← VCE Software Development U3O1

Looping through an array by index

Loops i through every index of arr, so arr[i] can be read or changed and you know each element's position. Used for totals, finding the largest value and editing in place.

Part of: VCE Software Development - U3O1 - Programming — Module 93, VCE Software Development - U4O1 - development and evaluation — Module 36, QCE Digital Solutions Unit 1 — Algorithms and Programming Techniques — Module 76, QCE Digital Solutions Unit 4 — Prototype Digital Data Exchanges — Module 26, WA Computer Science ATAR Year 11 — Control Structures and Data Types — Module 54, WA Computer Science ATAR Year 11 — Standard Algorithms — Module 2

Understanding Looping through an array by index in VCE Software Development U3O1

What it does: The total starts at 0 and each element, read by its index, is added in turn, giving the sum of the array. largest holds the biggest value seen so far and is updated whenever arr[i] beats it, so it ends as the maximum. range() needs a number. len(scores) gives the number of elements, so range(len(scores)) produces the indexes 0 to len-1. arr[i] is the element at position i. Using i alone would add up the index numbers, not the values. Assigning to arr[i] changes the list itself. Changing the loop variable item only changes a copy of the value, so the list stays the same.

Common mistakes: A running total must start at 0. Starting at 1 adds an extra 1 to the answer. It should be total = 0. Array elements are read with square brackets, not round ones. Round brackets try to call marks as a function. It should be biggest = marks[0].

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This module's key terms are also in a printable crossword (Modules 72–96) and word search (Modules 72–113).

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Learning
Duo
Which loop visits every index of the list scores?
for i in range(scores):for i in range(len(scores)):
Matches
for i in range(len(arr)): total = total + i
↓
Adds the index i instead of the element arr[i], so the total is of positions, not values. It should be total = total + arr[i].
Applying
Meanings
Term
for i in range(len(arr)):
Meaning
Loops i through every index of arr, so arr[i] can be read or changed and you know each element's position. Used for totals, finding the largest value and editing in place.
Practice
marks = [72, 85, 64, 91, 78]
Testing
Gaps
for i in (len(arr)):
Check Answer
Spots
1scores = [12, 8, 15, 9]
2total = 1

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Quick Questions

1. Which loop visits every index of the list scores?
A. for i in range(scores):
B. for i in range(len(scores)): ✓
range() needs a number. len(scores) gives the number of elements, so range(len(scores)) produces the indexes 0 to len-1.
2. A list arr has 5 elements. Which values does i take in: for i in range(len(arr)):
A. 0, 1, 2, 3, 4 ✓
B. 1, 2, 3, 4, 5
Indexes start at 0, so a list of 5 elements has indexes 0 to 4. range(5) stops before 5.
3. Which code adds up every element of arr into total?
A. for i in range(len(arr)): total = total + i
B. for i in range(len(arr)): total = total + arr[i] ✓
arr[i] is the element at position i. Using i alone would add up the index numbers, not the values.
4. Which line finds the largest element so far inside the loop?
A. if arr[i] > largest: ✓
B. if i > largest:
Compare the element arr[i] with largest. The index i is just a position, not a value in the array.
5. Which code doubles every element of arr in place?
A. for i in range(len(arr)): arr[i] = arr[i] * 2 ✓
B. for item in arr: item = item * 2
Assigning to arr[i] changes the list itself. Changing the loop variable item only changes a copy of the value, so the list stays the same.

Match the Pairs

for i in range(len(arr)): → Loops i through every index of arr, from 0 to the last position
arr[i] = arr[i] * 2 → Changes the element at position i in place by doubling it
Totalling array elements: start a total at 0, then add arr[i] on every pass → total = 0 for i in range(len(arr)): total = total + arr[i]
Finding the largest element: start with the first element and replace it whenever arr[i] is bigger → largest = arr[0] for i in range(len(arr)): if arr[i] > largest: largest = arr[i]
for i in range(len(arr)): total = total + i → Adds the index i instead of the element arr[i], so the total is of positions, not values. It should be total = total + arr[i].
for i in range(arr): arr[i] = arr[i] + 1 → range() needs a number, but arr is a list. It should be range(len(arr)) so i covers every index.

Key Term

for i in range(len(arr)):
Loops i through every index of arr, so arr[i] can be read or changed and you know each element's position. Used for totals, finding the largest value and editing in place.

Fill in the Blanks

1. for i in range(len(arr)):
Hint: Type the function that generates the index numbers to loop through.
2. total = 0 for i in range(len(arr)): total = total + arr[i]
Hint: Type the function that gives the number of elements in the array.
3. largest = arr[0] for i in range(len(arr)): if arr[i] > largest: largest = arr[i]
Hint: Type the comparison operator that checks whether the current element is bigger than the largest so far.
4. for i in range(len(arr)): arr[i] = arr[i] * 2
Hint: Type the loop variable that selects which position is being changed in place.
5. total = 0 for i in range(len(arr)): total += arr[i]
Hint: Type the operator that adds each element onto the running total.

Spot the Error

Question 1
A scores = [12, 8, 15, 9]
B total = 1
C for i in range(len(scores)):
D total = total + scores[i]
E print(total)
Line B — A running total must start at 0. Starting at 1 adds an extra 1 to the answer. It should be total = 0.
Question 2
A marks = [56, 91, 73, 84]
B biggest = marks(0)
C for i in range(len(marks)):
D if marks[i] > biggest:
E biggest = marks[i]
Line B — Array elements are read with square brackets, not round ones. Round brackets try to call marks as a function. It should be biggest = marks[0].
Question 3
A prices = [10, 20, 30]
B for i in range(prices):
C prices[i] = prices[i] * 2
D print(prices)
Line B — range() needs a whole number, but prices is a list. Use len(prices) to get the number of elements: for i in range(len(prices)):
Question 4
A temps = [21, 25, 19, 30]
B total = 0
C for i in range(len(temps)):
D total = total + i
E average = total / len(temps)
Line D — i is the index (position), not the element. Adding i totals 0, 1, 2, 3 instead of the temperatures. It should be total = total + temps[i].
Question 5
A nums = [4, 9, 2, 7]
B for i in range(len(nums)):
C nums = nums[i] + 1
D print(nums)
Line C — This replaces the whole array with a single number. To change one element in place, assign to nums[i]. It should be nums[i] = nums[i] + 1.

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