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Processing arrays with loops

Arrays become powerful when a loop processes each element. Common exam tasks include calculating a total or average, counting values that meet a condition, finding an extreme value and updating elements.

In Python, for score in scores: assigns each element to score in turn. The indented statements then process that current element before the loop moves to the next one.

Multiple choice1 mark

Which loop header processes every value in scores directly?

Aif score in scores:
Bwhile scores = score:
Cfor score in scores:
Dprint(scores):

Common patterns

Initialise an accumulator or counter before the loop. For a maximum, a safe pattern is to initialise from the first element rather than guessing a starting value.

highest = scores[0]
for score in scores:
    if score > highest:
        highest = score

This works even if every score is negative.

Be clear whether a loop variable stores an element or an index. In for score in scores:, score is the current value, so writing scores[score] would incorrectly treat that value as a position. When positions are required, for example to use matching positions in two arrays, loop through indices instead: for index in range(len(scores)):.

The example below combines an accumulator, a counter and an average. Before running it, predict total and above_twelve. Notice that the average is calculated after the loop, when the complete total is available.

Worked example

readings = [12, 15, 9, 17, 11]
total = 0
above_twelve = 0
for reading in readings:
    total = total + reading
    if reading > 12:
        above_twelve = above_twelve + 1
average = total / len(readings)
print(total, average, above_twelve)

Multiple choice1 mark

In for reading in readings:, what does reading contain on each repetition?

AThe length of the list
BThe index of the current element
CThe whole list
DThe current element's value

Supported practice: total and average

Process every value in scores using a loop. Add each score to the running total, then calculate the average after the loop using total / len(scores).

total = 0 is required initialisation for the accumulator. The expected total is 75 and the expected average is 15.0.

Coding task3 marks

# Calculate the total and average score.
scores = [18, 12, 15, 20, 10]
total = 0
# Add a loop and calculate average.

Multiple choice1 mark

Why can highest = 0 be an unsafe initial value when searching a list for its highest element?

AAll actual values might be negative
BZero cannot be stored
CThe loop will always be infinite
DLists cannot contain numbers

Debugging task: find the true minimum

The supplied temperatures are all negative, so the initial statement lowest = 0 is unsafe: zero is not an element in the data and would hide some incorrect comparison logic.

Initialise lowest from the first array element, loop through all temperatures and replace lowest only when a smaller value is found. The final result should be -9.

Coding task3 marks

# Find the lowest temperature without assuming it is positive or negative.
temperatures = [-4, -9, -2, -7]
lowest = 0
# Correct the initial value and use a loop with selection.

Independent practice: count values in an inclusive range

Count the stored scores from 40 to 60 inclusive. Loop through every value, combine both boundary comparisons with and, and increase in_range only when the condition is true.

Boundary values 40 and 60 must both be counted. The supplied list contains four qualifying scores.

Coding task3 marks

# Count scores from 40 to 60 inclusive.
scores = [39, 40, 52, 60, 61, 47]
in_range = 0
# Use a loop and a compound condition.

Written answer2 marks

Explain why total = 0 must be placed before, rather than inside, a loop that calculates the sum of an array.

Compare retaining the running total with resetting it on each repetition.

Students type their answer here.

Written answer4 marks

Design an algorithm to count how many names in an array have more than six characters. State the initialisation, loop, condition and update.

Use `len(name)` in your explanation or Python-style algorithm.

Students type their answer here.

Review

When processing an array, initialise once before the loop, visit every required element and update only when the rule says to. Initialise a minimum or maximum from real array data, and calculate an average only after the complete total is known.