Predefined macros - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of __FILE__ and __LINE__ macros

What is the output of this C program when compiled and run?

C

#include <stdio.h>

int main() {
    printf("File: %s, Line: %d\n", __FILE__, __LINE__);
    return 0;
}

A File: main.c, Line: 7

B File: main.c, Line: 6

C File: main.c, Line: 4

D File: main.c, Line: 5

💡 Hint

Remember that __LINE__ expands to the line number where it appears in the source code.

The __FILE__ macro expands to the current file name as a string. The __LINE__ macro expands to the line number where it is used. The printf statement is on line 5 in the code snippet, so it prints Line: 5.

❓ Predict Output

intermediate

Output of __DATE__ and __TIME__ macros

What will this program print when compiled and run?

C

#include <stdio.h>

int main() {
    printf("Compiled on %s at %s\n", __DATE__, __TIME__);
    return 0;
}

A Compiled on Jun 15 2024 at 10:00:00 (example)

B Compiled on 2024-06-15 at 10:00:00

C Compiled on Jun 15 2024 at 10:00:00

D Compiled on Jun 15 2024 at 10:00:00 (actual compile time)

💡 Hint

The __DATE__ macro expands to a string literal in the format "Mmm dd yyyy".

The __DATE__ macro expands to the date when the source file was compiled, in the format "Mmm dd yyyy" (e.g., "Jun 15 2024"). The __TIME__ macro expands to the time of compilation in "hh:mm:ss" format. The output matches option C.

❓ Predict Output

advanced

Output of __STDC__ macro

What is the output of this program if compiled with a standard C compiler?

C

#include <stdio.h>

int main() {
#ifdef __STDC__
    printf("Standard C compiler detected\n");
#else
    printf("Non-standard compiler\n");
#endif
    return 0;
}

A Standard C compiler detected

B Non-standard compiler

C Compilation error due to undefined macro

D No output

💡 Hint

The __STDC__ macro is defined by standard C compilers.

The __STDC__ macro is predefined by standard C compilers to indicate compliance with the ANSI C standard. The #ifdef check succeeds, so the program prints "Standard C compiler detected".

❓ Predict Output

advanced

Output of __func__ macro inside a function

What does this program print when run?

C

#include <stdio.h>

void greet() {
    printf("Function name: %s\n", __func__);
}

int main() {
    greet();
    return 0;
}

A Function name: main

B Function name: greet

C Function name: __func__

D Function name: (null)

💡 Hint

The __func__ macro expands to the name of the current function as a string.

The __func__ macro expands to the name of the function where it is used. Since it is inside the greet() function, it prints "greet".

🧠 Conceptual

expert

Behavior of __LINE__ macro with macro expansion

Consider the following code. What is the output when compiled and run?

C

#include <stdio.h>

#define PRINT_LINE() printf("Line: %d\n", __LINE__)

int main() {
    PRINT_LINE();
    return 0;
}

A Line: 9

B Line: 5

C Line: 8

D Line: 7

💡 Hint

The __LINE__ macro expands to the line number where it appears after macro expansion.

The __LINE__ macro expands to the line number in the source code where it is used after macro expansion. Since PRINT_LINE() is called on line 9, __LINE__ expands to 9, so the output is "Line: 9".

Practice

1. Which of the following predefined macros in C gives the current line number in the source code?

easy

A. __LINE__

B. __FILE__

C. __DATE__

D. __TIME__

Solution

1. Step 1: Understand predefined macros purpose

   Predefined macros provide info about the code or environment automatically.

2. Step 2: Identify macro for line number

   __LINE__ gives the current line number in the source file.

3. Final Answer:

   __LINE__ -> Option A

4. Quick Check:

   Line number macro = __LINE__ [OK]

Hint: Line number macro always has LINE in its name [OK]

Common Mistakes:

• Confusing __FILE__ with line number
• Using __DATE__ or __TIME__ for line info
• Thinking __LINE__ is a function

2. Which of the following is the correct syntax to print the current file name using a predefined macro in C?

easy

A. printf("File: %s", FILE);

B. printf("File: %s", __FILE__);

C. printf("File: %s", _FILE_);

D. printf("File: %s", file);

Solution

1. Step 1: Recall predefined macro for file name

   The macro __FILE__ holds the current source file name as a string.

2. Step 2: Check correct syntax usage

   Using printf("File: %s", __FILE__); correctly prints the file name.

3. Final Answer:

   printf("File: %s", __FILE__); -> Option B

4. Quick Check:

   File name macro syntax = __FILE__ [OK]

Hint: Predefined macros always have double underscores before and after [OK]

Common Mistakes:

• Omitting underscores in __FILE__
• Using FILE without underscores
• Using lowercase file instead of macro

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

#include <stdio.h>
int main() {
    printf("Date: %s\n", __DATE__);
    printf("Time: %s\n", __TIME__);
    return 0;
}

medium

A. Compilation error due to undefined macros

B. Date: __DATE__\nTime: __TIME__

C. Date: current date at compilation\nTime: current time at compilation

D. Date: 0000-00-00\nTime: 00:00:00

Solution

1. Step 1: Understand __DATE__ and __TIME__ macros

   These macros expand to strings representing the date and time when the code was compiled.

2. Step 2: Predict output of printf statements

   The program prints the compilation date and time as strings, not the macro names or errors.

3. Final Answer:

   Date: current date at compilation\nTime: current time at compilation -> Option C

4. Quick Check:

   __DATE__ and __TIME__ show compile date/time [OK]

Hint: __DATE__ and __TIME__ show compile time info as strings [OK]

Common Mistakes:

• Expecting runtime current date/time
• Thinking macros print their own names
• Assuming macros are undefined

4. Consider this C code snippet:

#include <stdio.h>
int main() {
    printf("Line: %d\n", LINE);
    return 0;
}

What is the problem with this code?

medium

A. LINE is not a predefined macro; it should be __LINE__

B. Missing semicolon after printf

C. printf format specifier %d is incorrect for LINE

D. No problem; code runs fine

Solution

1. Step 1: Identify the macro usage error

   The code uses LINE instead of the correct predefined macro __LINE__.

2. Step 2: Understand consequences

   Since LINE is undefined, this causes a compilation error.

3. Final Answer:

   LINE is not a predefined macro; it should be __LINE__ -> Option A

4. Quick Check:

   Predefined macros need double underscores [OK]

Hint: Predefined macros always have double underscores before and after [OK]

Common Mistakes:

• Using LINE without underscores
• Thinking %d is wrong for line number
• Ignoring missing macro error

5. You want to log the current file name, line number, and compilation date in your C program for debugging. Which code snippet correctly uses predefined macros to do this?

hard

A. printf("File: %s, Line: %d, Date: %s\n", _FILE_, _LINE_, _DATE_);

B. printf("File: %s, Line: %d, Date: %s\n", FILE, LINE, DATE);

C. printf("File: %s, Line: %d, Date: %s\n", "__FILE__", "__LINE__", "__DATE__");

D. printf("File: %s, Line: %d, Date: %s\n", __FILE__, __LINE__, __DATE__);

Solution

1. Step 1: Identify correct predefined macros

   The macros __FILE__, __LINE__, and __DATE__ provide file name, line number, and compile date respectively.

2. Step 2: Check correct usage in printf

   Using them directly without quotes and with correct underscores prints the desired info.

3. Final Answer:

   printf("File: %s, Line: %d, Date: %s\n", __FILE__, __LINE__, __DATE__); -> Option D

4. Quick Check:

   Use double underscore macros directly in printf [OK]

Hint: Use __FILE__, __LINE__, __DATE__ directly without quotes [OK]

Common Mistakes:

• Using macros without underscores
• Putting macros inside quotes (prints names, not values)
• Using single underscores instead of double