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--- python:dierbach:chapter_2_data_and_expressions [2016/07/28 20:42] – created mithat
+++ python:dierbach:chapter_2_data_and_expressions [2016/07/29 03:03] (current) – mithat
@@ Line 1: / Line 1: @@
 ~~SLIDESHOW~~
 ~~NOTOC~~
-====== Data and Expressions ======
 ===== Contents =====
@@ Line 14: / Line 12: @@
 ===== Literals =====
-{{http://lorempixel.com/480/360/cats}}
+{{http://mkonar.org/assets/img/cat-silhouette-01-trans-64px.png}}
 ===== What is a literal? =====
@@ Line 160: / Line 157: @@
 ===== Variables and Identifiers =====
-{{http://lorempixel.com/480/360/cats}}
+{{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
+>>> 2 * 3 / 4
+.5
+>>> 2 ** 3
+>>> 2 ** 3 ** 2  # exponentiation is r-to-l
+</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
+>>> int(3.9)   # convert float to int
+>>> float('99') # convert int to float
+.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>