Linear Search Algorithm in DSA Python - Build from Scratch

Linear Search Algorithm

📖 Scenario: You have a list of product IDs in a store's inventory. You want to find if a specific product ID is available.

🎯 Goal: Build a simple program that uses linear search to find the position of a product ID in the list.

📋 What You'll Learn

Create a list called product_ids with these exact values: 101, 203, 405, 502, 305

Create a variable called target_id and set it to 502

Use a for loop with variables index and product to iterate over enumerate(product_ids)

Inside the loop, use an if statement to check if product equals target_id

If found, set a variable found_index to index and break the loop

If not found after the loop, set found_index to -1

Print the value of found_index

💡 Why This Matters

🌍 Real World

Stores and warehouses often need to check if a product is available by searching through their inventory lists.

💼 Career

Understanding linear search helps in basic data lookup tasks and is a foundation for learning more efficient search algorithms.

1

Create the product ID list

Create a list called product_ids with these exact values: 101, 203, 405, 502, 305

DSA Python

# Create the list product_ids with the given values
# Your code here

Hint

Use square brackets [] to create the list and separate numbers with commas.

2

Set the target product ID

Create a variable called target_id and set it to 502

DSA Python

product_ids = [101, 203, 405, 502, 305]
# Create the variable target_id and set it to 502
# Your code here

Hint

Use the assignment operator = to set target_id to 502.

3

Implement linear search loop

Use a for loop with variables index and product to iterate over enumerate(product_ids). Inside the loop, use an if statement to check if product equals target_id. If found, set a variable found_index to index and break the loop. If not found after the loop, set found_index to -1.

DSA Python

product_ids = [101, 203, 405, 502, 305]
target_id = 502
# Use a for loop with index, product in enumerate(product_ids)
# Inside the loop, check if product == target_id
# If yes, set found_index = index and break
# After the loop, if not found, set found_index = -1
# Your code here

Hint

Initialize found_index to -1 before the loop. Use enumerate() to get both index and product.

4

Print the found index

Write print(found_index) to display the position of the target product ID in the list.

DSA Python

product_ids = [101, 203, 405, 502, 305]
target_id = 502
found_index = -1
for index, product in enumerate(product_ids):
    if product == target_id:
        found_index = index
        break
# Print the found_index
# Your code here

Hint

The output should be the index where 502 is found, which is 3.

Practice

1. What is the main idea behind the Linear Search Algorithm?

easy

A. Sort the list first then search using binary steps

B. Divide the list into halves repeatedly to find the target

C. Check each element one by one until the target is found or list ends

D. Use a hash table to find the target instantly

Solution

1. Step 1: Understand linear search process

   Linear search checks each element in order, one by one.

2. Step 2: Compare with other search methods

   Binary search divides list, hash uses extra memory, linear is simple sequential check.

3. Final Answer:

   Check each element one by one until the target is found or list ends -> Option C

4. Quick Check:

   Linear search = sequential check [OK]

Hint: Linear search checks items one by one [OK]

Common Mistakes:

• Confusing linear search with binary search
• Thinking linear search requires sorted list
• Assuming linear search is faster than binary search always

2. Which of the following is the correct syntax to start a linear search loop over a list arr in Python?

easy

A. for i to len(arr):

B. for i in range(arr):

C. for i in arr:

D. for i in range(len(arr)):

Solution

1. Step 1: Identify how to loop over indices

   To access elements by index, use range(len(arr)).

2. Step 2: Check each option's correctness

   for i in arr: loops over elements, not indices; B is invalid because range needs an integer; D is invalid syntax.

3. Final Answer:

   for i in range(len(arr)): -> Option D

4. Quick Check:

   Loop indices = range(len(arr)) [OK]

Hint: Use range(len(arr)) to loop indices [OK]

Common Mistakes:

• Using range(arr) instead of range(len(arr))
• Trying to loop with invalid syntax like 'for i to len(arr)'
• Confusing looping over elements vs indices

3. What will be the output of the following code?

def linear_search(arr, target):
    for i in range(len(arr)):
        if arr[i] == target:
            return i
    return -1

print(linear_search([4, 2, 7, 1, 3], 7))

medium

A. 2

B. 3

C. -1

D. 7

Solution

1. Step 1: Trace the loop and compare elements

   Check elements: arr[0]=4 (no), arr[1]=2 (no), arr[2]=7 (yes, matches target 7).

2. Step 2: Return index when found

   Function returns index 2 immediately when target found.

3. Final Answer:

   2 -> Option A

4. Quick Check:

   Index of 7 in list = 2 [OK]

Hint: Return index when target matches during loop [OK]

Common Mistakes:

• Returning element value instead of index
• Returning -1 even when target is present
• Confusing index with element value

4. Find the error in this linear search function:

def linear_search(arr, target):
    for i in range(len(arr)):
        if arr[i] = target:
            return i
    return -1

medium

A. Using '=' instead of '==' in the if condition

B. Missing colon ':' after for loop

C. Returning -1 instead of None when not found

D. Looping over range(len(arr)) instead of arr

Solution

1. Step 1: Identify syntax error in condition

   The if condition uses '=' which is assignment, not comparison.

2. Step 2: Correct operator for comparison

   Use '==' to compare values in if condition.

3. Final Answer:

   Using '=' instead of '==' in the if condition -> Option A

4. Quick Check:

   Comparison needs '==' not '=' [OK]

Hint: Use '==' for comparison, '=' is assignment [OK]

Common Mistakes:

• Using single '=' in if condition
• Confusing assignment and comparison operators
• Ignoring syntax errors causing runtime failure

5. You want to find all positions of a target number in a list using linear search. Which modification to the code below will achieve this?

def linear_search_all(arr, target):
    positions = []
    for i in range(len(arr)):
        if arr[i] == target:
            # What to do here?
    return positions

print(linear_search_all([1, 3, 7, 3, 9, 3], 3))

hard

A. Return i immediately when target is found

B. Append index i to positions list inside the if block

C. Break the loop after first match

D. Use positions = target instead of list

Solution

1. Step 1: Understand requirement to find all matches

   We must collect all indices where target appears, not stop at first.

2. Step 2: Append matching indices to list

   Inside if, add positions.append(i) to store all found positions.

3. Final Answer:

   Append index i to positions list inside the if block -> Option B

4. Quick Check:

   Collect all indices by appending [OK]

Hint: Append all matching indices to list [OK]

Common Mistakes:

• Returning immediately on first match
• Breaking loop early
• Assigning target to positions instead of appending