Selection and conditionals

Why did the pensioner pay the student price?

• A cinema charges £5 for children and over-65s, £7 for students and £10 for everyone else. A 70-year-old student was charged £7.
• Every rule in the program was correct. The rules were simply tested in the wrong order. This lesson is about how a program chooses a road, and why order is part of the design.

A condition is a True or False question

• A condition 条件 is an expression whose value is a bool: age >= 65, answer == "y", n != 0. The comparisons are <, <=, >, >=, == (equal) and != (not equal).
• Join conditions with and (both must be true), or (at least one) and not (flips it). 3 >= 3 and not (2 > 5) is True.

student = input("Student? y/n ") == "y"   # already a bool
print(type(student))                      # <class 'bool'>

if and else: one of two roads

• if condition: runs the indented block only when the condition is True. An else: block runs when it is False. Exactly one of the two runs.
• The indentation is the block. Lines indented under if belong to it; the first line back at the left edge runs whatever happened.

if total >= 50:
    fee = 0
else:
    fee = 5
print("Delivery:", fee)   # always runs

elif: the first true branch wins

• Selection 选择 with if … elif … else tests the conditions from top to bottom and runs only the first one that is True. All the others are skipped, even if they are also true.
• else catches whatever is left, so a full chain always takes exactly one road.

Trust the fall-through

• An elif is only reached when every test above it has failed. So elif score >= 70 does not need and score < 80: reaching that line already proves the score is below 80.
• Writing the extra test is longer, and each copy is one more place to make a mistake at the boundary.

if score >= 80:
    band = "A"
elif score >= 70:     # here score < 80 is already known
    band = "B"
elif score >= 60:
    band = "C"
else:
    band = "F"

Put the narrow rule first

• When two rules can both be true, the one written first decides. Put the rule that must win, usually the narrower one, above the wider one.
• Test one input from each road, and one input that matches two rules. That overlap case is where order bugs hide.

Combine conditions with care

• Python lets you chain comparisons: 0 <= mark <= 100 means 0 <= mark and mark <= 100.
• and stops as soon as the answer is known. n != 0 and total / n > 5 never divides by zero, because when n is 0 the left side is already False.

Marks that slip away

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