Editing Community Data Science Course (Spring 2019)/Day 2 Lecture

== Resources ==

* [[Python data types cheat sheet]]
* [[Python loops cheat sheet]]

== Lecture outline ==

=== Review: ways to access python ===

* python
* python file.py

=== Review Week 1 python material ===

* math: using python as a calculator
**addition, subtraction, multiplication, division
**division shows something different: <code>8/2</code> versus <code>2*2</code>
* <tt>type()</tt>
** there are different types of things in python (called objects)
** variables that "know about the decimal place" (int) and variables that don't (floats)
** variables that are strings.
* variables
** assignment of variables
** e.g., math with variables: scale up a recipe, into an assignment
** you can assign to a variable and it will replace the old value
* strings
** things within quotation marks
** adding strings with "concatenation" (smushing things together)
** e.g., <code>print("Hello" + name)</code>
** concatenating strings and integers doesn't work (e.g., <code>print(1 + "tommy")</code>)
** 1 is different than "1"; name is different than "name"
** convert "1" to 1 with the <code>int()</code> function
** single quotes versus double quotes (python doesn't care)
** you can also multiply strings! (although it's not clear why you want to)
** '''The type of a variable tells you what you can do with it'''
* Booleans
** comparisons (e.g., <code>1 == 1</code> or <code>1 == 0</code>)
*** you can compare strings (case sensitive!)
*** also >, <, and !=
** type() shows that the output of True or False is <code>bool</code>
** e.g., <code>"i" in "team"</code>
** e.g., <code>"i" not in "team"</code>
* <tt>if</tt>/<tt>elif</tt>/<tt>else</tt> ('''move to external file''')
** if, something that evaluates to a Boolean, and then colon
** e.g., <code>if "tom" in "tommyguy"</code>
** e.g., adding else example: <code>if brother_age > sister_age</code>
** e.g., temperature range
** e.g., adding elif: fix the bug in the previous program if they were the same age
** indent with spaces (we use 4 spaces!)

===Exercise:===

The code below lets a user enter their age, and it prints their age back to the screen.

1) Copy the code to a file and run it. 

2) Modify the file to take the user's age and do the following:

a) If the age entered is younger than your age, print <code>You are younger than me!</code>

b) If the age entered is older than your age, print <code>You are older than me!</code>

c) If the age entered is your age, print <code>You are the same age as me!</code>


    input_age = input("Enter your age in years: ")
    print("User entered " + input_age)

=== Lists ===

* purpose
** Stores things ''in order''
* initialization
** making a list called my list: <code>my_list = ["a", "b", "c"]</code>
** comma separated elements. in python they can be a mix of any kind of types
** <code>type(my_list)</code>
* <tt>len()</tt> review
* accessing elements
** indexing like my_list[0]
** indexing starts from the front and we ''start counting at 0''
** we go from the end with negative numbers
** what happens if we try to move outside of the range? ('''error!'')
* adding elements
** using the the <code>my_list.append()</code> function
** the <code>.append()</code> function is a special kind of function that lists know about
** compare to <code>print</code> which is a global function.
* changing elements
** replacing elements like <code>my_list[0] = "foo"</code>
* finding elements in list
** e.g., <code>"z" in my_list</code>
* slicing lists
** the colon inside the [] is the ''slicing syntax''
** e.g., <code>my_list[0:2]</code> is 0th up to, but not including, the 2nd
** e.g., <code>my_list[2:]</code> 
** e.g., <code>my_list[:2]</code>
** e.g., <code>my_list[:]</code>
* strings are like lists
** we can slice lists
** len()
*** <code>len("")</code> length of the empty string
* many other interesting functions for lists
** e.g., <code>min()</code> and <code>max()</code>
** e.g., create a list of names and sort it <code>names.sort()</code>
** search "python lists" in any search engine to see many more functions that list knows about.

===Exercise===

Figure out what the list function <code>pop</code> does. Confirm with your neighbor. Write an example to be sure.

=== loops and more flow control ===

* <tt>for</tt> loops
** e.g., <code>for name in names: print name</code>
** e.g., <code>for name in names: print 'hello ' + name</code>
** Super powerful because it can do something many many times. Data science is about doing tedious things very quickly. <code>for</code> is the workhorse that makes this possible.
** Look and see what name is after we're done looping.
** ''Move to editor.''
* <tt>if</tt> statements inside <tt>for</tt> loops
** e.g., <code>if name[0] in "AEIOU"</code> then print "starts with a vowel"
** show we can test things outside the loop to show how the comparisons are working
** add an else statement to capture words that start with a consonant
** append to a list within a for loop
** create a counter within a for loop (keep track)
* nested <tt>for</tt> loops
* <tt>range()</tt>
* <tt>while</tt> loops
* infinite loops and how to stop them
* <tt>if</tt> statements inside <tt>while</tt> loops
* <tt>break</tt>