Review

• Return floating point values without modification
• print values “nicely” e.g. using {foo:0.02f}
• The formatting won’t modify the value – it will just change how it looks
  • So you can see it “nicely”, but still have all of the precision available.

div/mod and debugging

• Look at IC-3, Task 2
  • Make sure you understand the problem and can solve it by hand.
    • If you can’t solve it without a computer, then you can’t solve it with.
    • Be sure to ask me for clarification if the problem description doesn’t make sense.
  • Use helper variables
  • make a mistake for middle digit x // 10
  • Observe the incorrect result.
  • Use the debugger to narrow down where the problem is.

input

• Explain IC-4, Task 3 better
• Add I/O to IC-3, Task 2 or 3 as an example

Function interfaces

• Emphasize the importance of not changing the interface.
  • e.g., can’t switch to three parameters for the sum_digits problem.
• Discuss how tests work
  • My tests are written to call the function as described. If you change the interface, then the code won’t run..
  • Try to call a 1-param function with 2 parameters.

Conditionals

Boolean Values

• Remember:
  • values are groups of 1s and 0s in RAM.
  • type tells us how to interpret the 1s and 0s.
    • The set/range of values
    • The set of legal operations
• A boolean type with only two values
  • traditionally True and False
• They are not strings. Written without quotation marks
• Also need to be capitalized.

Boolean Expressions

• Remember: An expression is a set of operations that can evaluate to a value
  • 4 + 3*2
• A _boolean expression` is an expression that evaluates to a boolean value.
• A boolean operator is an operator that evaluates to either True or False
• == is the equality operator. It is either True or False
  • 5 == 5 is True
  • 5 == 6 is False
• Emphasize difference between assignment (=) and equality (==)
• Other comparisons
  • x != y # x is not equal to y
  • x > y # x is greater than y
  • x < y # x is less than y
  • x >= y # x is greater than or equal to y
  • x <= y # x is less than or equal to y
• Logical operators

x = True
y = False
print(x or y)
print(x and y)
print(not x)

• in: 'a' in 'apple'
• Can’t do this: 'a' in 'dog' or 'cat'. Need 'a' in 'dog' or 'a' in 'cat'

If/else

def even_or_odd(num : int) -> str:
  if (num % 2 == 0):
    return 'even'
  else:
    return 'odd'

x = 6
print(f"{x} is {even_or_odd(x)}.")

• The else clause is optional — Sometimes you don’t need it.
• You can nest conditional statements (even_odd_neither).
  • Notice the nested indentation

• ‘You get a B!’

• With if-else chain, order can be important.
  • is_leap_year

Old notes

• if
• if-else
  • max_value
• if-elsif-else
  • too hot / too cold / just right
• if-elsif-elsif-elsif-else
  • Letter grades
• Indentation matters
  • Spaces different than tabs
• = vs ==
• Operator chaining a < b < c
  • somewhat unique to Python
• and / or / not
  • != as shortcut for not x == y
• implicit vs explicit ranges
  • i.e., choosing order of if statements carefully
  • Look at letter grades again.
• chain of if vs. else-if
• nested if
  • Compute bonus for different structures
    • Structure A: 10% for sales over 10,000
    • Structure B: $100 if sales is a multiple of 5.
• integers and floats can be compared. (One is coerced)
• strings can be compared using ==, <, <=, etc.
• can’t directly compare strings and integers
• Avoid comparing float using ==
  • Why?
• is vs ==
• watch for if x = 9: (instead of ==)

  • Example

    def print_discount(has_discount: bool):
      if (has_discount = True):
          print('You have a discount!')
      else 
          print(f"No discount.  (Your status {has_discount})")
          
    print_discount(False)
    

  • Actually, this is a syntax error. Need to use :=.
  • The requirement of the : is to avoid this common mistake.
• Order of operations
  • precedence: not, and, or
  • not x or y is evaluated as (not x) or y.
  • x == 5 or y == 10 and z != 10 is evaluated as (x == 5) or ((y == 10) and (z != 10)) because and has precedence over or.
  • When in doubt, just use parentheses.

• conditional/ ternary operator

  x = 18
  parity = 'even' if x %2 == 0 else 'odd'
  

• Expression trees