Hash function concept in Data Structures Theory - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show what a hash function does to a key.

Data Structures Theory

hashed_value = hash([1])

Drag options to blanks, or click blank then click option'

Alist

Bvalue

Cindex

Dkey

Attempts:

3 left

💡 Hint

Common Mistakes

Using the value instead of the key as input to the hash function.

A hash function takes a key as input and returns a hashed value. This hashed value is used to find the data quickly.

2fill in blank

medium

Complete the code to create a hash table entry using a hash function.

Data Structures Theory

index = hash(key) [1] table_size

Drag options to blanks, or click blank then click option'

A+

B/

C%

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using division (/) instead of modulo (%) causes wrong index calculation.

The modulo operator % is used to keep the hash value within the table size, so the index fits in the table.

3fill in blank

hard

Fix the error in the hash function usage to get a valid index.

Data Structures Theory

index = hash([1]) % table_size

Drag options to blanks, or click blank then click option'

Aindex

Bkey

Ctable_size

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Applying hash to the value or index instead of the key.

The hash function must be applied to the key, not the value or other variables, to get a correct index.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps keys to their hash indexes.

Data Structures Theory

{key: hash(key) [1] table_size for key [2] keys}

Drag options to blanks, or click blank then click option'

A%

Bin

C*

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of '%' or wrong loop keyword.

The modulo operator % keeps the hash within table size. The keyword in is used to loop over keys.

5fill in blank

hard

Fill all three blanks to create a hash table lookup that checks if a key's hash index matches a stored index.

Data Structures Theory

if hash([1]) [2] table_size == [3]:

Drag options to blanks, or click blank then click option'

Akey

B%

Cstored_index

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using value instead of key or wrong operator instead of modulo.

The key is hashed and modulo applied to get the index. This index is compared to the stored index to find the correct slot.

Practice

1. What is the main purpose of a hash function in data structures?

easy

A. To encrypt data for security

B. To sort data in alphabetical order

C. To compress data into smaller files

D. To convert data into a fixed-size number for quick access

Solution

1. Step 1: Understand the role of a hash function

   A hash function takes input data and produces a fixed-size number called a hash code.

2. Step 2: Identify the purpose in data structures

   This fixed-size number helps quickly find or organize data, improving access speed.

3. Final Answer:

   To convert data into a fixed-size number for quick access -> Option D

4. Quick Check:

   Hash function = fixed-size number for quick access [OK]

Hint: Hash functions create quick lookup numbers [OK]

Common Mistakes:

• Confusing hashing with sorting
• Thinking hash functions compress data
• Assuming hash functions encrypt data

2. Which of the following is a correct property of a good hash function?

easy

A. It always produces the same output for the same input

B. It changes output randomly each time for the same input

C. It produces outputs larger than the input size

D. It sorts the input data before hashing

Solution

1. Step 1: Recall the determinism property of hash functions

   A good hash function must always produce the same output for the same input to be reliable.

2. Step 2: Evaluate other options

   Random outputs or sorting inputs are not properties of hash functions; output size is fixed, not necessarily larger.

3. Final Answer:

   It always produces the same output for the same input -> Option A

4. Quick Check:

   Deterministic output = same input same output [OK]

Hint: Good hash functions are consistent, not random [OK]

Common Mistakes:

• Thinking hash outputs change each time
• Believing hash functions sort data
• Assuming output size varies unpredictably

3. Consider a hash function that sums the ASCII values of characters in a string and then takes modulo 10. What is the hash value of "abc"?

medium

A. 4

B. 3

C. 5

D. 6

Solution

1. Step 1: Calculate ASCII values of each character

   'a' = 97, 'b' = 98, 'c' = 99; sum = 97 + 98 + 99 = 294.

2. Step 2: Apply modulo 10 to the sum

   294 % 10 = 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   (97+98+99) mod 10 = 4 [OK]

Hint: Sum ASCII codes then mod 10 for hash [OK]

Common Mistakes:

• Adding character positions instead of ASCII
• Forgetting to apply modulo operation
• Calculating modulo with wrong divisor

4. A hash function is defined as hash(x) = (3 * x + 7) % 5. What is the error if the function returns values outside 0 to 4?

medium

A. The multiplication factor should be negative

B. The function should add 5 instead of 7

C. The modulo operation is missing or incorrect

D. The input x must be a string, not a number

Solution

1. Step 1: Understand modulo operation range

   Modulo 5 should limit output to 0 through 4 only.

2. Step 2: Identify cause of out-of-range values

   If outputs are outside 0-4, the modulo operation is either missing or applied incorrectly.

3. Final Answer:

   The modulo operation is missing or incorrect -> Option C

4. Quick Check:

   Modulo ensures output range 0-4 [OK]

Hint: Modulo keeps hash values in range [OK]

Common Mistakes:

• Ignoring modulo effect on output range
• Changing constants without reason
• Confusing input type requirements

5. You want to design a hash function for a phone book that minimizes collisions. Which approach is best?

hard

A. Use the length of the phone number as the hash value

B. Use the sum of digits in the phone number modulo a prime number

C. Use only the first digit of the phone number as the hash

D. Use the phone number reversed without modulo

Solution

1. Step 1: Understand collision minimization

   Minimizing collisions means spreading hash values evenly across possible outputs.

2. Step 2: Evaluate each option's effectiveness

   Using sum of digits modulo a prime number distributes values better than using only first digit, length, or reversed number without modulo.

3. Final Answer:

   Use the sum of digits in the phone number modulo a prime number -> Option B

4. Quick Check:

   Sum digits mod prime reduces collisions [OK]

Hint: Sum digits mod prime spreads hash values well [OK]

Common Mistakes:

• Using only part of input causing many collisions
• Ignoring modulo operation
• Choosing non-prime modulo numbers