cel shading as PDF and Python description

Chapter 2
Python for Everybody

ed values such as numbers, letters, and strings, are ca
nstants” because their value does not change
meric constants are as you expect
ng constants use single quotes (')
ouble quotes (")
>>> print(123)
123
>>> print(98.6)
98.6
>>> print('Hell
Hello world

annot use reserved words as variable names / identifi
e await else import pass
break except in raise
class finally is retur
continue for lambda try
def from nonlocal while
rt del global not with
c elif if or yield

able is a named place in the memory where a programmer ca
nd later retrieve the data using the variable “name”
ammers get to choose the names of the variables
an change the contents of a variable in a later statement
12.2
x
14
y
x = 12.2
y = 14

able is a named place in the memory where a programmer ca
nd later retrieve the data using the variable “name”
ammers get to choose the names of the variables
an change the contents of a variable in a later statement
12.2
x
14
y
100
x = 12.2
y = 14
x = 100

t start with a letter or underscore _
t consist of letters, numbers, and underscores
e Sensitive
ood: spam eggs spam23 _speed
ad: 23spam #sign var.12
ifferent: spam Spam SPAM

ce we programmers are given a choice in how we cho
able names, there is a bit of “best practice”
name variables to help us remember what we intend
hem (“mnemonic” = “memory aid”)
s can confuse beginning students because well-named
ables often “sound” so good that they must be keywor
http://en.wikipedia.org/wiki/Mnemonic

ocd = 35.0
afd = 12.50
afd = x1q3z9ocd * x1q3z9afd
x1q3p9afd)
is this bit of
de doing?

ocd = 35.0
afd = 12.50
x1q3p9afd)
a = 35.0
b = 12.50
c = a * b
print(c)
are these bits
code doing?

ocd = 35.0
afd = 12.50
x1q3p9afd)
hours = 35.0
rate = 12.50
pay = hours * rate
print(pay)
a = 35.0
b = 12.50
c = a * b
print(c)
are these bits
code doing?

2
x + 2
nt(x)
ble Operator Constant Functi
Assignment statemen
Assignment with exp
Print statement

sign a value to a variable using the assignment statem
ignment statement consists of an expression on the
and side and a variable to store the result
x = 3.9 * x * ( 1 - x )

x = 3.9 * x * ( 1 -
0.6
x
ide is an expression.
expression is evaluated, the
aced in (assigned to) x.
0.6 0
0.4
0.936
is a memory location
ore a value (0.6)

x = 3.9 * x * ( 1 -
0.6 0.93
x
0.4
0.936
de is an expression. Once the
is evaluated, the result is
ssigned to) the variable on the
., x).
s a memory location used to
e. The value stored in a
be updated by replacing the
.6) with a new value (0.936).
0.6 0

cel shading as PDF and Python description

use of the lack of mathematical
ols on computer keyboards - we
computer-speak” to express the
c math operations
sk is multiplication
nentiation (raise to a power) looks
ent than in math
O
Operator
A
+
Su
-
Mul
*
D
/
**
Re
%

xx = 2
xx = xx + 2
print(xx)
yy = 440 * 12
print(yy)
zz = yy / 1000
print(zz)
>>> jj = 23
>>> kk = jj % 5
>>> print(kk)
3
>>> print(4 ** 3)
64
Operator
+
-
*
/
**
%
5 23
4 R 3
20
3

n we string operators together - Python must know whi
first
s called “operator precedence”
h operator “takes precedence” over the others?
x = 1 + 2 * 3 - 4 / 5 ** 6

ecedence rule to lowest precedence rule:
ntheses are always respected
onentiation (raise to a power)
plication, Division, and Remainder
tion and Subtraction
to right
Paren
Po
Multip
Add
Left to

1 + 2 ** 3 /
1 + 8 / 4 *
1 + 2 * 5
1 + 10
11
x = 1 + 2 ** 3 / 4 * 5
print(x)
Parenthesis
Power
Multiplication
Addition
Left to Right

ember the rules top to bottom
n writing code - use parentheses
n writing code - keep mathematical expressions simple
hey are easy to understand
k long series of mathematical operations up to make th
clear
Parenth
Powe
Multiplic
Additi
Left to R

hon variables, literals, and
ants have a “type”
n knows the difference between
eger number and a string
xample “+” means “addition” if
thing is a number and
atenate” if something is a string
>>> ddd = 1 + 4
>>> print(ddd)
5
>>> eee = 'hello '
>>> print(eee)
hello there
concatenate = put

n knows what “type”
thing is
e operations are
bited
annot “add 1” to a string
an ask Python what type
thing is by using the
function
>>> eee = 'hello ' + 't
>>> eee = eee + 1
Traceback (most recent
File "<stdin>", line 1,
<module>
TypeError: can only con
str (not "int") to str
>>> type(eee)
<class'str'>
>>> type('hello')
<class'str'>
>>> type(1)
<class'int'>
>>>

bers have two main types
gers are whole numbers:
-2, 0, 1, 100, 401233
ating Point Numbers have
al parts: -2.5 , 0.0, 98.6, 14.0
e are other number types - they
ariations on float and integer
>>> xx = 1
>>> type (xx
<class 'int'
>>> temp = 9
>>> type(tem
<class'float
>>> type(1)
<class 'int'
>>> type(1.0
<class'float
>>>

n you put an integer and
ng point in an
ssion, the integer is
itly converted to a float
an control this with the
n functions int() and
>>> print(float(99)
199.0
>>> i = 42
>>> type(i)
<class'int'>
>>> f = float(i)
>>> print(f)
42.0
>>> type(f)
<class'float'>
>>>

division produces a floating
sult
>>> print(10 / 2)
5.0
>>> print(9 / 2)
4.5
>>> print(99 / 100
0.99
>>> print(10.0 / 2
5.0
>>> print(99.0 / 1
0.99
as different in Python 2.x

an also use int() and
to convert between
s and integers
will get an error if the string
not contain numeric
cters
>>> sval = '123'
>>> type(sval)
<class 'str'>
>>> print(sval + 1)
Traceback (most recent cal
File "<stdin>", line 1,
TypeError: can only concat
(not "int") to str
>>> ival = int(sval)
>>> type(ival)
<class 'int'>
>>> print(ival + 1)
124
>>> nsv = 'hello bob'
>>> niv = int(nsv)
Traceback (most recent cal
File "<stdin>", line 1, in
ValueError: invalid litera
with base 10: 'x'

can instruct Python to
se and read data from
user using the input()
ction
e input() function
urns a string
nam = input('Who are
print('Welcome', nam)
Who are you? Chuc
Welcome Chuck

e want to read a number
m the user, we must
vert it from a string to a
mber using a type
version function
er we will deal with bad
ut data
inp = input('Europe fl
usf = int(inp) + 1
print('US floor', usf)
Europe floor? 0
US floor 1

ing after a # is ignored by Python
comment?
scribe what is going to happen in a sequence of code
cument who wrote the code or other ancillary informat
n off a line of code - perhaps temporarily

# Get the name of the file and open it
name = input('Enter file:')
handle = open(name, 'r')
# Count word frequency
counts = dict()
for line in handle:
words = line.split()
for word in words:
counts[word] = counts.get(word,0) + 1
# Find the most common word
bigcount = None
bigword = None
for word,count in counts.items():
if bigcount is None or count > bigcount:
bigword = word
bigcount = count
# All done
print(bigword, bigcount)

pe
served words
riables (mnemonic)
erators
erator precedence
• Integer Division
• Conversion between
• User input
• Comments (#)

e
Write a program to prompt the user for hours
and rate per hour to compute gross pay.
Enter Hours: 35
Enter Rate: 2.75
Pay: 96.25

Acknowledgements / Contributions
re Copyright 2010- Charles R. Severance of the
ichigan School of Information and made available
ve Commons Attribution 4.0 License. Please
st slide in all copies of the document to comply
tion requirements of the license. If you make a
ee to add your name and organization to the list of
this page as you republish the materials.
ment: Charles Severance, University of Michigan
mation
Contributors and Translators here
...

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