Decisions

An if asks a question that is either True or False, and runs one path or the other.

One sign carries most of this block. One equals puts a value in. Two equals asks a question. Mark = 70 stores 70. Mark == 70 asks whether Mark is 70, and gives True or False.

When there are several tests in a row, Python checks them top to bottom and stops at the first one that is True. A cinema sells three tickets: Child under 12, Adult under 60, and Senior for everyone else. Pick an age and step through it.

Try 12. It is not under 12, so the first test is False and the Adult test catches it.

Comparing, and the signs that change from pseudocode

if, elif, else, and the indentation is the ENDIF

Every if line ends with a colon, and the lines it controls are indented underneath. There is no ENDIF: the indentation shows where the branch stops.

elif means "else, if". Use as many as you need. else has no test: it catches everything the tests above did not.

CASE OF in pseudocode becomes an elif chain in Python.

The first True test wins, so put the narrowest first

Both programs below use the same three tests. Only the order changed.

5 is under 60, so the wrong order stops at the first test and never asks about 12. A child pays the adult price. Grade bands work the same way: test Mark >= 80 before Mark >= 70.

Comparing text: capitals, and numbers that are still text

"yes" == "Yes" is False. Capital letters count. Convert first, then compare with one spelling:

Text compares letter by letter, not by value. As text, "9" > "10" is True, because the character 9 comes after the character 1. Forget int() on two ages and your program names the wrong person as older, with no error at all.

To swap two boxes you need a third

Putting three numbers in order means swapping two of them when they are the wrong way round. A = B then B = A does not work: the first line wipes out the old A. Keep it somewhere first:

You will use exactly this in bubble sort, much later in the course.

Six ways this block goes wrong

• TRAPUsing one equals in a test.

  if Mark = 70: stops with a SyntaxError. A test needs ==.

• Tests in the wrong order.

  The wider test catches everything first. Narrowest test at the top.

• Comparing numbers that are still text.

  "9" > "10" is True. Convert with int() as you read.

• Forgetting the colon.

  Every if, elif and else line ends with :.

• Capitals in text answers.

  Use .upper() on what was typed, then compare with capitals.

• Writing AND or <>.

  Python wants and and !=.

What you do and don’t need

Test the edges, not just one easy value

A grade program that works for 72 can still be wrong at 70. Run it with a value from every band and with the exact boundary values: 69, 70, 79, 80. The mistakes in decisions hide at the boundaries, usually a > that should have been >=.