← VCE Software Development U3O1

Linear search in Python

An algorithm that uses a loop to check each item in a list in turn. It stops when it finds the target or reaches the end of the list, often returning the index or -1.

Part of: VCE Software Development - U3O1 - Programming — Module 145, 9-10 Digital Technologies: Modular programs and algorithms — Module 10, IGCSE Computer Science 0478 — 7c Pseudocode and Flowcharts — Module 112

Understanding Linear search in Python in VCE Software Development U3O1

What it does: Returning i inside the loop ends the function straight away and reports the index where the target was found. range() needs a number, so use len(scores) to get how many items there are. range(scores) would cause a TypeError. Jumping to the middle and halving is binary search. Linear search checks items one at a time from the start. -1 is not a valid index, so it clearly means 'not found'. 0 is the first index, so it would look like a match at the start of the list. == compares two values. A single = assigns a value and causes a SyntaxError inside an if statement.

Common mistakes: A single = assigns a value, so it causes a syntax error inside an if. Use == to compare: if nums[i] == target:. With != the function returns the index of the first item that is NOT the goal. Linear search must check for a match using ==.

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Learning

Duo

Which code correctly starts a linear search through the list scores?

for i in range(scores):for i in range(len(scores)):

Matches

if items[i] = target:

↓

Uses = (assignment) instead of == (comparison), which causes a syntax error. It should be ==.

Applying

Meanings

Term

linear search

Meaning

An algorithm that uses a loop to check each item in a list in turn. It stops when it finds the target or reaches the end of the list, often returning the index or -1.

Practice

def linear_search(items, target):

Testing

Gaps

i in range(len(items)):

Check Answer

Spots

2 for i in range(len(nums)):

3 if nums[i] = target:

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Quick Questions

1. Which code correctly starts a linear search through the list scores?

A. for i in range(scores):

B. for i in range(len(scores)): ✓

range() needs a number, so use len(scores) to get how many items there are. range(scores) would cause a TypeError.

2. What does a linear search do?

A. Jumps to the middle of the list and halves the search area each time

B. Checks each item in order until it finds the target or reaches the end ✓

Jumping to the middle and halving is binary search. Linear search checks items one at a time from the start.

3. A linear search finds the target at index 2. What should the function do next?

A. return i straight away ✓

B. keep looping to the end of the list

Linear search stops as soon as it finds the target. Returning immediately ends the function and the loop.

4. Which value does a linear search commonly return when the target is NOT in the list?

A. -1 ✓

B. 0

-1 is not a valid index, so it clearly means 'not found'. 0 is the first index, so it would look like a match at the start of the list.

5. Which comparison checks whether the current item matches the target?

A. if names[i] == target: ✓

B. if names[i] = target:

== compares two values. A single = assigns a value and causes a SyntaxError inside an if statement.

Match the Pairs

for i in range(len(items)): → Loops through every index of the list so each item can be checked in turn

if items[i] == target: → Compares the current item with the value being searched for

When the target is found, the search stops and gives back its position (index) → return i

If the loop finishes without finding the target, the search returns -1 to show it is not in the list → return -1

if items[i] = target: → Uses = (assignment) instead of == (comparison), which causes a syntax error. It should be ==.

for i in range(len(items)): if items[i] == target: return i return -1 → return -1 is inside the loop, so the search gives up after checking only the first item. It should be after the loop, not inside it.

Key Term

linear search

An algorithm that uses a loop to check each item in a list in turn. It stops when it finds the target or reaches the end of the list, often returning the index or -1.

Fill in the Blanks

1. for i in range(len(items)): if items[i] == target: return i

Hint: Type the keyword that starts a loop so every item is checked in turn.

2. for i in range(len(items)): if items[i] == target: return i

Hint: Type the operator that compares two values to see if they are equal.

3. for i in range(len(items)): if items[i] == target: return i return -1

Hint: Type the value returned when the loop ends without finding the target.

4. for i in range(len(items)): if items[i] == target: break

Hint: Type the keyword that stops the loop early once the target is found.

5. for i in range(len(items)): if items[i] == target: return i

Hint: Type the built-in function that gives how many items are in the list.

Spot the Error

Question 1

A def find(nums, target):

B for i in range(len(nums)):

C if nums[i] = target:

D return i

E return -1

Line C — A single = assigns a value, so it causes a syntax error inside an if. Use == to compare: if nums[i] == target:

Question 2

A def find(ids, goal):

B for i in range(len(ids)):

C if ids[i] != goal:

D return i

E return -1

Line C — With != the function returns the index of the first item that is NOT the goal. Linear search must check for a match using ==.

Question 3

A def find(pets, name)

B for i in range(len(pets)):

C if pets[i] == name:

D return i

E return -1

Line A — A function definition must end with a colon. It should be def find(pets, name):

Question 4

A def find(pets, name):

B for i in range(pets):

C if pets[i] == name:

D return i

E return -1

Line B — range() needs a number, not a list. Use len(pets) to get the length: for i in range(len(pets)):

Question 5

A def search(nums, target):

B for i in range(len(nums)):

C if nums[i] == target:

D return target

E return -1

Line D — Linear search should return the index where the target was found. Returning target just gives back the value we already knew. It should be return i.

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