Ranges & Controls

Announcements

• Problem Set 2 is posted and due on Monday next week at 10 pm
  • Feedback for PS1 will be ready by next week!
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  • 10% for 24 hours late submission.
  • 20% for 48 hours late submission.
  • 50% for a week late submission.
  • Beyound 1 week may not earn points
• We will be concluding chapter 2 of the text today. Hope eveyone is reading the text along
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Review Question

What is the printed value of A in the code below?

>>> A = 10
>>> B = 5 % 3
>>> C = A * B ** B
>>> A -= B + A // 2
>>> A, B, C = C, A, B
>>> print(A)
??

Boolean Expressions

• Python defines two types of operators that work with Boolean data: relational operators and logical operators

• Relational operators compare values of other types and produce a True/False result:

  ==	Equals				!=	Not equals
  <	Less than				<=	Less than or equal too
  >	Greater than				>=	Greater than or equal to

• Be careful! == compares two booleans. A single = assigns a variable. The odds are high you’ll use one when you meant the other at least once this semester!

The Vulcan Way

• Logical operators act on Boolean pairings

  Operator	Description
  A and B	True if both terms True, False otherwise
  A or B	True if any term is True, False otherwise
  not A	True if A False, False if A True (opposite)

Shorting the Circuit

• Python evaluates and and or operators using a strategy called short-circuit mode
• Only evaluates the right operand if it actually needs to

  • Example: if n=0, then the x % n == 0 is never actually checked in the statement

    n != 0 and x % n == 0

    since n != 0 already is False and False and anything is always False

• Can use short-circuit to prevent errors: the above x % n == 0 statement would have erred out if n=0

Input: input

• To retrieve data from a user, we can use Python’s built-in input() function

• The form will generally look like:

  variable = input(prompt_text)

  • variable is the variable name you want to assign the user’s typed input to
  • prompt_text is the string that will be displayed on the screen to communicate to the user what they should be doing

• The input() function always returns a string

  • If you want to get an integer from the user, you will need to convert it yourself after retrieving it

    num = int(input('Pick a number between 1 and 10: '))

Sentinels

• Many programs in programming can be approached similarly by using a particular idiom or pattern
• A common one with repetition is to use a variable to keep track of the loop state until something particular happens (the sentinel is triggered)

Running a Program

• Python programs specify what part of the code is supposed to be executed when a program is run using a few special lines at the end of the program

  if __name__ == '__main__':
      function_to_run()

  • function_to_run is the name of whatever function you want to execute when the program is run directly

• Patterns of this sort are commonly called boilerplate

  • Less important to fully understand all the pieces of it now
  • Is important to understand what it is doing and how to implement it correctly

Sample Program

NUM_ODDS = 100 # Constant, so using all caps

def print_odds():
    """
    Prints the first NUM_ODDS odd numbers 
    starting at 1.
    """
    value = 1
    for i in range(NUM_ODDS):
        print(value)
        value += 2

if __name__ == '__main__':
    print_odds()

Visiting the library(ies)

• A huge strength of Python is that it offers a very large number of code collections called libraries
  • Can save you the effort from writing that code yourself!
• Requires you to import that library, which can take several different forms

  • Most common is to use import to grab everything in a library

    import math

    • You must then access any function in that library using its fully qualified name, which includes the library name (var = math.sqrt(4))

  • Can also use from ... import to grab specific functions from the library

    from math import sqrt

    • You do not need to use the library name then when you call it (var = sqrt(4))

Useful math definitions

Extra if functionality

if condition_1:
    # Run this code if 
    # condition 1 is true
elif condition_2:
    # This code runs if 
    # condition 1 is false
    # but condition 2 is true
elif condition_3:
    # Runs if both condition 1
    # and 2 fail but condition
    # 3 is true
else:
    # This code runs if 
    # all above conditions 
    # fail

For Loop Anatomy

• We have already seen for loops in Karel, but let’s expand on their use:

• The more general syntax of a for loop looks like:

  for variable_name in sequence:
      # code to loop over

  • variable_name is a variable name that will be assigned every value in the sequence over the course of the loop
  • sequence is any expression that produces a variable that supports iteration
    • We’ve already seen range() fill this role
    • Any sequence works though! So strings and lists can also be looped over!

The range() iterable

• Need an easy way to produce or describe a range of numeric values
• The built-in range() function handles this and produces the needed iterable object
• Takes 1, 2, or 3 arguments:
  • Start (default 0): where to start the sequence at
  • Stop (mandatory): the sequence ends just below this value (does not include it!)
  • Step (default 1): what value the sequence counts by
    
    

For ranging examples

• Providing just a stop argument:

  for n in range(5):
      print(n)

• Providing a start and stop:

  for n in range(1,11):
      print(n)

• Providing a start, stop, and step:

  for n in range(10,0,-1):
      print(n)

Iterating over sequences

• We can also use a for loop to iterate directly over a sequence of values

• We can loop through a string or list to examine each individual character or element

• Example of looping through a word to count occurrences of a given letter:

  def count_letters(letter, string):
      count = 0
      for character in string:
          if character == letter:
              count += 1
      return count

Test Yourself

• It is easy to make mistakes when writing code
  • Syntax mistakes
  • Typos or operations mistakes
  • Overall conceptual mistakes
• Given that making mistakes is entirely natural, it is also imperative that looking for mistakes become entirely natural
• You known what the desired goal is for basically every problem in this class. Test your code!

Way of Testing

Assertions

• You can use Python’s assert statement to write test functions, which take the form:

  assert condition

  where condition is any operation that returns a True or False

• Assert statements “expect” a condition to yield a True, and if they do, nothing happens

  • No news is good news in this case

• If an assert condition evaluates to False, an error is raised

• Naming your test functions starting with the word test_ will make them automatically discoverable by other tools

Testing Example

• Suppose we wanted to write some checks of the count_letters function from earlier

def test_count_letters():
    """ Runs several tests on the function count_letters """
    assert count_letters("z", "banana") == 0
    assert count_letters("a", "strawberry") == 1
    assert count_letters("A", "apple") == 0
    assert count_letters("e", "eerie") == 3