Python Coding Challenge - Question with Answer (ID -040726)

 

Code Explanation:

🔹 Line 1: Import reduce

from functools import reduce

reduce() is imported from the functools module.

👉 reduce() repeatedly applies a function to the elements of an iterable until only one final value remains.

Think of it as:

Value1 + Value2

      ↓

 Result + Value3

      ↓

 Result + Value4

      ↓

 Final Result

🔹 Line 2: Create a List

nums = [1, 2, 3, 4]

A list containing four numbers is created.

Current list:

[1, 2, 3, 4]

🔹 Line 3: Call reduce()

result = reduce(lambda x, y: x + y * 2, nums)

The lambda function is:

lambda x, y: x + y * 2

It means:

Take the previous result (x)

+

Double the next number (y)

Formula:

x + (y * 2)

🔹 Step 1: First Iteration

Initially:

x = 1

y = 2

Calculation:

1 + (2 × 2)

1 + 4

Result:

5

Current result becomes:

5

🔹 Step 2: Second Iteration

Now:

x = 5

y = 3

Calculation:

5 + (3 × 2)

5 + 6

Result:

11

Current result:

11

🔹 Step 3: Third Iteration

Now:

x = 11

y = 4

Calculation:

11 + (4 × 2)

11 + 8

Result:

19

Current result:

19

🔹 Line 4: Print Result

print(result)

Python prints:

19

⚡ Complete Execution Flow

Initial list:

[1, 2, 3, 4]

↓

First step:

1 + (2 × 2)

↓

5

↓

Second step:

5 + (3 × 2)

↓

11

↓

Third step:

11 + (4 × 2)

↓

19

↓

Final Output:

19

📊 Iteration Table

Iteration x y Calculation Result

1 1 2 1 + (2×2) 5

2 5 3 5 + (3×2) 11

3 11 4 11 + (4×2) 19

❌ Common Mistake

Many developers think reduce() calculates:

1 + 2 + 3 + 4

which is:

10

❌ Wrong.

The lambda doubles every new element:

x + y * 2

Because * has higher precedence than +, Python evaluates:

x + (y * 2)

not

(x + y) * 2

💡 Memory Flow

nums

[1] → [2] → [3] → [4]

      ↓

1 + 4 = 5

      ↓

5 + 6 = 11

      ↓

11 + 8 = 19

      ↓

result = 19

🎯 Final Result

19

✅ Correct Answer

19