Making Decisions: Quick Revision

A brief note on Making Decisions. Read it before a test. Try each question at the end before you open its answer.

Comparing values

• A comparison gives True or False. One = stores a value; two == ask a question.
• Text is compared letter by letter, and capital letters come before small letters.
• A comparison is a value, so you can keep it: can_vote = age >= 18.

if, elif and else

• if condition: is followed by indented lines, four spaces in. They run only when the condition is True.
• else: runs when the condition is False. Only one of the two parts runs.
• elif means "else if". Python asks the conditions from the top and stops at the first True.
• The else at the end catches everything that is left. Put the most special condition first.
• number % 2 == 0 is True for an even number.

and, or and not

• A and B needs both to be True. A or B needs at least one. not A turns the answer around.
• Python works out not first, then and, then or. Use brackets when you mix and with or.
• A range is two comparisons joined with and, or a chain: 13 <= age <= 19.
• x in ("red", "blue") asks whether x is one of the values in the group.
• An inner if is asked only when the outer condition is True. The indent shows which else belongs to which if.

• not (A and B) is the same as (not A) or (not B). not (A or B) is the same as (not A) and (not B).

Testing every branch

• A test table has the input, the expected answer and the program's answer.
• Test one value in each branch, and the values on both sides of every edge.
• The usual bugs are the wrong order, an edge that is one off, a missing else, and = written for ==.
• With three yes/no questions there are 8 cases. Each new question doubles the number.

A worked example

A school library fines late books. A book returned on time costs nothing. Up to 7 days late costs 2 rupees a day. More than 7 days late costs 5 rupees for every day.

1. Is days_late <= 0 true? 9 is not, so Python moves on.
2. Is days_late <= 7 true? 9 is not, so Python moves on again.
3. The else runs. The fine is 9 * 5, which is 45 rupees.
4. Test the edges. The edge between the two charges is 7 and 8 days. Test both, with one value from each branch.

The edge rows catch a slip at once. With days_late < 7 in place of days_late <= 7, a book 7 days late would cost 35 rupees, not 14 rupees.

Practice with new numbers

A program to try

The program gives advice for the walk to school from the temperature and the rain.

Mistakes to avoid

• = for ==. if x = 5: stops with SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?
• A missing colon. if x == 5 with no colon stops with SyntaxError: expected ':'.
• A missing indent. A line under if that is not pushed in gives IndentationError: expected an indented block after 'if' statement.
• Half a comparison. if age > 10 and < 15: is a SyntaxError. Each comparison needs both sides: age > 10 and age < 15.
• day == "Sat" or "Sun". It is always True, because "Sun" on its own counts as true. Write day == "Sat" or day == "Sun", or day in ("Sat", "Sun").
• The wrong order in a ladder. If marks >= 50 comes before marks >= 80, nobody gets the top grade.

Check yourself

1. Is "Mango" < "apple" True or False? Why?

2. Tanvi writes if size == "S" or "M": to find small and medium shirts. Every size passes the test. Why, and how should she write it?

3. A thermostat program has if t > 30: for hot, elif t > 20: for warm and else: for cool, with t in degrees Celsius. Which values of t should its test table hold?

4. Write not (x >= 5 and y >= 5) without the word not.

5. An outer if a > 0: holds an inner if b > 0: that prints P, with an inner else that prints Q. The outer else prints R. What is printed when a is -2 and b is 5?