Parameter passing techniques are a favorite topic in compiler design exams. Questions often test your understanding using tricky programs.

In this post, we’ll:

• Understand call by name and copy restore
• Solve a real exam-level question step by step

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🔵 Given Program

void func(int a, int b)
{
    a = a + b;
    b = a + b;
    a = b - a;
    b = a - b;
}main()
{
    int j, i = 4;
    int array[10] = {1,2,3,4,5,6,7,8,9,0};    func(i, array[i]);    for(j = 0; j < 10; j++)
        printf("%d", array[j]);
}

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🔴 Concept 1: Call by Name

🧠 Key Idea:

• Arguments are re-evaluated every time they are used
• array[i] changes dynamically as i changes

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🔍 Step-by-Step Execution

Initial:

• i = 4
• array = {1,2,3,4,5,6,7,8,9,0}

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Step 1:

a = a + b → i = i + array[i] = 4 + 5 = 9

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Step 2:

b = a + b → array[i] = i + array[i]
→ array[9] = 9 + 0 = 9

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Step 3:

a = b - a → i = array[i] - i
→ i = 9 - 9 = 0

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Step 4:

b = a - b → array[i] = i - array[i]
→ array[0] = 0 - 1 = -1

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✅ Final Array (Call by Name)

-1 2 3 4 5 6 7 8 9 9

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🔴 Concept 2: Copy Restore (Call by Value-Result)

🧠 Key Idea:

• First: values are copied into parameters
• Then: operations happen on local variables
• Finally: values are copied back

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🔍 Step-by-Step Execution

Initial:

• a = 4, b = 5 (since array[4] = 5)

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Step 1:

a = a + b = 4 + 5 = 9

Step 2:

b = a + b = 9 + 5 = 14

Step 3:

a = b - a = 14 - 9 = 5

Step 4:

b = a - b = 5 - 14 = -9

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🔁 Copy Back:

• i = a = 5
• array[4] = b = -9

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✅ Final Array (Copy Restore)

1 2 3 4 -9 6 7 8 9 0

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🎯 Final Answer

👉 Call by Name:
-1 2 3 4 5 6 7 8 9 9

👉 Copy Restore:
1 2 3 4 -9 6 7 8 9 0

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⚡ Exam Tricks

• Call by Name → dynamic evaluation (very tricky)
• Copy Restore → copy → operate → copy back
• Watch for changing indices like array[i]

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🧠 Quick Comparison

Feature	Call by Name	Copy Restore
Evaluation	Every use	Once (copy)
Behavior	Dynamic	Static
Complexity	High	Medium
Exam Trick	Index changes	Copy back matters

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🎯 Conclusion

Understanding parameter passing techniques is crucial for solving tricky compiler design questions. Always track variable changes carefully, especially in call by name.