Writing first C program - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C program?

Look at the code below. What will it print when run?

C

#include <stdio.h>

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

A Compilation error

B Hello, C World!

C Hello, C World!\n

D hello, c world!

💡 Hint

Remember that printf prints the string exactly, and \n means new line.

The program prints the string inside printf exactly, including the new line. So it outputs 'Hello, C World!' followed by a new line.

❓ Predict Output

intermediate

What is the output of this C program with variables?

What will this program print when run?

C

#include <stdio.h>

int main() {
    int a = 5;
    int b = 3;
    printf("Sum is %d\n", a + b);
    return 0;
}

A Sum is 8

B Sum is 53

C Compilation error

D Sum is %d

💡 Hint

The %d in printf is replaced by the integer value of a + b.

The program adds 5 and 3 to get 8, then prints 'Sum is 8' followed by a new line.

❓ Predict Output

advanced

What error does this C program produce?

What error will this program cause when compiled?

C

#include <stdio.h>

int main() {
    printf("Missing semicolon")
    return 0;
}

A No error, prints 'Missing semicolon'

B Runtime error

C Syntax error: expected ';' before 'return'

D Linker error

💡 Hint

Check if all statements end with a semicolon.

The printf statement is missing a semicolon at the end, causing a syntax error during compilation.

❓ Predict Output

advanced

What is the output of this C program with a loop?

What will this program print when run?

C

#include <stdio.h>

int main() {
    for (int i = 1; i <= 3; i++) {
        printf("%d ", i);
    }
    return 0;
}

A 1 2 3

B 123

C 1 2 3\n

D Compilation error

💡 Hint

The loop prints numbers 1 to 3 separated by spaces.

The loop prints '1 ', then '2 ', then '3 ', all on the same line with spaces.

🧠 Conceptual

expert

How many items are printed by this C program?

How many times does the printf statement run and print a number?

C

#include <stdio.h>

int main() {
    int count = 0;
    for (int i = 0; i < 5; i++) {
        if (i % 2 == 0) {
            printf("%d ", i);
            count++;
        }
    }
    printf("\nCount: %d", count);
    return 0;
}

A 2

B 5

C 4

D 3

💡 Hint

Count how many numbers from 0 to 4 are even.

The loop prints numbers 0, 2, and 4 because they are even. So printf runs 3 times.

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