All subjects · Computer Science · WCP02 Practical Programming and Problem-solving

Operations on data structures: create, insert, update, retrieve, delete, traverse

Edexcel IAL Computer Science · WCP02 Practical Programming and Problem-solving

Whatever the structure, you need the same six operations. Knowing the Python for each one, for lists and dictionaries, saves time in the exam.

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The six operations (spec 8.2.4)

Operation	List	Dictionary
Create	items = [] or list()	d = {} or dict()
Insert	items.append(x) (end); items.insert(i, x) (at index i)	d[key] = value (new key)
Update	items[i] = x	d[key] = value (existing key)
Retrieve	items[i]; items[a:b]	d[key]; d.get(key, default)
Delete	del items[i]; items.pop(i); items.pop() removes the last	del d[key]
Traverse	for x in items: or for i in range(len(items)):	for k, v in d.items():

Lists in action

queue = ["Ana", "Ben"]
queue.append("Cai")
queue.insert(0, "Dev")
queue[1] = "Ann"
removed = queue.pop()
del queue[0]
print(queue, removed)
print(queue + ["Eve"])

Output:

['Ann', 'Ben'] Cai
['Ann', 'Ben', 'Eve']

Deleting while traversing

Never delete items from a list while a for loop is going through it: the indexes shift and items are skipped. Build a new list of the items to keep instead.

nums = [4, 4, 7, 4]
for n in nums:
    if n == 4:
        nums.remove(n)
print(nums)
nums = [4, 4, 7, 4]
kept = []
for n in nums:
    if n != 4:
        kept.append(n)
print(kept)

Output:

[7, 4]
[7]

Copies and references

b = a does not copy a list: both names refer to the same list. Use copy.copy(a) (or a[:]) for a shallow copy, and copy.deepcopy(a) for a 2D list. The sample paper rewarded leaving the original data unchanged.

import copy
a = [[1, 2], [3, 4]]
b = a
c = copy.deepcopy(a)
a[0][0] = 99
print(b[0][0], c[0][0])

Output:

99 1

Worked example

Starting from tasks = ["wash", "cook"], insert "shop" at the front, change "cook" to "bake", then remove "wash".

tasks.insert(0, "shop") gives ["shop", "wash", "cook"]
tasks[2] = "bake"
del tasks[1]
Result: ['shop', 'bake']

Answer: ['shop', 'bake']

Key idea: append/insert add, [i] = updates, [i] and get() retrieve, del and pop() remove, for loops traverse; do not delete during a for loop; b = a shares a list, deepcopy copies it.

Check yourself

What is displayed when this code runs? ```python x = [5, 6, 7] x.insert(1, 9) x.append(1) print(x) ```
- [5, 6, 9, 7, 1]
- [9, 5, 6, 7, 1]
- [5, 9, 6, 7, 1]
- [1, 5, 9, 6, 7]
Show answer
[5, 9, 6, 7, 1]. insert(1, 9) puts 9 at index 1 and append adds 1 at the end: [5, 9, 6, 7, 1].
What is displayed when this code runs? ```python stack = [3, 8, 2, 6] a = stack.pop() b = stack.pop(0) print(a + b, len(stack)) ```
Show answer
9 2. pop() removes the last item (6) and pop(0) the first (3): 9, leaving 2 items: 9 2.
What is displayed when this code runs? ```python marks = {"Ana": 7, "Ben": 4, "Cai": 9} del marks["Ben"] marks["Dee"] = 5 print(list(marks.keys())) ```
- ['Ana', 'Ben', 'Cai', 'Dee']
- ['Dee', 'Ana', 'Cai']
- ['Ana', 'Cai', 'Dee']
- ['Ana', 'Cai']
Show answer
['Ana', 'Cai', 'Dee']. Ben is removed and Dee is added at the end: ['Ana', 'Cai', 'Dee'].
What is displayed when this code runs? ```python a = [1, 2, 3] b = a b.append(4) print(len(a)) ```
Show answer
4. b = a makes both names refer to the same list, so a also has 4 items: 4.
What is displayed when this code runs? ```python import copy a = [1, 2, 3] b = copy.copy(a) b.append(4) print(len(a)) ```
Show answer
3. copy.copy makes a new list, so a still has 3 items.
What is displayed when this code runs? ```python prices = {"tea": 2, "bun": 3, "jam": 4} for item in prices: prices[item] = prices[item] * 2 print(prices["bun"]) ```
Show answer
6. Each value is updated during the traversal; bun becomes 6.
This code should remove every negative number from readings, giving [4, 7, 2]. Which line completes it? ```python readings = [4, -1, -3, 7, 2] kept = [] for r in readings: __________ kept.append(r) readings = kept print(readings) ```
- `if r < 0:`
- `if r > 0 and r < 7:`
- `if readings[r] >= 0:`
- `if r >= 0:`
Show answer
`if r >= 0:`. Keep the items that are not negative.

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Last updated 8 November 2026. Spotted a mistake? Tell us.