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Algorithms consolidation

This worksheet brings together strings, arrays, subprograms, searching and sorting. The questions move from recognition to program completion and applied exam-style reasoning. Read each scenario carefully and choose methods because they fit the data and requirement.

Key ideas to recall

A linear search checks items in order and can be used on unsorted data.
A binary search repeatedly checks the middle item and discards half of the remaining search area. Its data must already be sorted.
A bubble sort compares adjacent values and swaps those in the wrong order.
An insertion sort takes the next unsorted value and inserts it into the correct place in a growing sorted section.
A merge sort splits data into smaller sublists, then merges sorted sublists back together.
String characters and array-style list elements can be selected using an index; Python starts indexing at 0.
A function uses return when its result needs to be stored or used elsewhere in a program.

Multiple choice1 mark

A list of unsorted emergency call identifiers must be checked once for a target. Which search is immediately applicable?

ABinary search
BMerge sort
CInsertion sort
DLinear search

Multiple choice1 mark

Which algorithm repeatedly inserts the next unsorted value into a growing sorted section?

AInsertion sort
BBubble sort
CLinear search
DBinary search

Written answer3 marks

Apply one complete ascending bubble-sort pass to [5, 1, 4, 2]. Show the list after each swap.

Compare adjacent values from left to right. Swap them only when the left value is larger.

Students type their answer here.

Integrated practice: strings, arrays and functions

Complete count_initial(names, target_initial) so it returns how many names begin with the required character. Loop through every name, use index 0 to select its first character and increase count when it matches the parameter.

Replace pass with the required decision. The function must work for different lists and initials, so do not hard-code "A" or the answer 2.

Coding task4 marks

# Complete the function to return the number of names beginning with target_initial.
def count_initial(names, target_initial):
    count = 0
    for name in names:
        pass
    return count

answer = count_initial(["Aisha", "Ben", "Amir", "Cara"], "A")

Written answer4 marks

A school keeps student surnames in alphabetical order and frequently searches for a surname. Recommend a search algorithm and explain its steps for this scenario.

Name the prerequisite already satisfied, then explain how the search area changes.

Students type their answer here.

Read and run: finding the longest string

This function combines an array parameter, initialisation from the first element, iteration, selection and a returned result. Predict the word returned before running it.

Notice that longest stores an actual string rather than merely its length. The comparison uses lengths, but the function returns the selected element from the array.

Worked example

def find_longest(words):
    longest = words[0]
    for word in words:
        if len(word) > len(longest):
            longest = word
    return longest

result = find_longest(["oak", "willow", "ash", "beech"])
print(result)

Written answer5 marks

Explain how the find_longest function works. Refer to its parameter, initial value, loop, selection and returned value.

Follow the data through the function rather than translating each line separately.

Students type their answer here.

Multiple choice1 mark

Why is longest = words[0] safer than longest = "" when the task is to return an element from a non-empty list?

AIt automatically sorts the words
BIt guarantees the starting candidate is an actual list element
CIt makes every string the same length
DIt prevents the loop from running

Debugging challenge: a premature return

The supplied contains function should return True when target appears anywhere in values, and False only after every element has been checked.

At present, the else returns False after the first non-match, ending the function too early. Move the unsuccessful return to the correct position while preserving the successful return inside the loop.

Coding task4 marks

# Debug the function. It should return True when target is present and False otherwise.
def contains(values, target):
    for value in values:
        if value == target:
            return True
        else:
            return False

present = contains([4, 8, 12], 12)
absent = contains([4, 8, 12], 7)

Written answer2 marks

Why did the original contains function fail when the target appeared after the first element?

Identify exactly when the incorrect return executed and its effect on the loop.

Students type their answer here.

Written answer3 marks

A student says that learning the steps of searching and sorting means memorising one exact Python program. Evaluate this claim for OCR J277.

Distinguish understanding, applying and recognising an algorithm from recalling one implementation.

Students type their answer here.

Block B checkpoint

You should now be able to manipulate strings, store and process array-style data, design reusable subprograms, and apply or recognise OCR's required searching and sorting algorithms. Focus on the data movement and decisions: different valid code can express the same algorithmic method.