Class 9 · Unit 5: Python · Lesson 5.5 · 40 min

Lists, and doing something to every item

Five days, five variables. Then the teacher asks for a month: thirty-one variables. There's a better box.

Today you will: Store many values in one list · Do something to every item with a loop · Build a running total and a mean

Story

Hiba has five temperatures on the board — Monday to Friday — and a program with five variables in it: mon, tue, wed, thu, fri. It works. She is pleased with it.

Then her teacher asks for the average across a month.

Thirty-one variables. Thirty-one lines of typing, and thirty more + signs. She has written the same instruction over and over, which is exactly what a computer is supposed to do for her.

There is a better container, and a way to say do this to every item in it.

Watch

One name, many values — — scan code 9.5.5 in the printed book.

Warm up your fingers

Today's symbols are the square brackets [ ] and the comma ,.

[ and ] are to the right of P. No Shift — that is what makes them different from { and }.
typing.com: the symbols lessons, then a free practice round.
Then type this line four times, with no mistakes: temps = [12, 15, 18, 21, 19]

You have now met every symbol this unit needs: ( ) " " _ = : [ ] and the operators. From here the difficulty is thinking, not typing.

On the laptop

You need: Thonny · save as 05-average.py

Mission 1: A list and its positions (6 min)

☐1

Predict the four outputs first, then run:

temps = [12, 15, 18, 21, 19]
print(temps)
print(temps[0])
print(temps[4])
print(len(temps))

☐2
Predict: what will print(temps[5]) do? Run it: IndexError: list index out of range. Positions start at 0. Delete the line.

Mission 2: Every item, in turn (8 min)

☐3

A running total, then the mean:

total = 0
for t in temps:
   total += t
print("Total:", total)
print("Average:", round(total / len(temps), 1))

total += t means add t to what's already in total. You should get 85 and 17.0.

☐4
Check it in one line: print(sum(temps) / len(temps)).

Mission 3: Repeat until something changes (4 min)

☐5
A while loop runs as long as a condition is true:
```
count = 5
while count > 0:
   print(count)
   count -= 1
print("Done")
```
Forget count -= 1 and it runs for ever — press Stop. Every programmer does it once. Save: five programs kept.

Stuck?

IndexError? Count from zero. Total always 85? total = 0 must be above the loop. Won't stop? A while with no -= 1 — press Stop. Only the last number prints? Check the indentation.

Finished early?

Find the highest temperature without max(): start with highest = temps[0], loop, and replace it whenever you meet a bigger one. Check against max(temps).

No laptop today?

Trace the running total on paper in two columns, t and total, one row per time round. How many rows, and why? What would print for a one-item list, [12]?

Now you know

A list of five temperatures in numbered positions 0 to 4, with position 5 giving an IndexError; below, a running total starting at 0 grows to 12, 27, 45, 66 and 85, which divided by 5 gives a mean of 17.0.

A list holds many values under one name: temps = [12, 15, 18, 21, 19].
Positions start at 0. The last item is at len(list) - 1, and asking beyond it gives an IndexError.
for item in list: does the same thing to every item, in order.
+= adds to what a variable already holds; -= takes away. They are shorthand for x = x + 1 and x = x - 1.
A running total needs a pot set to 0 before the loop.
while condition: repeats as long as the condition is true — and repeats for ever if nothing inside ever makes it false.
Every model you trained this year learned from a list of examples. This is the shape data has.

Debate it

To find the average, the program needed all five numbers, not one.

A model that grades apples needs many photographs; a model that predicts rainfall needs many years. But somebody chooses which five days, or which five hundred photographs, go into the list. Who is left out of a list, and what does that do to the average? You have answered a version of this question every year since Class 6 — say what has changed in your answer.

Check yourself — practice, not a test

1. In temps = [12, 15, 18, 21, 19], what does temps[1] give?
- ○ 12
- ○ 15
- ○ 1
- ○ 19
2. What does this print? python nums = [4, 6, 10] total = 0 for n in nums: total += n print(total)
- ○ 0
- ○ 20
- ○ 10
- ○ 4 6 10
3. Find the bug. This should print the total, but prints 10: python nums = [4, 6, 10] for n in nums: total = 0 total += n print(total)
- ○ The list is wrong
- ○ total = 0 is inside the loop, so it resets each time
- ○ += should be =
- ○ print should be indented
4. len([3, 5, 7]) gives 2, because counting starts at zero.
- ○ True
- ○ False
5. A loop that never ends because its condition stays true is called an ______ loop.
6. Put these in the order they must appear to total a list correctly:
- 1. for n in nums:
- 2. total = 0
- 3. print(total)
- 4. total += n

Remember

A list holds many; a loop visits each one. Count from zero, and set the pot to 0 first.

Story

Hiba has five temperatures on the board — Monday to Friday — and a program with five variables in it: mon, tue, wed, thu, fri. It works. She is pleased with it.

Then her teacher asks for the average across a month.

Thirty-one variables. Thirty-one lines of typing, and thirty more + signs. She has written the same instruction over and over, which is exactly what a computer is supposed to do for her.

There is a better container, and a way to say do this to every item in it.

Warm up your fingers

Today's symbols are the square brackets [ ] and the comma ,.

[ and ] are to the right of P. No Shift — that is what makes them different from { and }.
typing.com: the symbols lessons, then a free practice round.
Then type this line four times, with no mistakes: temps = [12, 15, 18, 21, 19]

You have now met every symbol this unit needs: ( ) " " _ = : [ ] and the operators. From here the difficulty is thinking, not typing.

On the laptop

You need: Thonny · save as 05-average.py

Mission 1: A list and its positions (6 min)

☐1

Predict the four outputs first, then run:

temps = [12, 15, 18, 21, 19]
print(temps)
print(temps[0])
print(temps[4])
print(len(temps))

☐2
Predict: what will print(temps[5]) do? Run it: IndexError: list index out of range. Positions start at 0. Delete the line.

Mission 2: Every item, in turn (8 min)

☐3

A running total, then the mean:

total = 0
for t in temps:
   total += t
print("Total:", total)
print("Average:", round(total / len(temps), 1))

total += t means add t to what's already in total. You should get 85 and 17.0.

☐4
Check it in one line: print(sum(temps) / len(temps)).

Mission 3: Repeat until something changes (4 min)

☐5
A while loop runs as long as a condition is true:
```
count = 5
while count > 0:
   print(count)
   count -= 1
print("Done")
```
Forget count -= 1 and it runs for ever — press Stop. Every programmer does it once. Save: five programs kept.

Stuck?

Finished early?

Find the highest temperature without max(): start with highest = temps[0], loop, and replace it whenever you meet a bigger one. Check against max(temps).

No laptop today?

Trace the running total on paper in two columns, t and total, one row per time round. How many rows, and why? What would print for a one-item list, [12]?

Now you know

A list holds many values under one name: temps = [12, 15, 18, 21, 19].

Positions start at 0. The last item is at len(list) - 1, and asking beyond it gives an IndexError.

for item in list: does the same thing to every item, in order.

+= adds to what a variable already holds; -= takes away. They are shorthand for x = x + 1 and x = x - 1.

A running total needs a pot set to 0 before the loop.

while condition: repeats as long as the condition is true — and repeats for ever if nothing inside ever makes it false.

Every model you trained this year learned from a list of examples. This is the shape data has.

Debate it

To find the average, the program needed all five numbers, not one.

Check yourself — practice, not a test

1. In temps = [12, 15, 18, 21, 19], what does temps[1] give?

○ 12
○ 15
○ 1
○ 19

2. What does this print? python nums = [4, 6, 10] total = 0 for n in nums: total += n print(total)

○ 0
○ 20
○ 10
○ 4 6 10

3. Find the bug. This should print the total, but prints 10: python nums = [4, 6, 10] for n in nums: total = 0 total += n print(total)

○ The list is wrong
○ total = 0 is inside the loop, so it resets each time
○ += should be =
○ print should be indented

4. len([3, 5, 7]) gives 2, because counting starts at zero.

○ True
○ False

5. A loop that never ends because its condition stays true is called an ______ loop.

6. Put these in the order they must appear to total a list correctly:

1. for n in nums:
2. total = 0
3. print(total)
4. total += n