Lists and Functions: Quick Revision

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

Making and reading a list

• A list is written in square brackets, with commas: marks = [56, 72, 91]. It can hold any values, a mix, or none: [].
• Positions start at 0. marks[0] is 56. A negative position counts from the end, so marks[-1] is 91.
• A slice a[1:3] starts at position 1 and stops before position 3. A slice is always a list.
• len(a) counts the items. x in a asks whether x is in the list. a.index(x) gives its position.

Changing a list

• These methods change the list itself and give back None. So never write a = a.sort().
• sorted(a) makes a new sorted list, and the original stays as it was.
• A list can change; a string cannot.
• b = a gives the same list a second name. b = a.copy() makes a real copy.
• A list of lists is a table: table[row][col] is one cell.

Lists and loops

• for item in a: goes through the items, one in each round.
• sum(), max(), min() and len() give quick answers.
• To count, use a counter and an if. To build a new list, start with [] and append.
• Do not change a list while you loop over it. Build a new one instead.
• Two lists in the same order work like a table. for i in range(len(names)): reads both at position i.

Functions

• def name(parameters): and the indented lines under it define a function. Defining does not run it.
• name(arguments) calls it. A function must be defined before the line that calls it.
• return gives a value back and ends the function. print only shows. A function without return gives back None.
• A parameter can have a default: def tax(price, rate=5):. Parameters with defaults come last.
• A variable made inside a function is local. It disappears when the function ends. Use parameters in and return out.
• Test a function with a table of cases and a loop that prints PASS or FAIL. Test both sides of every edge.

Fixing your own programs

• Read an error message from the bottom. Make the fault small, and follow the program by hand.
• Print the values inside a loop to watch them, and remove those prints when you are done.
• Change one thing at a time, and test the edges again.

A worked example

What does this program show? The heights are in centimetres.

1. The call. values gets the list of heights, and limit gets 140.
2. The loop. Follow count round by round.

3. The answer. return count gives back 3, and print shows 3. Three children are taller than 140 cm.

Practice with new numbers

A program to try

Five athletes jump on sports day. Two lists in the same order hold the names and the jumps in centimetres. Two small functions do the thinking.

Mistakes to avoid

• Counting positions from 1. The first item is a[0]. Asking for a position that does not exist stops with IndexError: list index out of range.
• a = a.sort(). sort() gives back None, so a is lost. Then a[0] stops with TypeError: 'NoneType' object is not subscriptable. Write a.sort() on its own line.
• print instead of return. The caller gets None. Then double(4) + 1 stops with TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'.
• A missing argument. For def speed(distance, time):, the call speed(100) stops with TypeError: speed() missing 1 required positional argument: 'time'.
• b = a as a copy. Both names are the same list, so a change through b shows in a. Use a.copy().

Check yourself

1. Lata makes days = ["Mon", "Tue", "Wed", "Thu", "Fri"]. Which day is days[3], and which slice gives only Tue and Wed?

2. Kiran writes a function double(n) whose only line is print(n * 2). Then he writes x = double(4) + 1. What goes wrong, and how should the function end?

3. A program has a = [3, 1, 2], then b = a, then b.append(5). What is in a now?

4. A program runs to the end with no error message, but its total is wrong. Which kind of fault is it, and what are two ways to find it?

5. A function is_teen(age) should give True for ages from 13 to 19 years, and False otherwise. Which ages would you test, and why?