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--- python:dierbach:chapter_2_data_and_expressions [2016/07/29 01:36] – mithat
+++ python:dierbach:chapter_2_data_and_expressions [2016/07/29 03:03] (current) – mithat
@@ Line 2: / Line 2: @@
 ~~NOTOC~~
-====== Data and Expressions ======
-Mithat Konar\\
-based on Dierbach's //[[http://www.wiley.com/WileyCDA/WileyTitle/productCd-EHEP002046.html|Introduction to Computer Science using Python]]//.
 ===== Contents =====
@@ Line 231: / Line 228: @@
   * Get a list of keywords:
-<code>
+<code python>
 >>> help()
 help> keywords
-...
+# table of keywords appears
 help> quit
 >>>
@@ Line 287: / Line 284: @@
 print('Tab: $', format(tab - amt_certificate, '.2f'))
 print('(negative amount indicates unused amount of gift certificate)')
 </code>
 ===== Operators =====
@@ Line 298: / Line 296: @@
 ===== Arithmetic operators =====
-Demonstration of simple table syntax.
+^symbol      ^operator         ^example         ^result  ^
-^  Right^Left  ^  Center  ^Default^
+|-x          |negation         |-10             |-10     |
-|     12|12    |    12    |12     |
+|x + y       |addition         |10 + 25         |35      |
-|    123|123   |   123    |123    |
+|x - y       |subtraction      |10 - 25         |-15     |
-|      1|1     |    1     |1      |
+|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>