DS4UX (Spring 2016)/Day 2 lecture

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Part 1: Review[edit]

  • math: using python as a calculator
    • addition, subtraction, multiplication, division
    • division shows something different: 8/2 versus 2*2
  • type()
    • 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
    • assignment of variaibles
    • 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 "concatination" (smushing things together)
    • e.g., print("Hello" + name)
    • concatenating strings and integers don't work (e.g., print(1 + "mako"))
    • 1 is different than "1"; name is different than "name"
    • single quotes versus double quotes (python doesn't care)
    • you can also multiply strings! (although it's not clear why you want to weird)
  • booleans
    • comparisons (e.g., 1 == 1 or 1 == 0)
      • you can compare strings (case sensative!)
      • also >, <, and !=
    • type() shows that the output of True or False is bool
    • e.g., "i" in "team"
    • e.g., "i" not in "team"
  • if/elif/else (move to external file)
    • if, something that evaluates to a boolean, and then colon
    • e.g., if "mako" in "makoshark"
    • e.g., adding else example: if brother_age > sister_age
    • e.g., temperature range (e.g., if temp<65 is cold; temp>80 is hot; otherwise, just right)
    • e.g., adding elif: fix the bug in the previous program if they were the same age
    • indent with spaces (we use 4 spaces!)
  • functions
    • has a parentheses
    • we've already learnd examples of this: exit(), help(), type()

Part 2: New programming concepts[edit]

Lists[edit]

  • purpose
    • Stores things in order
  • initialization
    • making a list called my list: my_list = ["a", "b", "c"]
    • comma separated elements. in python they can be a mix of any kind of types
    • type(my_list)
  • len() review
  • accessing elements
    • indexing like my_list[0]
    • indexing starts from the front and we start counting at 0 (now you understand all the zeros we've been using)
    • 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 my_list.append() function
    • the .append() function is a special kind of function that lists know about
  • changing elements
    • replacing elements like my_list[0] = "foo"
  • finding elements in list
    • e.g., "z" in my_list
  • slicing lists
    • the colon inside the [] is the slicing syntax
    • e.g., my_list[0:2] is 0th up to, but not including, the 2nd
    • e.g., my_list[2:]
    • e.g., my_list[:2]
    • e.g., my_list[:]
  • strings are like lists
    • we can slice lists
    • len()
      • len("") length of the empty string
  • many other interesting functions for lists
    • e.g., min() and max()
    • e.g., create a list of names and sort it names.sort()

loops and more flow control[edit]

See also: working within loops.

  • for loops
    • e.g., for name in names: print name
    • e.g., for name in names: print 'hello ' + name
    • Super powerful because it can do something many many times. Data science is about doing tedious things very quickly. For is the workhorse that makes this possible.
    • Look and see name is after we're done looping.
    • Move to editor.
  • if statements inside for loops
    • e.g., if name[0] in "AEIOU" 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)
    • build up a sentence
  • nested for loops
  • range()
  • while loops
  • infinite loops
  • if statements inside while loops
  • break
  • input()

modules[edit]

  • purpose
  • builtins
  • import random
  • random.randint
  • random.choice
  • random.sample

walk through favorite_buildings.py[edit]

Where favorite_buildings.py from http://jtmorgan.net/ds4ux/week2/others/favorite_buildings.py is the grand finale and synthesis of lecture material.