~~SLIDESHOW~~
~~NOTOC~~

===== Contents =====

  * Literals
  * Variables and identifiers
  * Operators
  * Expressions
  * Data types

===== Literals =====

{{http://mkonar.org/assets/img/cat-silhouette-01-trans-64px.png}}

===== What is a literal? =====

  * **literal**: a sequence of one or more characters that stands for itself.
  * **numeric literal**: contains only the digits 0–9, a sign character (1 or 2) and a optional decimal point.
    * integer versus floating point value
    * ''%%e%%'' for scientific notation
    * ''%%3%%'', ''%%-42%%'', ''%%+98.6%%'', ''%%6.033e23%%''

===== Number ranges =====

  * Floating point values have limited range and precision.
  * IEEE 754: ±10<sup>-308</sup> to ±10<sup>308</sup> with 16 to 17 digits of precision.
  * **arithmetic overflow**: results when operation causes number to be “too big”.

<code python>
>>> 1.5e200 * 2.0e210
inf
</code>
  * **arithmetic underflow**: results when operation causes number to be “too small”.

<code python>
>>> 1.0e-300 / 1.0e100
0.0
</code>
===== Floating point numbers are an approximation =====

  * Limited precision means actual values are approximations.

<code python>
>>> 6 * 1/10
0.6
>>> 6 * (1/10)
0.6000000000000001
</code>
===== Formatting output =====

  * ''%%format()%%'' produces a string version of a value with a specific number of decimal places.
  * **format specifier** says how many places and format.

<code python>
>>> 12/5
2.4
>>> format(12/5, '.2f')      # 2 places, floating point
'2.40'
>>> format(2 ** 100, '.6e')  # 6 places, scientific notation
'1.267651e 1 30'
>>> format(13402.25, ',.2f') # optional comma
'13,402.25'
</code>
===== String literals =====

  * Use either single-quote or double-quote.

<code python>
greeting = 'Hi there'
"Vernacular nepotism is fractious."
'A'
"A"
</code>
  * Clever use:

<code python>
msg = 'She said, "Yellow."'
msg = "She's making a radio."
</code>
===== Character codes =====

  * Characters need to be encoded as (binary) numbers.
  * Python uses [[http://www.w3schools.com/charsets/ref_html_utf8.asp|UTF-8]] by default.
  * ''%%ord()%%'' translates a single character into its UTF-8 code.
  * ''%%chr()%%'' converts a UTF-8 code into a character.

<code python>
>>> ord('1')
49
>>> ord('2')
50
>>> ord('a')
97
>>> ord('A')
65
</code>
===== Control characters =====

  * **control characters**: non-printing characters used to control display of output.
  * Represented with **escape sequences** (backslash in Python).
  * Most popular escape sequence is the newline control: ''%%'\n'%%''.

<code python>
>>> print('Eat\nMore\nElderberries')
Eat
More
Elderberries
</code>
===== String formatting =====

  * Format strings with ''%%format( value, format_specifier )%%''

<code python>
format('Hello', '<16')  # left aligned field 16 characters wide
'Hello           '

format('Hello', '>16')  # right aligned field 16 characters wide
'           Hello'

format('Hello', '.>16') # fill with '.'
'...........Hello'
</code>
===== Implicit Line Joining =====

  * What to do with long lines in source code?
  * **implicit joining**: you can break the line at some points without messing up logic or syntax.

<code python>
print('Name:', student_name, 'Address:', student_address,
      'Number of Credits:', total_credits, 'GPA:', current_gpa)
</code>
  * Doesn’t work to break up a string.

===== Explicit Line Joining =====

  * **explicit joining** (backslash) can be used in some situations where implicit joining won’t.

<code python>
num_seconds = num_hours * 60 * 60 + \
              num_minutes * 60 
</code>
  * Also doesn’t work to break up strings.

===== Silly encoding example =====

<code python>
# This program displays the Unicode encoding for 'Hello World!

# program greeting
print('The Unicode encoding for "Hello World!" is:')

# output results
print(ord('H'), ord('e'), ord('l'), ord('l'), ord('o'), ord(' '),
      ord('W'), ord('o'), ord('r'), ord('l'), ord('d'), ord('!'))
</code>
===== Variables and Identifiers =====

{{http://mkonar.org/assets/img/cat-silhouette-01-trans-64px.png}}

===== What is a variable? =====

  * **variable**: a name associated with a value.

<code>
       +--------+
num -->|   10   |
       +--------+
</code>
===== Assignment =====

  * **assignment**: take the value resulting from the expression on the right and copy it into the variable on the left.
  * **assignment operator**: ''%%=%%'' in Python.
    * ''%%=%%'' is //not// “equals”!

<code python>
foo = 7
num = 1 + foo
foo = foo + 1
</code>
===== Variable reassignment =====

<code python>
num = 10    #        +--------+
k = num     # num -->|   10   |
            #   k -->|        | 
            #        +--------+
</code>
<code python>
k = 20      #        +--------+
            # num -->|   10   |
            #        +--------+
            #        +--------+
            #   k -->|   20   |
            #        +--------+
</code>
===== id() =====

  * Use ''%%id()%%'' to see if two variables are pointing to the same thing.

<code python>
>>> num = 10
k = num
>>> id(num)
??? # some number
>>> id(k)
??? # same number as above
k = 20
>>> id(num)
??? # same as before
>>> id(k)
??? # different
</code>
===== What is an identifier? =====

  * **identifier**: a sequence of one or more characters used to provide a name for a given program element.
  * **identifier**: names of things.
  * Rules of the name:
    * letters, digits, and the underscore character only (no spaces)
    * can’t start with a digit
    * don’t use underscore for the first character for now
    * can be as long as you want

===== Keywords and help =====

  * **keyword** an identifier that has predefined meaning in a programming language.
  * You can’t use a keyword as your own identifier in Python.
  * Get a list of keywords:

<code python>
>>> help()
help> keywords
# table of keywords appears
help> quit
>>> 
</code>
  * Things you think should be keywords are not!
  * Check “special” words using ''%%'testword' in dir(__builtins__)%%''

===== Example: Restaurant Tab Calculation =====

RestaurantTab.py

<code python>
# Restaurant Tab Calculation Program
# This program will calculate a restaurant tab with a gift certificate

# initialization
tax = 0.08

# program greeting
print('This program will calculate a restaurant tab for a couple with')
print('a gift certificate, with a restaurant tax of', tax * 100, '%\n')

# get amount of gift certificate
amt_certificate = float(input('Enter amount of the gift certificate: '))

# cost of ordered items
print('Enter ordered items for person 1')

appetizer_per1 = float(input('Appetizier: '))
entree_per1 = float(input('Entree: '))
drinks_per1 = float(input('Drinks: '))
dessert_per1 = float(input('Dessert: '))

print('\nEnter ordered items for person 2')

appetizer_per2 = float(input('Appetizier: '))
entree_per2 = float(input('Entree: '))
drinks_per2 = float(input('Drinks: '))
dessert_per2 = float(input('Dessert: '))

# total items
amt_person1 = appetizer_per1 + entree_per1 + drinks_per1 + dessert_per1
amt_person2 = appetizer_per2 + entree_per2 + drinks_per2 + dessert_per2

# compute tab with tax
items_cost = amt_person1 + amt_person2
tab = items_cost + items_cost * tax

# display amount owe
print('\nOrdered items: $', format(items_cost, '.2f'))
print('Restaurant tax: $', format(items_cost * tax, '.2f'))
print('Tab: $', format(tab - amt_certificate, '.2f'))
print('(negative amount indicates unused amount of gift certificate)')

</code>
===== Operators =====

{{http://mkonar.org/assets/img/cat-silhouette-01-trans-64px.png}}

===== What Is an Operator? =====

**operator**: a symbol that represents an operation that may be performed on one or more operands. **binary operator**: takes two operands **unary operator**: takes one operand

===== Arithmetic operators =====

^symbol      ^operator         ^example         ^result  ^
|-x          |negation         |-10             |-10     |
|x + y       |addition         |10 + 25         |35      |
|x - y       |subtraction      |10 - 25         |-15     |
|x * y       |multiplication   |10 * 5          |50      |
|x / y       |division         |25 / 10         |2.5     |
|x %%//%% y  |truncating div.  |25 %%//%% 10    |2       |
|            |                 |25 %%//%% 10.0  |2.0     |
|x % y       |modulus          |25 % 10         |2.0     |
|x %%**%% y  |exponentiation   |10 %%**%% 2     |100     |

===== Example: Your Place in the Universe =====

PlaceInUniverse.py

<code python>
# Your Place in the Universe Program

# This program will determine the approximate number of atoms that a person is
# person consists of and the percent of the universe that they comprise

# Initialization
num_atoms_universe = 10e80
weight_avg_person = 70  # 70 kg (154 lbs)
num_atoms_avg_person = 7e27

# Program greeting
print('This program will determine your place in the universe.')

# Prompt for user's weight
weight_lbs = int(input('Enter your weight in pounds: '))

# Convert weight to kilograms
weight_kg = 2.2 * weight_lbs

# Determine number atoms and percentage of universe
num_atoms = (weight_kg / weight_avg_person) * num_atoms_avg_person
percent_of_universe = (num_atoms / num_atoms_universe) * 100

# Display results
print('You contain approximately', format(num_atoms, '.2e'), 'atoms')
print('Therefore, you comprise', format(percent_of_universe, '.2e'),
      '% of the universe')

</code>
===== Expressions and Data Types =====

{{http://mkonar.org/assets/img/cat-silhouette-01-trans-64px.png}}

===== What is an expression? =====

  * **expression**: a combination of symbols that evaluates to a value.
  * ''%%4 + (3 * k)%%'' is an expression.
    * Has two subexpressions:
      * ''%%4%%''
      * ''%%(3 * k)%%''
        * Has two **subexpressions**:
          * ''%%3%%''
          * ''%%k%%''

===== Operator precedence and associativity =====

  * **operator precedence** determines which operations happen first.
  * **operator associativity** determines the order when two operands have the same precedence.

===== Python’s rules =====

  * Similar to math.
  * So far we have:

^operator       ^associativity  ^
|%%**%%         |right to left  |
|- (negation)   |left to right  |
|*, /, %%//%%, %|left to right  |
|+, -           |left to right  |

===== Examples =====

<code python>
>>> 6 - 3 + 2
5
>>> 2 * 3 / 4
1.5
>>> 2 ** 3
8
>>> 2 ** 3 ** 2  # exponentiation is r-to-l
512
</code>
===== What is a data type? =====

  * Python differentiates between types of data.
  * So far we have seen character strings and numbers.
  * **data type**: a set of values and a set of operators that may be applied to those values.
  * ''%%+%%'' on a number is different than on a string.
  * can’t do ''%%/%%'' on a string.

===== Python’s basic types =====

  * Python has many [[https://docs.python.org/3/library/stdtypes.html|built-in types]].
    * ''%%string%%'' : character strings
    * ''%%int%%'' : integer values
    * ''%%float%%'' : floating point (i.e., real) values
    * ''%%bool%%'' : ''%%True%%'' or ''%%False%%''
  * Use ''%%type()%%'' to get the type of a variable or literal.

<code python>
>>> type(66)
<class 'int'>
>>> type(3.1)
<class 'float'>
>>> type(True)
<class 'bool'>
>>> type(3.1)
<class 'float'>
</code>
===== Static versus dynamic typing =====

  * Python is a **dynamically typed** language.
  * The same variable can store different type values at different times.

<code python>
x = 41 + 1
print(x)
x = 'I ate a donut.'
print(x)
x = False
print(x)
</code>
  * In **statically typed** languages, once a variable is associated with a type, the type can’t be changed.

===== Mixed-type expressions =====

  * Example: ''%%3 + 1.2%%''

===== Type coercion =====

  * **type coercion**: the implicit (and automatic) conversion of one type to another.

''%%3 + 1.2%%'' -> ''%%3.0 + 1.2%%'' -> 4.2

===== Type conversion =====

  * **type conversion**: explicitly converting one data type to another.

<code python>
>>> str(42)    # convert into to string
'42'
>>> int('42')  # convert string to int
42
>>> int(3.9)   # convert float to int
3
>>> float('99') # convert int to float
99.0
>>> int('99.9') # nope
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '99.9'
</code>
===== Example: Temperature Conversion Program =====

TempConversion.py

<code python>
# Temperature Conversion Program (Fahrenheit to Celsius)

# This program will convert a temperature entered in Fahrenheit
# to the equivalent degrees in Celsius

# program greeting
print('This program will convert degrees Fahrenheit to degrees Celsius')

# get temperature in Fahrenheit
fahrenheit = float(input('Enter degrees Fahrenheit: '))

# calc degrees Celsius
celsius = (fahrenheit - 32) * 5 / 9

# output degrees Celsius
print(fahrenheit, 'degrees Fahrenheit equals',
       format(celsius, '.1f'), 'degrees Celsius')

</code>