Use Python lists and strings, including indexing, length, looping over items, and basic operations like append and slicing

What this dot point is asking

SEAB wants you to use Python lists and strings: creating them, indexing items, finding their length, looping over them, and basic operations such as append and slicing. The central idea is that both a list and a string are ordered sequences whose items have positions (indexes) starting at $0$, so you can reach any item by its index and process every item with a loop.

The answer

Creating a list

A list stores several values in order, written in square brackets:

scores = [40, 75, 60, 90]
names = ["Ravi", "Mei", "Tom"]

A list can hold numbers, strings, or a mix, and it can grow or shrink.

Indexing starts at zero

Each item has an index showing its position, starting at $0$:

scores[0]    # 40  (first item)
scores[3]    # 90  (fourth item)

The last item of a list of length $n$ is at index $n - 1$. A negative index counts from the end: scores[-1] is the last item.

Length with len()

len() gives the number of items in a list or characters in a string:

len(scores)    # 4
len("cat")     # 3

Looping over items

A for loop visits each item in turn:

for score in scores:
    print(score)

Adding to a list

append adds an item to the end:

scores.append(55)    # scores is now [40, 75, 60, 90, 55]

Strings are sequences too

A string is a sequence of characters, indexed the same way:

word = "COMPUTER"
word[0]      # 'C'
len(word)    # 8

Slicing

A slice takes part of a list or string with [start:stop]. The start is included, the stop is excluded:

word[0:4]    # 'COMP'  (characters 0,1,2,3)
scores[1:3]  # [75, 60]

Examples in context

Example 1. A register of names. A teacher stores student names in a list and loops over it to print the register. Adding a new student is a single append, and len(names) gives the class size at any time without counting by hand. Because the names keep their order, the first to register is always names[0], which makes it easy to process them in the order they arrived.

Example 2. Checking a password length. A login program uses len(password) to reject passwords shorter than eight characters, and could slice password[0:3] to show a masked hint such as the first three letters. Treating the password as a sequence of characters makes both the length check and the slice short and clear, and the same indexing ideas apply to any text the program reads.

Try this

Q1. For data = [10, 20, 30], state the value of data[1]. [1 mark]

• Cue. $20$, the second item (indexing starts at $0$).

Q2. Write a line that adds the number $99$ to the end of a list called nums. [2 marks]

• Cue. nums.append(99).

Q3. State what len("HELLO") returns and why. [2 marks]

• Cue. $5$, because the string has five characters and len counts them.