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Free GCSE Computer Science guide
Find a value by its list index, loop through a collection and reuse a calculation in a function. Follow the workshop's five-number list and sumUp example, then check your understanding with original questions.
Content owner: Michael Print · Written for GCSE learners · Checked against official specifications
A list groups values in an ordered collection. You can access an item by its position, change an item or add another one. In Python, the first index is 0.
A function groups a reusable block of code under a name. This page's sumUp function accepts two values and returns their inclusive range total so the calling code can use the result.
nums = [4, 5, 8, 1, 3]
print(nums[0]) # 4
print(nums[1]) # 5
print(nums[4]) # 3
print(len(nums)) # 5Index 0
4
Index 1
5
Index 2
8
Index 3
1
Index 4
3
nums[0] is 4 and nums[4] is 3. The index identifies a position; it is not the value at that position. The list has five items, so len(nums) is 5, but the last non-negative index is 4.
Ordered means the items keep their positions. It does not mean sorted into numerical order: 8 is followed by 1 in this list.
nums = [4, 5, 8, 1, 3]
nums.append(7)
print(nums) # [4, 5, 8, 1, 3, 7]
print(len(nums)) # 6append() adds one item at the end of the existing list. It does not sort the values. After this append, 7 is at index 5 and the length is 6.
nums = [4, 5, 8, 1, 3]
for num in nums:
print(num)The loop assigns each item in turn to num, then prints it. The output is 4, 5, 8, 1 and 3, each on a separate line. Here num holds the item's value, rather than its index.
The workshop's sumUp function adds each integer from start to end, including both endpoints. For this example, supply integer arguments with start <= end.
def sumUp(start, end):
total = 0
for i in range(start, end + 1):
total = total + i
return total
result = sumUp(4, 8)
print(result) # 30def sumUp(start, end):start and end are parameter names. Defining the function does not run its body.sumUp(4, 8)start and the argument 8 supplies end.total = 0range(start, end + 1)end.return totalresult receives it, and the separate print(result) displays it.Python terminology distinguishes parameters in the definition from arguments at the call. AQA's examinable terminology uses “parameter” for both; follow the wording of your exam question.
Worked example
The running total starts at zero. Adding 4 gives 4; adding 5 gives 9; then the totals become 15, 22 and 30. The return value is 30, because 4 + 5 + 6 + 7 + 8 = 30.
Call sumUp(1, 10) with different arguments and the same function returns 55, matching the calculation in the selection and loops guide.
| i | total after addition |
|---|---|
| 4 | 4 |
| 5 | 9 |
| 6 | 15 |
| 7 | 22 |
| 8 | 30 |
A short captioned explanation from Michael's Python fundamentals workshop. Use the examples on this page to practise the idea afterwards.
The list [4, 5, 8, 1, 3] stores five ordered values. Its first index is zero, so index 0 gives 4, index 1 gives 5 and index 4 gives 3. The last index is one less than the list length.
A short captioned explanation from Michael's Python fundamentals workshop. Use the examples on this page to practise the idea afterwards.
This function takes start and end, adds their inclusive integer range and returns the total. Calling sumUp(4, 8) supplies 4 and 8 and returns 30.
These independently written questions use the original five-item list unless an append is explicitly stated. The marking guidance is indicative.
For nums = [4, 5, 8, 1, 3], state the values of nums[0], nums[1], nums[4] and len(nums).
Indicative answer: nums[0] is 4; nums[1] is 5; nums[4] is 3; len(nums) is 5. Award one mark for each correct value.
The program executes nums.append(7) on the original list. State the new list, its length and the index of the appended item.
Indicative answer: The list becomes [4, 5, 8, 1, 3, 7] (1); its length is 6 (1); the appended item is at index 5 (1).
For the call result = sumUp(4, 8), identify the parameter names and supplied arguments. Explain the calculation, state the return value and state what print(result) outputs.
Indicative answer: The parameter names are start and end (1); the supplied arguments are 4 and 8 respectively (1). The function adds 4 + 5 + 6 + 7 + 8 (1), returning 30 (1). That value is assigned to result and the print outputs 30 (1).
A learner changes the function's loop to for i in range(start, end):. State what sumUp(4, 8) now returns, explain the error and repair the range to include both endpoints.
Indicative answer: It returns 22 (1), because the range stops before 8 and adds only 4, 5, 6 and 7 (1). Restore range(start, end + 1) (1); the corrected call returns 30 (1).
These examples practise one-dimensional indexed collections and functions. They do not cover every array or subprogram requirement, such as two-dimensional arrays. Use the index bounds and terminology stated by your board.
These are independently written explanations and practice questions. CompSciTutoring.co.uk is not affiliated with or endorsed by an examination board. The marking guidance is indicative; always check the syllabus for your examination year.
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Use the free examples first, then explore one-to-one tuition if guided practice would help.