Map phase explained in Hadoop - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main role of the Map phase in Hadoop?

In Hadoop's MapReduce, what does the Map phase primarily do?

A It combines all intermediate data into a single output file.

B It processes input data and produces intermediate key-value pairs.

C It sorts the final output data before saving.

D It distributes the output data to different storage nodes.

💡 Hint

Think about what happens right after data is read in Hadoop.

The Map phase reads input data and transforms it into intermediate key-value pairs for further processing.

❓ Predict Output

intermediate

Output of a simple Map function in Hadoop

Given a Map function that reads lines of text and outputs each word with count 1, what is the output for the input line: "apple banana apple"?

Hadoop

def map(line):
    words = line.split()
    for word in words:
        print(f"{word}\t1")

map("apple banana apple")

A

apple	1
banana	1
apple	1

B apple banana apple 1

C

apple	3
banana	1

D

1	apple
1	banana
1	apple

💡 Hint

Each word is output separately with count 1.

The Map function outputs each word with a count of 1, so repeated words appear multiple times with count 1.

❓ data_output

advanced

Intermediate key-value pairs from Map phase

What intermediate key-value pairs does the Map phase produce from the input data: ["cat dog", "dog mouse", "cat mouse"] using a word count Map function?

Hadoop

input_data = ["cat dog", "dog mouse", "cat mouse"]
intermediate = []
for line in input_data:
    words = line.split()
    for word in words:
        intermediate.append((word, 1))
print(intermediate)

A [('cat', 1), ('dog', 1), ('mouse', 1)]

B [('cat', 2), ('dog', 2), ('mouse', 2)]

C [('cat dog', 1), ('dog mouse', 1), ('cat mouse', 1)]

D [('cat', 1), ('dog', 1), ('dog', 1), ('mouse', 1), ('cat', 1), ('mouse', 1)]

💡 Hint

Each word in each line is output with count 1 separately.

The Map phase outputs each word occurrence as a separate key-value pair with count 1, so duplicates appear multiple times.

🔧 Debug

advanced

Identify the error in this Map function

What error will this Map function cause when processing input lines?

def map(line):
    words = line.split()
    for word in words
        print(f"{word}\t1")

A IndentationError due to wrong indentation

B TypeError because split() returns None

C SyntaxError due to missing colon after for loop

D NameError because 'word' is not defined

💡 Hint

Check the for loop syntax carefully.

The for loop is missing a colon at the end, causing a SyntaxError.

🚀 Application

expert

How does the Map phase handle large datasets in Hadoop?

In Hadoop, when processing a very large dataset, how does the Map phase efficiently handle the data?

A It splits the input data into chunks and processes each chunk in parallel on different nodes.

B It loads the entire dataset into memory on a single node before processing.

C It sends all data to the Reduce phase without processing.

D It compresses the data first and then processes it sequentially.

💡 Hint

Think about how Hadoop uses multiple computers to speed up processing.

The Map phase splits data into chunks called splits and processes them in parallel on different nodes to handle large datasets efficiently.