Writing first C program - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the #include <stdio.h> line in a C program?

It tells the compiler to include the Standard Input Output library, which allows the program to use functions like printf() to display text on the screen.

beginner

What does the int main() function represent in a C program?

It is the starting point of the program where execution begins. The program runs the code inside main() first.

beginner

Why do we use return 0; at the end of the main() function?

It tells the operating system that the program finished successfully. Returning 0 usually means no errors happened.

beginner

What does the printf() function do?

It prints text or variables to the screen so the user can see output from the program.

beginner

Why do we put a semicolon ; at the end of most lines in C?

The semicolon tells the compiler that the statement is finished, like a full stop in a sentence.

What is the correct way to start the main function in a C program?

A void main()

B start()

C int main()

D function main()

The main function in C starts with int main() which returns an integer.

Which header file is needed to use printf()?

A <stdlib.h>

B <stdio.h>

C <string.h>

D <math.h>

printf() is declared in the stdio.h header file.

What does return 0; mean in the main() function?

A Program ended with no errors

B Program has an error

C Start the program

D Pause the program

Returning 0 means the program finished successfully without errors.

Which symbol ends a statement in C?

A ,

B .

C :

D ;

A semicolon ; ends a statement in C.

What will this code print?

printf("Hello, World!\n");

A Hello, World!

B Hello World

C Hello, World!\n

D Error

It prints the text Hello, World! followed by a new line.

Explain the basic structure of a simple C program that prints text on the screen.

Think about what each part does and how they work together.

Why is the main() function important in every C program?

Consider what happens when you run a C program.

Practice

1. What is the purpose of the int main() function in a C program?

easy

A. It prints messages to the screen.

B. It is the starting point where the program begins execution.

C. It declares variables for the program.

D. It ends the program immediately.

Solution

1. Step 1: Understand the role of main()

   The main() function is where the program starts running in C.

2. Step 2: Identify the correct purpose

   It is not for declaring variables or printing messages but for starting the program.

3. Final Answer:

   It is the starting point where the program begins execution. -> Option B

4. Quick Check:

   Program starts at main() [OK]

Hint: Remember: C programs always start at main() [OK]

Common Mistakes:

• Thinking main() prints output
• Confusing main() with variable declaration
• Believing main() ends the program immediately

2. Which of the following is the correct syntax to print "Hello, World!" in C?

easy

A. printf("Hello, World!");

B. printf("Hello, World!")

C. print("Hello, World!");

D. echo("Hello, World!");

Solution

1. Step 1: Identify the correct function for output

   In C, printf is used to print text to the screen.

2. Step 2: Check syntax details

   Each statement must end with a semicolon ;. So printf("Hello, World!"); is correct.

3. Final Answer:

   printf("Hello, World!"); -> Option A

4. Quick Check:

   Use printf with semicolon [OK]

Hint: Always end printf with a semicolon [OK]

Common Mistakes:

• Missing semicolon at the end
• Using print instead of printf
• Using echo which is not C syntax

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

#include 
int main() {
    printf("Welcome to C programming!\n");
    return 0;
}

medium

A. No output

B. Welcome to C programming!\n

C. Syntax Error

D. Welcome to C programming!

Solution

1. Step 1: Understand the printf statement

   The string inside printf includes \n which means new line after printing.

2. Step 2: Predict the output

   The program prints "Welcome to C programming!" followed by a new line.

3. Final Answer:

   Welcome to C programming! -> Option D

4. Quick Check:

   \n creates new line, output is plain text [OK]

Hint: \n means new line, not literal text [OK]

Common Mistakes:

• Thinking \n prints as text
• Expecting syntax error due to \n
• Ignoring the return statement effect

4. Identify the error in the following C program:

#include 
int main() {
    printf("Hello World")
    return 0;
}

medium

A. Incorrect header file included

B. Missing return type for main

C. Missing semicolon after printf statement

D. Missing main function

Solution

1. Step 1: Check each statement for syntax

   The printf statement is missing a semicolon at the end.

2. Step 2: Verify other parts

   The main function has correct return type and header file is correct.

3. Final Answer:

   Missing semicolon after printf statement -> Option C

4. Quick Check:

   Every statement ends with ; [OK]

Hint: Look for missing semicolons after statements [OK]

Common Mistakes:

• Ignoring missing semicolon errors
• Thinking return type is missing
• Assuming wrong header file causes error

5. You want to write a C program that prints two lines: "Hello!" and "Welcome to C." Which code snippet correctly does this?

hard

A. #include
int main() {
printf("Hello!\n");
printf("Welcome to C.\n");
return 0;
}

B. #include
int main() {
printf("Hello!Welcome to C.\n");
return 0;
}

C. #include
int main() {
printf("Hello!");
printf("Welcome to C.\n");
return 0;
}

D. #include
int main() {
printf("Hello!");
printf("Welcome to C.");
return 0;
}

Solution

1. Step 1: Understand how to print two lines

   Each line should end with a newline character \n to move to the next line.

2. Step 2: Check each option for correct newlines

   #include int main() { printf("Hello!\n"); printf("Welcome to C.\n"); return 0; } prints "Hello!" with \n and "Welcome to C." with \n, so two lines appear.

3. Final Answer:

   Two printf statements, each ending with \n -> Option A

4. Quick Check:

   Use \n at end of each line for separate lines [OK]

Hint: Use \n after each line to print on new lines [OK]

Common Mistakes:

• Missing \n causing lines to join
• Putting \n only on first line
• Not returning 0 at end of main