Unit 2 Day 10: For-Each Loop Limitation
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Unit 2: Selection & Iteration
Day 10 Practice - Section 2.10
Focus: For-Each Loop Limitations
Practice Question
Consider the following code segment intended to double all values in an array:
int[] nums = {2, 4, 6, 8};
for (int val : nums)
{
val = val * 2;
}
System.out.println(nums[0] + " " + nums[2]);What is printed as a result of executing this code segment?
Difficulty: Medium - Time: 2-3 minutes - AP Skill: 2.B - Determine output
What This Tests: This question tests a critical limitation of for-each loops: you cannot use them to modify array elements. The loop variable is a copy of each element, not a reference to it.
Why the Array Doesn't Change
In a for-each loop, the loop variable val receives a copy of each array element. When you modify val, you're only changing the copy - the original array remains untouched.
| Iteration | val (copy) | val after val = val * 2 | nums array |
|---|---|---|---|
| 1 | 2 | 4 | {2, 4, 6, 8} - unchanged! |
| 2 | 4 | 8 | {2, 4, 6, 8} - unchanged! |
| 3 | 6 | 12 | {2, 4, 6, 8} - unchanged! |
| 4 | 8 | 16 | {2, 4, 6, 8} - unchanged! |
Output: 2 6 (original values at indices 0 and 2)
How to Actually Modify Array Elements
To modify array elements, you must use an indexed for loop:
for (int i = 0; i < nums.length; i++)
{
nums[i] = nums[i] * 2; // This DOES modify the array
}
// nums is now {4, 8, 12, 16}Common Mistakes
Mistake: Answer A (4 12)
This assumes the for-each loop successfully modified the array. It didn't - val is just a copy of each element.
Mistake: Answer E (compile-time error)
The code compiles and runs fine - it just doesn't do what you might expect. Assigning to the loop variable is legal, but pointless for primitives.
AP Exam Strategy
For-Each Loop Rules
Use for-each when: Reading/accessing all elements without modification
Use indexed for loop when:
- You need to modify array elements
- You need the index value
- You need to traverse in reverse or skip elements
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