Register storage class - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of code using register variable

What is the output of this C code snippet that uses a register variable inside a loop?

C

#include <stdio.h>

int main() {
    register int i;
    int sum = 0;
    for(i = 1; i <= 5; i++) {
        sum += i;
    }
    printf("%d", sum);
    return 0;
}

A Undefined behavior

B 10

C 15

D Compilation error due to register variable

💡 Hint

Remember that register variables are stored in CPU registers for faster access but behave like normal variables in this context.

The register keyword suggests storing the variable in a CPU register for speed, but it does not change the logic. The loop sums numbers 1 to 5, so output is 15.

❓ Predict Output

intermediate

Effect of register keyword on address operator

What happens when you try to print the address of a register variable in C?

C

#include <stdio.h>

int main() {
    register int x = 10;
    printf("%p", (void*)&x);
    return 0;
}

A Compilation error: cannot take address of register variable

B Prints the memory address of x

C Runtime error

D Prints 0

💡 Hint

The register keyword restricts taking the address of the variable.

Register variables are stored in CPU registers and their address cannot be taken, so the compiler throws an error when & operator is used.

🔧 Debug

advanced

Identify the error with register variable usage

What error will this code produce and why?

C

#include <stdio.h>

int main() {
    register int count = 5;
    int *ptr = &count;
    printf("%d", *ptr);
    return 0;
}

A Prints 5

B Compilation error: cannot take address of register variable

C Runtime segmentation fault

D Undefined behavior but compiles

💡 Hint

Check if the address of a register variable can be taken.

Taking the address of a register variable is not allowed, so the compiler will produce an error at the line int *ptr = &count;

🧠 Conceptual

advanced

Behavior of register variables in recursive functions

Consider a recursive function using a register variable as a counter. What is the behavior of the register variable across recursive calls?

C

int factorial(int n) {
    register int count = 0;
    count++;
    if (n <= 1) return 1;
    return n * factorial(n - 1);
}

A The count variable is shared globally among all calls

B The register variable count keeps incrementing across all recursive calls

C Compilation error due to register variable in recursion

D Each recursive call has its own separate register variable count initialized to 0

💡 Hint

Think about variable scope and lifetime in recursive calls.

Each recursive call creates a new instance of the register variable count initialized to 0, so count does not accumulate across calls.

❓ Predict Output

expert

Number of items in a register variable array

What is the output of this code that declares a register array and prints its size?

C

#include <stdio.h>

int main() {
    register int arr[4] = {1, 2, 3, 4};
    printf("%lu", sizeof(arr)/sizeof(arr[0]));
    return 0;
}

A Compilation error: register variable cannot be an array

B 4

C 1

D Undefined behavior

💡 Hint

Check if arrays can be declared with register storage class.

In C, register storage class cannot be applied to arrays, so this code causes a compilation error.

Practice

1. What is the main purpose of the register storage class in C?

easy

A. To suggest storing the variable in CPU registers for faster access

B. To allocate memory on the heap

C. To make the variable global

D. To allow the variable to be accessed outside the function

Solution

1. Step 1: Understand the role of register keyword

   The register keyword suggests to the compiler that the variable should be stored in CPU registers for faster access.

2. Step 2: Compare with other storage classes

   Unlike global or heap variables, register variables are local and optimized for speed, not for scope or memory allocation.

3. Final Answer:

   To suggest storing the variable in CPU registers for faster access -> Option A

4. Quick Check:

   register means fast access [OK]

Hint: Remember: register means fast access in CPU registers [OK]

Common Mistakes:

• Thinking register allocates memory on the heap
• Assuming register variables are global
• Believing register variables can be accessed outside functions

2. Which of the following is the correct syntax to declare a register variable in C?

easy

A. int count register;

B. int register count;

C. register = int count;

D. register int count;

Solution

1. Step 1: Recall correct syntax for register declaration

   The correct syntax places the keyword register before the type: register int count;.

2. Step 2: Check other options for syntax errors

   Options A, B, and C have incorrect order or invalid assignment syntax.

3. Final Answer:

   register int count; -> Option D

4. Quick Check:

   Keyword before type = correct syntax [OK]

Hint: Put 'register' before the type name [OK]

Common Mistakes:

• Placing 'register' after the variable name
• Using assignment '=' in declaration
• Mixing order of type and storage class

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

void func() {
    register int x = 5;
    // printf("%p", &x); // Uncommenting this line causes error
    printf("%d", x);
}
int main() {
    func();
    return 0;
}

medium

A. Garbage value

B. Compilation error due to &x

C. 5

D. Runtime error

Solution

1. Step 1: Understand register variable usage

   The variable x is declared as register int, so it cannot have its address taken.

2. Step 2: Analyze the code behavior

   The commented line with &x would cause a compilation error if uncommented. But as is, the code prints the value of x, which is 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Printing register variable value works fine [OK]

Hint: Register variables can't use & but can print value [OK]

Common Mistakes:

• Trying to print address of register variable
• Expecting runtime error instead of compile error
• Confusing garbage value with initialized value

4. Identify the error in the following code snippet:

void example() {
    register int a = 10;
    int *p = &a;
    printf("%d", *p);
}

medium

A. Register variable cannot be initialized

B. Cannot take address of register variable

C. Pointer declaration is invalid

D. No error, code runs fine

Solution

1. Step 1: Check usage of register variable

   The variable a is declared as register int, so its address cannot be taken.

2. Step 2: Identify the error in pointer assignment

   The line int *p = &a; tries to take the address of a register variable, which causes a compilation error.

3. Final Answer:

   Cannot take address of register variable -> Option B

4. Quick Check:

   Address of register variable forbidden [OK]

Hint: Register variables have no address, so & operator causes error [OK]

Common Mistakes:

• Thinking register variables can't be initialized
• Assuming pointer declaration is wrong
• Believing code runs without error

5. Consider this code snippet:

void count() {
    register int i;
    for(i = 0; i < 3; i++) {
        printf("%d ", i);
    }
}
int main() {
    count();
    return 0;
}

What is the main advantage of using register for variable i here?

hard

A. It suggests faster access to i since it is frequently used

B. It allows taking the address of i inside the loop

C. It makes i a global variable

D. It automatically initializes i to zero

Solution

1. Step 1: Analyze the role of register in loops

   Loop variables like i are used repeatedly, so storing them in CPU registers speeds up access.

2. Step 2: Check other options for correctness

   It allows taking the address of i inside the loop is wrong because you cannot take the address of a register variable. It makes i a global variable is wrong because i is local, not global. It automatically initializes i to zero is wrong because register does not initialize variables.

3. Final Answer:

   It suggests faster access to i since it is frequently used -> Option A

4. Quick Check:

   Register = fast access for frequent vars [OK]

Hint: Register speeds up frequent variable access like loop counters [OK]

Common Mistakes:

• Thinking register allows & operator
• Assuming register makes variable global
• Believing register initializes variables automatically