Decisions Lab · Lecture 3.1

Grade machine: which branch runs, and why order matters

Move the score. Python checks the conditions from the top and stops at the first True. Then switch to the wrong order and watch whole grades disappear.

score = 72

Order

A ≥ B ≥ C ≥

The path Python takes

Every score from 0 to 100 with this code

050100

Boundary tests: the scores where bugs hide

What to notice

Only one branch of an if / elif / else chain ever runs.
Conditions after the first True are not checked at all.
With the loosest condition first, A and B can never be printed: the code runs without error but is wrong.
Test the edges: exactly 80, 79, 65, 64, 50, 49.

Try this

Predict the grade for 65 before moving the slider there. Is 65 a B or a C? Switch to the wrong order. How many of the 101 scores still get the correct grade? Set B ≥ 85 (higher than A). Which grade can never appear now, and why? Change the operators in your head from >= to >. Which boundary rows would change?

and · or · not: build conditions from smaller ones

Flip the two switches. Every expression below is recalculated. Click an expression to see its full truth table.

p =

q =

Your own expression (use p and q)

Truth table:

Real use: who can enter?

age = 20

has_id = True

What to notice

and is True only when both sides are True; or when at least one is.
not (p and q) always equals not p or not q. Compare their columns.
Each side of and must be a complete condition: age >= 18 and <= 60 is a SyntaxError.

Try this

Find the only switch setting where p and q is True. Type an expression that is always True, whatever the switches. (Hint: p or not p.) Find an age where age >= 18 or has_id lets in someone who should not enter.

Spam rule tester: write a rule, test it on an inbox

Pick suspicious words. The rule flags a message if it contains them. See which spam it catches, which it misses, and which normal messages it wrongly flags.

Suspicious words (click to switch on or off)

Combine with use .lower()

Spam caught

Spam missed

False alarms

Test your own message

The inbox (label = what a human decided)

What to notice

More words with or catch more spam and raise more false alarms.
Switching to and makes the rule strict: few false alarms, but most spam slips through.
Some spam (FR33 g1ft) beats every word rule. Lecture 6 lets the computer learn the rule from examples instead.

Try this

Find a set of words that catches every spam message. How many false alarms does it cost? Switch off .lower(). Which messages change, and why? Find the smallest set of words with zero false alarms that still catches at least 4 spam messages.

match-case: one value, many fixed options

Type a phone number. Python looks at the first three characters and tries each case from the top, just like an elif chain.

number =

Remove the else: (a common mistake)

Output

The same decision with if / elif

What to notice

number[:3] is the first three characters, as text: "017", not the number 17.
| inside a case means or; case _: catches everything else, like else.
Without the else:, a short number prints INV and then an operator name.

Try this

Pick 017-1111111. It passes the length check. Is it really a valid number? What extra check would catch it? Tick Remove the else and pick the 10-digit number. Explain the two lines of output. Which operator would a number starting with 011 get, and which line decides that?