Header files and include directive - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to include the standard input-output header file.

C

#include [1]

int main() {
    printf("Hello, world!\n");
    return 0;
}

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

A"stdio.h"

B<stdio.h>

C<stdlib.h>

D"stdlib.h"

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes instead of angle brackets for standard headers.

Including the wrong header file like stdlib.h.

The standard input-output header file is included using angle brackets: #include .

2fill in blank

medium

Complete the code to include a user-defined header file named "myheader.h".

C

#include [1]

void greet();

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

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

A"myheader.h"

B<myheader.h>

C<stdio.h>

D"stdio.h"

Attempts:

3 left

💡 Hint

Common Mistakes

Using angle brackets for user-defined headers.

Including the wrong header file.

User-defined headers are included using double quotes: #include "myheader.h".

3fill in blank

hard

Fix the error in the include directive to correctly include the math header.

C

#include [1]

int main() {
    double result = sqrt(16.0);
    return 0;
}

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

A<math>

B"math.h"

C<math.h>

D"math"

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the .h extension.

Using quotes instead of angle brackets.

The math header is a standard library header and must be included with angle brackets: #include .

4fill in blank

hard

Fill both blanks to include the header and declare a function prototype.

C

#include [1]

[2] greet();

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

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

A"myheader.h"

Bvoid

Cint

D<stdio.h>

Attempts:

3 left

💡 Hint

Common Mistakes

Using angle brackets for user headers.

Omitting the return type in function prototype.

User header included with quotes and function prototype declared with return type void.

5fill in blank

hard

Fill all three blanks to create a header guard in a header file.

C

#ifndef [1]
#define [2]

void greet();

#endif // [3]

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

AMYHEADER_H

DGREET_H

Attempts:

3 left

💡 Hint

Common Mistakes

Using different macro names in the guard.

Omitting the header guard entirely.

Header guards use the same unique macro name to prevent multiple inclusions.

Practice

1. What is the main purpose of a header file in C programming?

easy

A. To store declarations and share them between source files

B. To execute the main program logic

C. To compile the program faster

D. To store variable values during runtime

Solution

1. Step 1: Understand what header files contain

   Header files usually contain declarations like function prototypes and macros, not executable code.

2. Step 2: Understand their role in sharing code

   They allow multiple source files to share these declarations, so the compiler knows about functions and variables.

3. Final Answer:

   To store declarations and share them between source files -> Option A

4. Quick Check:

   Header files = declarations shared [OK]

Hint: Header files hold declarations, not code [OK]

Common Mistakes:

• Thinking header files contain executable code
• Confusing header files with source files
• Believing header files speed up compilation
• Assuming header files store variable values

2. Which of the following is the correct syntax to include a system header file named stdio.h?

easy

A. #include "stdio.h"

B. #include

C. include

D. #include stdio.h

Solution

1. Step 1: Recall syntax for system headers

   System headers use angle brackets <> to tell the compiler to look in system directories.

2. Step 2: Check each option

   #include uses #include with <stdio.h>, which is correct syntax. #include "stdio.h" uses quotes which are for user headers. Options C and D miss the # or angle brackets.

3. Final Answer:

   #include <stdio.h> -> Option B

4. Quick Check:

   System headers use <> [OK]

Hint: Use < > for system headers, " " for your own [OK]

Common Mistakes:

• Using quotes for system headers
• Omitting the # in #include
• Not using angle brackets or quotes
• Writing include without #

3. What will be the output of this C program?

#include <stdio.h>
#include "myheader.h"

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

Assuming myheader.h contains:

void printMessage() {
    printf("Hello from header!\n");
}

medium

A. No output

B. Compilation error: function not declared

C. Runtime error

D. Hello from header!

Solution

1. Step 1: Understand #include "myheader.h" effect

   The content of myheader.h is inserted, so printMessage() is defined before main.

2. Step 2: Check function call and output

   printMessage() prints "Hello from header!\n", so the program outputs that line.

3. Final Answer:

   Hello from header! -> Option D

4. Quick Check:

   Included function runs and prints message [OK]

Hint: Including header inserts code, so functions are available [OK]

Common Mistakes:

• Assuming function is undeclared without prototype
• Thinking header files can't contain function definitions
• Confusing compile and runtime errors
• Ignoring the newline in output

4. Identify the error in this code snippet:

#include 
#include 

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

Assuming myheader.h is a user-defined header file in the project folder.

medium

A. Function printHello() is not defined

B. Missing semicolon after #include directives

C. Using angle brackets for a user-defined header file

D. Missing main function return type

Solution

1. Step 1: Check #include syntax for user headers

   User headers should be included with quotes "myheader.h" to search local directories first.

2. Step 2: Understand effect of wrong include syntax

   Using angle brackets makes compiler look in system directories, so it may not find myheader.h causing an error.

3. Final Answer:

   Using angle brackets for a user-defined header file -> Option C

4. Quick Check:

   User headers use "" not <> [OK]

Hint: Use "" for your headers, < > for system headers [OK]

Common Mistakes:

• Using < > for user headers
• Thinking #include needs semicolons
• Assuming printHello() is always undefined
• Ignoring main return type

5. You want to create a header file mathutils.h that declares a function int add(int a, int b); and use it in two source files main.c and calc.c. Which is the best practice to avoid multiple definition errors?

hard

A. Put only the function declaration in mathutils.h and define the function in one source file

B. Define the function in both source files and include mathutils.h everywhere

C. Put the function definition in mathutils.h and include it in both source files

D. Include mathutils.h only in main.c and define function in calc.c

Solution

1. Step 1: Understand declarations vs definitions in headers

   Headers should contain declarations (function prototypes), not definitions (function bodies), to avoid multiple definitions.

2. Step 2: Apply to multiple source files

   Define the function in one source file (e.g., calc.c) and declare it in mathutils.h included by both files.

3. Final Answer:

   Put only the function declaration in mathutils.h and define the function in one source file -> Option A

4. Quick Check:

   Declarations in headers, definitions in one source file [OK]

Hint: Declare in header, define once in source file [OK]

Common Mistakes:

• Defining functions in headers causing multiple definitions
• Not including header in all source files needing declaration
• Defining function in multiple source files
• Including header only in one source file