Predict the output of the following C#.NET code (pay close attention to post-increment and pre-increment on the same variable): int i, j = 1, k; for (i = 0; i < 5; i++) { k = j++ + ++j; Console.Write(k + " "); }
-
A8 4 16 12 20
-
B4 8 12 16 20
-
C4 8 16 32 64
-
D2 4 6 8 10
-
E1 3 5 7 9
Answer
Correct Answer: 4 8 12 16 20
Explanation
Introduction / Context:This question tests evaluation order and side effects of the post-increment (x++) and pre-increment (++x) operators in C#. Understanding how the same variable changes within one expression is vital to avoid subtle bugs.
Given Data / Assumptions:
- Initial values: j = 1; loop executes 5 times.
- Expression: k = j++ + ++j inside the loop.
- C# left-to-right operand evaluation for the + operator.
Concept / Approach:For j++: the value used is the old value, then j increments by 1. For ++j: j is first incremented, then the new value is used. The binary + evaluates its left operand first, then right operand, applying side effects in that order.
Step-by-Step Solution:
Iteration 1: j = 1 → left j++ yields 1 (j becomes 2), right ++j makes j = 3 (yields 3) → k = 1 + 3 = 4; print 4; j ends as 3.Iteration 2: j = 3 → left 3 (j→4), right ++j = 5 → k = 8; print 8; j ends as 5.Iteration 3: j = 5 → left 5 (j→6), right ++j = 7 → k = 12; print 12; j ends as 7.Iteration 4: j = 7 → left 7 (j→8), right ++j = 9 → k = 16; print 16; j ends as 9.Iteration 5: j = 9 → left 9 (j→10), right ++j = 11 → k = 20; print 20; j ends as 11.Verification / Alternative check:Insert diagnostic prints after each operand or run in a debugger to observe j's changes stepwise.
Why Other Options Are Wrong:They reflect misordered evaluation or arithmetic sequences not produced by this side-effect pattern.
Common Pitfalls:Assuming unspecified order of evaluation (in C#, + evaluates left to right) or misinterpreting pre/post increment semantics.
Final Answer:4 8 12 16 20