Why Collision Handling Using Open Addressing Linear Probing in DSA Rust?

The Big Idea

What if your mailbox could magically find the next free slot without you searching endlessly?

The Scenario

Imagine you have a small mailbox with numbered slots for letters. When two letters arrive for the same slot number, you have to decide where to put the second letter. If you try to put it manually without a plan, you might lose letters or waste time searching for empty slots.

The Problem

Manually checking each slot for availability is slow and confusing. You might forget which slots are taken or accidentally overwrite letters. This causes errors and wastes time, especially when many letters arrive at once.

The Solution

Open Addressing with Linear Probing solves this by checking the next slot in order until it finds an empty one. This way, collisions are handled smoothly without losing data, and the process is fast and predictable.

Before vs After

✗ Before

fn insert(&mut self, key: usize) {
    let h = key % self.size;
    if self.slots[h] != None {
        // Manually search for empty slot
        let mut i = (h + 1) % self.size;
        while self.slots[i] != None {
            i = (i + 1) % self.size;
        }
        self.slots[i] = Some(key);
    } else {
        self.slots[h] = Some(key);
    }
}

✓ After

fn insert(&mut self, key: usize) {
    let mut index = key % self.size;
    while self.slots[index] != None {
        index = (index + 1) % self.size;
    }
    self.slots[index] = Some(key);
}

What It Enables

This method enables efficient and reliable storage of data even when multiple items map to the same slot, keeping operations fast and simple.

Real Life Example

Think of a parking lot where cars have assigned spots. If a spot is taken, the driver looks for the next available spot in order. This avoids confusion and keeps parking organized.

Key Takeaways

Manual collision handling is slow and error-prone.

Linear probing checks next slots in order to find empty space.

This method keeps data organized and access fast.

Practice

1. What is the main idea behind linear probing in collision handling for hash tables?

easy

A. Use a secondary hash function to find a new slot.

B. Use a linked list to store multiple values at the same index.

C. Double the size of the hash table when a collision happens.

D. If a collision occurs, check the next slot sequentially until an empty slot is found.

Solution

1. Step 1: Understand collision handling methods

   Linear probing is a method where, upon collision, the algorithm checks the next slot in the array sequentially.

2. Step 2: Compare with other methods

   Other methods include chaining (linked lists), resizing, or double hashing, but linear probing specifically uses sequential slots.

3. Final Answer:

   If a collision occurs, check the next slot sequentially until an empty slot is found. -> Option D

4. Quick Check:

   Linear probing = sequential slot check [OK]

Hint: Linear probing means try next slot one by one [OK]

Common Mistakes:

• Confusing linear probing with chaining
• Thinking linear probing uses a second hash function
• Assuming table size doubles on collision

2. Which of the following Rust code snippets correctly implements linear probing to find the next index after a collision in a hash table of size table_size?

easy

A. let next_index = (current_index + 1) % table_size;

B. let next_index = (current_index * 2) % table_size;

C. let next_index = (current_index - 1) % table_size;

D. let next_index = (current_index + 3) % table_size;

Solution

1. Step 1: Understand linear probing index calculation

   Linear probing moves to the next slot by adding 1 to the current index and wrapping around using modulo.

2. Step 2: Check each option

   let next_index = (current_index + 1) % table_size; adds 1 and uses modulo, which is correct. Others multiply or subtract, which are not standard linear probing steps.

3. Final Answer:

   let next_index = (current_index + 1) % table_size; -> Option A

4. Quick Check:

   Linear probing = increment index by 1 modulo table size [OK]

Hint: Linear probing increments index by 1 modulo table size [OK]

Common Mistakes:

• Using multiplication instead of addition
• Subtracting index without handling negative wrap
• Skipping modulo operation causing out-of-bounds

3. Consider the following Rust code snippet for inserting keys into a hash table using linear probing. What will be the final state of the hash table array after inserting keys [5, 15, 25] with table size 10 and hash function key % 10?

let mut table = vec![None; 10];
let keys = vec![5, 15, 25];
for &key in &keys {
    let mut index = key % 10;
    while table[index].is_some() {
        index = (index + 1) % 10;
    }
    table[index] = Some(key);
}
println!("{:?}", table);

medium

A. [None, None, None, None, None, Some(5), Some(15), None, Some(25), None]

B. [None, None, None, None, None, Some(5), None, Some(15), Some(25), None]

C. [None, None, None, None, None, Some(5), Some(15), Some(25), None, None]

D. [Some(5), Some(15), Some(25), None, None, None, None, None, None, None]

Solution

1. Step 1: Calculate initial indices for keys

   Keys 5, 15, 25 all hash to index 5 (5 % 10 = 5, 15 % 10 = 5, 25 % 10 = 5).

2. Step 2: Insert keys using linear probing

   Insert 5 at index 5. For 15, index 5 is occupied, so move to 6. Insert 15 at 6. For 25, indices 5 and 6 are occupied, move to 7 and insert 25.

3. Final Answer:

   [None, None, None, None, None, Some(5), Some(15), Some(25), None, None] -> Option C

4. Quick Check:

   Linear probing places keys at 5, 6, 7 [OK]

Hint: Keys with same hash go to next free slot sequentially [OK]

Common Mistakes:

• Placing all keys at index 5 ignoring collisions
• Skipping modulo wrap-around
• Inserting keys at wrong indices after collision

4. The following Rust code attempts to insert keys into a hash table using linear probing. However, it sometimes causes an infinite loop. What is the bug?

let mut table = vec![None; 5];
let keys = vec![1, 6, 11];
for &key in &keys {
    let mut index = key % 5;
    while table[index].is_some() {
        index = (index + 1) % 5;
    }
    table[index] = Some(key);
}

medium

A. The code does not check if the table is full, causing infinite loop when no empty slot exists.

B. The modulo operator is used incorrectly; it should be key % 6.

C. The loop should break after checking only once to avoid infinite loop.

D. The vector size is too large, causing index out of bounds.

Solution

1. Step 1: Analyze the while loop condition

   The loop continues while the slot is occupied, moving to next index with wrap-around.

2. Step 2: Identify missing termination condition

   If the table is full, the loop cycles endlessly because no empty slot is found and no exit condition exists.

3. Final Answer:

   The code does not check if the table is full, causing infinite loop when no empty slot exists. -> Option A

4. Quick Check:

   Missing full-table check causes infinite loop [OK]

Hint: Always check for full table to avoid infinite loops [OK]

Common Mistakes:

• Ignoring full table condition
• Changing modulo divisor incorrectly
• Breaking loop too early causing wrong insertion

5. You have a hash table of size 7 using linear probing for collision handling. You insert keys [10, 20, 15, 7, 32] with hash function key % 7. After all insertions, what is the state of the table? Represent empty slots as null. Explain your answer step-by-step.

medium

A. [10, 20, 15, 7, 32, null, null]

B. [7, 15, null, 10, 32, null, 20]

C. [null, 15, 32, 7, 10, 20, null]

D. [null, 15, 32, null, 10, null, 7]

Solution

1. Step 1: Calculate initial indices for each key

   10 % 7 = 3, 20 % 7 = 6, 15 % 7 = 1, 7 % 7 = 0, 32 % 7 = 4.

2. Step 2: Insert keys using linear probing

   All hashed indices are empty, so place: 10 at 3, 20 at 6, 15 at 1, 7 at 0, 32 at 4.

3. Step 3: Final table state

   Indices 2 and 5 remain null: [7, 15, null, 10, 32, null, 20].

4. Final Answer:

   [7, 15, null, 10, 32, null, 20] -> Option B

5. Quick Check:

   Insert keys at hashed index or next free slot [OK]

Hint: Calculate hash, insert or probe next free slot [OK]

Common Mistakes:

• Not applying modulo correctly
• Placing keys at wrong indices after collision
• Ignoring empty slots in final output