Lifetime and scope comparison - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a variable with block scope.

C

void function() {
    [1] int x = 5;
}

Drag options to blanks, or click blank then click option'

Aauto

Bstatic

Cextern

Dregister

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' inside a function makes the variable retain its value between calls, changing its lifetime.

The auto keyword declares a variable with block scope and automatic storage duration by default in C.

2fill in blank

medium

Complete the code to declare a variable with static lifetime inside a function.

C

void function() {
    [1] int count = 0;
    count++;
    printf("%d", count);
}

Drag options to blanks, or click blank then click option'

Aauto

Bstatic

Cregister

Dextern

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'auto' inside a function resets the variable each call.

The static keyword inside a function gives the variable static lifetime, so it keeps its value between function calls.

3fill in blank

hard

Fix the error in the code to correctly declare an external variable.

C

[1] int count;

void function() {
    count = 5;
}

Drag options to blanks, or click blank then click option'

Astatic

Bauto

Cextern

Dregister

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'static' limits the variable to the current file scope.

The extern keyword declares a variable defined elsewhere, allowing access across files or scopes.

4fill in blank

hard

Fill both blanks to declare a global variable with internal linkage and initialize it.

C

[1] int [2] = 10;

Drag options to blanks, or click blank then click option'

Astatic

Bextern

Ccount

Dauto

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'extern' at global scope declares but does not define the variable.

static at global scope gives internal linkage, and count is the variable name.

5fill in blank

hard

Fill all three blanks to create a function that uses a static local variable and prints it.

C

void [1]() {
    [2] int [3] = 0;
    [3]++;
    printf("%d", [3]);
}

Drag options to blanks, or click blank then click option'

Aincrement

Bstatic

Ccounter

Dauto

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'auto' resets the variable each call.

The function name is increment, the variable is declared static to keep its value, and the variable name is counter.

Practice

1. Which of the following best describes the scope of a variable in C?

easy

A. The time duration the variable exists in memory

B. The amount of memory the variable uses

C. The part of the program where the variable can be accessed

D. The data type of the variable

Solution

1. Step 1: Understand the meaning of scope

   Scope means where in the code a variable can be used or accessed.

2. Step 2: Differentiate scope from lifetime

   Lifetime is about how long the variable exists in memory, not where it can be used.

3. Final Answer:

   The part of the program where the variable can be accessed -> Option C

4. Quick Check:

   Scope = where variable is accessible [OK]

Hint: Scope means where variable is visible in code [OK]

Common Mistakes:

• Confusing scope with lifetime
• Thinking scope is about memory size
• Mixing scope with data type

2. Which declaration correctly creates a variable with local scope and static lifetime inside a function?

easy

A. static int count = 0;

B. int static count = 0;

C. auto static int count = 0;

D. register static int count = 0;

Solution

1. Step 1: Recall correct syntax for static local variable

   The keyword static comes before the type: static int count = 0;.

2. Step 2: Identify invalid syntax options

   int static count = 0; has incorrect order. auto static int count = 0; and register static int count = 0; use invalid combinations of storage class specifiers.

3. Final Answer:

   static int count = 0; -> Option A

4. Quick Check:

   Static local variable syntax = static before type [OK]

Hint: Put 'static' before type for static local variable [OK]

Common Mistakes:

• Placing 'static' after type
• Combining 'auto' and 'static' incorrectly
• Using 'register' with 'static'

3. What is the output of this C code?

void func() {
    static int x = 0;
    x++;
    printf("%d ", x);
}

int main() {
    for(int i = 0; i < 3; i++) {
        func();
    }
    return 0;
}

medium

A. 1 2 3

B. 1 1 1

C. 0 1 2

D. 3 3 3

Solution

1. Step 1: Understand static variable behavior inside function

   Static variable x keeps its value between function calls and starts at 0.

2. Step 2: Trace the loop and function calls

   Each call increments x by 1 and prints it: first 1, then 2, then 3.

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Static variable increments across calls = 1 2 3 [OK]

Hint: Static variables keep value between calls [OK]

Common Mistakes:

• Assuming static resets each call
• Confusing initial value of static variable
• Ignoring the loop effect

4. Identify the error in this code snippet related to variable scope:

int main() {
    if (1) {
        int a = 5;
    }
    printf("%d", a);
    return 0;
}

medium

A. Variable 'a' should be declared static

B. Missing semicolon after variable declaration

C. printf format specifier is incorrect

D. Variable 'a' is used outside its scope

Solution

1. Step 1: Check where 'a' is declared and used

   'a' is declared inside the if-block and cannot be accessed outside it.

2. Step 2: Understand scope rules

   Variables declared inside braces have local scope limited to those braces, so 'a' is out of scope in printf.

3. Final Answer:

   Variable 'a' is used outside its scope -> Option D

4. Quick Check:

   Using variable outside its block scope causes error [OK]

Hint: Variables only visible inside their braces [OK]

Common Mistakes:

• Trying to use variable outside its block
• Thinking static fixes scope errors
• Ignoring variable lifetime vs scope

5. Consider this code:

void counter() {
    static int count = 0;
    int temp = 0;
    count++;
    temp++;
    printf("%d %d\n", count, temp);
}

int main() {
    for (int i = 0; i < 3; i++) {
        counter();
    }
    return 0;
}

What will be the output and why?

hard

A. 1 1
2 2
3 3
Because both variables keep their values between calls

B. 1 1
2 1
3 1
Because 'count' is static and 'temp' is local each call

C. 1 0
1 0
1 0
Because 'temp' is never incremented properly

D. 0 0
0 0
0 0
Because variables are not initialized

Solution

1. Step 1: Analyze 'count' variable behavior

   'count' is static, so it keeps incrementing across calls: 1, 2, 3.

2. Step 2: Analyze 'temp' variable behavior

   'temp' is local and reinitialized to 0 each call, so it increments to 1 every time.

3. Step 3: Understand output pattern

   Output lines are: "1 1", "2 1", "3 1" matching the behavior above.

4. Final Answer:

   1 1 2 1 3 1 Because 'count' is static and 'temp' is local each call -> Option B

5. Quick Check:

   Static keeps value; local resets each call [OK]

Hint: Static persists, local resets each function call [OK]

Common Mistakes:

• Assuming local variable keeps value between calls
• Confusing initialization of static vs local
• Ignoring that temp resets every call