Enhanced for loop in Java - Time & Space Complexity

Time Complexity: Enhanced for loop

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run an enhanced for loop changes as the list size grows.

How does the number of steps grow when we loop over more items?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int sum = 0;
int[] numbers = {1, 2, 3, 4, 5};
for (int num : numbers) {
    sum += num;
}
System.out.println(sum);
    

This code adds up all numbers in an array using an enhanced for loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop goes through each number in the array once.
• How many times: Exactly once for each item in the array.

How Execution Grows With Input

As the array gets bigger, the loop runs more times, one for each item.

Input Size (n)	Approx. Operations
10	10 additions
100	100 additions
1000	1000 additions

Pattern observation: The number of steps grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to finish grows in a straight line as the list gets bigger.

Common Mistake

[X] Wrong: "The enhanced for loop runs faster than a regular for loop because it looks simpler."

[OK] Correct: Both loops do the same number of steps; the difference is only in how we write them, not how fast they run.

Interview Connect

Understanding how loops grow with input size helps you explain your code clearly and shows you know how to think about efficiency.

Self-Check

"What if we nested one enhanced for loop inside another over the same array? How would the time complexity change?"

Practice

1. What is the main purpose of the enhanced for loop in Java?

easy

A. To create infinite loops

B. To iterate over all elements in an array or collection easily

C. To declare variables with enhanced types

D. To replace if-else statements

Solution

1. Step 1: Understand the enhanced for loop usage

   The enhanced for loop is designed to simplify iterating over arrays or collections without manual index management.

2. Step 2: Compare options with the loop's purpose

   Only To iterate over all elements in an array or collection easily correctly describes this purpose; others describe unrelated concepts.

3. Final Answer:

   To iterate over all elements in an array or collection easily -> Option B

4. Quick Check:

   Enhanced for loop = easy iteration [OK]

Hint: Enhanced for loop = easy way to visit each item [OK]

Common Mistakes:

• Thinking it declares variables
• Confusing with infinite loops
• Using it to replace conditional statements

2. Which of the following is the correct syntax for an enhanced for loop iterating over an integer array nums?

easy

A. foreach (int num : nums) { System.out.println(num); }

B. for int num in nums { System.out.println(num); }

C. for (int num : nums) { System.out.println(num); }

D. for (num : nums) { System.out.println(num); }

Solution

1. Step 1: Recall Java enhanced for loop syntax

   The correct syntax uses: for (type variable : arrayOrCollection) { ... }

2. Step 2: Check each option

   for (int num : nums) { System.out.println(num); } matches the correct syntax. Options A, B, and C use incorrect keywords or missing parentheses.

3. Final Answer:

   for (int num : nums) { System.out.println(num); } -> Option C

4. Quick Check:

   Correct syntax uses ':' inside parentheses [OK]

Hint: Remember: for (type var : collection) { } [OK]

Common Mistakes:

• Using 'in' instead of ':'
• Omitting parentheses
• Using 'foreach' keyword in Java

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

int[] numbers = {1, 2, 3};
int sum = 0;
for (int n : numbers) {
    sum += n;
}
System.out.println(sum);

medium

A. 6

B. 123

C. 0

D. Compilation error

Solution

1. Step 1: Understand the loop operation

   The loop adds each element of the array (1, 2, 3) to the variable sum.

2. Step 2: Calculate the sum

   sum = 0 + 1 + 2 + 3 = 6

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum of 1, 2, 3 = 6 [OK]

Hint: Sum all elements inside the loop [OK]

Common Mistakes:

• Concatenating numbers instead of adding
• Forgetting to initialize sum
• Syntax errors in loop

4. Identify the error in the following code snippet:

String[] fruits = {"apple", "banana", "cherry"};
for (String fruit : fruits) {
    System.out.println(fruits);
}

medium

A. Inside the loop, it should print 'fruit' instead of 'fruits'

B. The loop variable should be 'fruits' not 'fruit'

C. The array declaration is incorrect

D. Enhanced for loop cannot be used with arrays

Solution

1. Step 1: Analyze the loop variable and print statement

   The loop variable is 'fruit', representing each element. Printing 'fruits' prints the whole array reference each time.

2. Step 2: Correct the print statement

   To print each fruit, use 'System.out.println(fruit);' inside the loop.

3. Final Answer:

   Inside the loop, it should print 'fruit' instead of 'fruits' -> Option A

4. Quick Check:

   Print loop variable, not array name [OK]

Hint: Print the loop variable, not the array name [OK]

Common Mistakes:

• Printing the array variable inside the loop
• Confusing loop variable with array name
• Incorrect array declaration

5. Given a list of integers, which enhanced for loop code correctly counts how many numbers are greater than 10?

List<Integer> nums = List.of(5, 12, 7, 20, 10);
int count = 0;
// Which code correctly counts numbers > 10?

hard

A. for (Integer n : nums) { if (n >= 10) count++; }

B. for (int n : nums) { if (n >= 10) count++; }

C. for (n : nums) { if (n > 10) count++; }

D. for (int n : nums) { if (n > 10) count++; }

Solution

1. Step 1: Understand the data and requirement

   List.of(5,12,7,20,10). Numbers greater than 10: 12,20 (count=2).

2. Step 2: Recall enhanced for loop with List<Integer>

   Supports int n via automatic unboxing of Integer to int.

3. Step 3: Check each option

   A: >=10 counts 12,20,10 (3) incorrect.
   B: int n >10 correct syntax, condition, count=2.
   C: Missing type for n: compile error.
   D: >=10 incorrect condition.

4. Final Answer:

   for (int n : nums) { if (n > 10) count++; } -> Option D

5. Quick Check:

   int n works with List<Integer> via unboxing [OK]

Hint: int n works for List<Integer> via autounboxing [OK]

Common Mistakes:

• Using >= instead of >
• Missing type declaration for loop variable
• Wrong comparison operator