Parameter passing mechanisms in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is pass by value in parameter passing?

Pass by value means the function receives a copy of the argument's value. Changes inside the function do not affect the original variable.

beginner

Explain pass by reference.

Pass by reference means the function receives a reference (or address) to the original variable. Changes inside the function affect the original variable.

intermediate

What is the difference between pass by value and pass by reference?

Pass by value sends a copy of data, so changes don't affect the original. Pass by reference sends the actual location, so changes affect the original variable.

intermediate

What does pass by value-result mean?

Pass by value-result copies the argument's value to the function (like pass by value) and then copies it back to the original variable after the function finishes.

advanced

Describe pass by name parameter passing.

Pass by name delays evaluation of the argument until it is used inside the function, substituting the argument expression directly. It behaves like textual substitution.

Which parameter passing mechanism allows the function to modify the caller's variable directly?

A Pass by value-result

B Pass by reference

C Pass by value

D Pass by name

Pass by reference passes the actual address, so changes inside the function affect the original variable.

In which parameter passing method is the argument evaluated only when used inside the function?

A Pass by value

B Pass by value-result

C Pass by reference

D Pass by name

Pass by name delays evaluation until the argument is used, substituting the expression directly.

What happens to the original variable in pass by value-result after the function finishes?

A It is updated with the function's local copy

B It is passed by reference

C It is deleted

D It remains unchanged

Pass by value-result copies back the local changes to the original variable after the function ends.

Which parameter passing mechanism is safest to avoid unintended side effects?

A Pass by name

B Pass by reference

C Pass by value

D Pass by value-result

Pass by value uses copies, so the original data cannot be changed by the function.

Which mechanism can cause unexpected behavior due to multiple evaluations of the argument?

A Pass by name

B Pass by reference

C Pass by value

D Pass by value-result

Pass by name substitutes the argument expression directly, so if used multiple times, it evaluates multiple times.

Explain the main differences between pass by value and pass by reference with examples.

Think about whether the function can change the caller's variable.

Describe pass by value-result and pass by name, and discuss when each might be useful.

Consider copying values vs delaying evaluation.

Practice

1. Which parameter passing mechanism sends a copy of the argument to the function, so changes inside the function do not affect the original variable?

easy

A. Pass by name

B. Pass by reference

C. Pass by value

D. Pass by result

Solution

1. Step 1: Understand pass by value

   Pass by value means the function receives a copy of the argument, so any changes inside the function do not affect the original variable.

2. Step 2: Compare with other mechanisms

   Pass by reference sends the original variable, so changes affect it. Pass by name and result behave differently and are less common.

3. Final Answer:

   Pass by value -> Option C

4. Quick Check:

   Copy sent, original unchanged = Pass by value [OK]

Hint: Copy means pass by value, original means pass by reference [OK]

Common Mistakes:

• Confusing pass by value with pass by reference
• Thinking pass by name sends a copy
• Assuming pass by result is the same as pass by value

2. Which of the following is the correct syntax to declare a function parameter passed by reference in C++?

easy

A. void func(int &x);

B. void func(int x);

C. void func(ref int x);

D. void func(*int x);

Solution

1. Step 1: Recall C++ reference syntax

   In C++, passing by reference uses the ampersand (&) symbol after the type in the parameter list.

2. Step 2: Analyze options

   void func(int &x); uses int &x which is correct. void func(int x); passes by value, C is invalid syntax, and D is pointer syntax, not reference.

3. Final Answer:

   void func(int &x); -> Option A

4. Quick Check:

   C++ reference uses & in parameter [OK]

Hint: Look for & symbol in parameter for C++ reference [OK]

Common Mistakes:

• Confusing pointer (*) with reference (&)
• Using invalid 'ref' keyword in C++
• Assuming no symbol means pass by reference

3. Consider the following pseudocode using pass by reference:

function increment(x) {
  x = x + 1
}

var a = 5
increment(a)
print(a)

What will be the output?

medium

A. 5

B. Undefined

C. Error

D. 6

Solution

1. Step 1: Understand pass by reference effect

   Since x is passed by reference, the function changes the original variable a by adding 1.

2. Step 2: Trace the code execution

   a starts at 5, increment adds 1 making it 6, then print outputs 6.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   Pass by reference changes original = 6 [OK]

Hint: Pass by reference changes original variable [OK]

Common Mistakes:

• Assuming pass by reference behaves like pass by value
• Thinking the original variable remains unchanged
• Ignoring the effect of increment inside the function

4. Identify the error in this code snippet using pass by value:

function swap(x, y) {
  temp = x
  x = y
  y = temp
}

var a = 3
var b = 4
swap(a, b)
print(a, b)

medium

A. Print statement is incorrect

B. swap function does not swap original variables

C. Variables a and b are not declared

D. Syntax error in function definition

Solution

1. Step 1: Understand pass by value behavior in swap

   Since parameters are passed by value, swap changes only local copies x and y, not original a and b.

2. Step 2: Check the output effect

   After swap call, a and b remain 3 and 4, so swap does not work as intended.

3. Final Answer:

   swap function does not swap original variables -> Option B

4. Quick Check:

   Pass by value means original unchanged [OK]

Hint: Pass by value swaps local copies only [OK]

Common Mistakes:

• Assuming swap changes original variables with pass by value
• Looking for syntax errors where none exist
• Ignoring parameter passing mechanism effect

5. You want to write a function that modifies a list of numbers by doubling each element. Which parameter passing mechanism is best to use to ensure the original list is changed efficiently?

hard

A. Pass by reference

B. Pass by value

C. Pass by name

D. Pass by result

Solution

1. Step 1: Consider efficiency and effect on original list

   Pass by value copies the entire list, which is inefficient and changes won't affect original.

2. Step 2: Choose mechanism that modifies original efficiently

   Pass by reference sends the original list, allowing direct modification without copying.

3. Final Answer:

   Pass by reference -> Option A

4. Quick Check:

   Modify original efficiently = Pass by reference [OK]

Hint: Modify original data efficiently with pass by reference [OK]

Common Mistakes:

• Choosing pass by value and expecting original change
• Confusing pass by name with pass by reference
• Ignoring efficiency of copying large data