
What is message[::5]
pi[0] * (pi[1:2] + pi[2:3])
Extra! Mind Muddlers
CompSci 001
What is pi[pi[2]]?
4.13
How many nested pi's before pi[…pi[0]…] produces an error?
Functioning in Python
Some basic, built-in functions:
abs
bool
absolute value
max
float
min
int
of lists
sum
range
round
these change data from
one type to another
long
list
str
creates lists
only as accurately as it can!
These are the most important:
CompSci 001
help
dir
4.14
Functioning in Python
Far more are available in separate files, or modules:
import math
accesses math.py's functions
math.sqrt( 1764 )
dir(math)
lists all of math.py's functions
from math import *
pi
same, but without typing math.
all of the time…
sin( pi/2 )
CompSci 001
help()
help modules
4.15
Functioning in Python
# my own function!
def dbl( x ):
""" returns double its input, x """
return 2*x
CompSci 001
4.16
Functioning in Python
# my own function!
def dbl( x ):
""" returns double its input, x """
return 2*x
keywords
Some of Python's baggage…
def starts the function
return stops it immediately
and sends back the return value
Comments
They begin with #
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Docstrings
They become part of python's built-in help system!
With each function be sure to include one that
(1)
describes overall what the function does, and
(2) explains what the inputs mean/are
4.17
Functioning in Python
# is it dis-0 or dis-O, anyway?
def undo(s):
""" this "undoes" its string input, s """
return 'de' + s
>>> undo('caf')
>>> undo(undo('caf'))
CompSci 001
strings, lists, numbers … all data are fair
game
4.18
random thoughts
import random
random.choice( range(0,100) )
These are the most important:
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help
dir
4.19
DNA
Our DNA's nucleotides:
http://genome.ucsc.edu/cgi-bin/hgGateway
AGCT
CompSci 001
4.20