Using scanf for input - Mini Project: Build & Apply

Using scanf for input

📖 Scenario: You are creating a simple program that asks a user for their age and height.

🎯 Goal: Build a C program that uses scanf to read an integer and a float from the user.

📋 What You'll Learn

Use scanf to read an integer value for age

Use scanf to read a float value for height

Print the values back to the user

💡 Why This Matters

🌍 Real World

Many programs need to get information from users, like age or height, to personalize the experience.

💼 Career

Understanding how to read user input with <code>scanf</code> is a basic skill for C programmers working on interactive applications.

1

Create variables for age and height

Declare an integer variable called age and a float variable called height.

C

// Declare variables for age and height
// Your code here

Hint

Use int for whole numbers and float for decimal numbers.

2

Use scanf to read age

Add a printf statement to ask the user to enter their age. Then use scanf with "%d" to read the integer into age.

C

#include 

int main() {
    int age;
    float height;
    // Ask for age and read it
    // Your code here
    return 0;
}

Hint

Remember to use &age to pass the address to scanf.

3

Use scanf to read height

Add a printf statement to ask the user to enter their height. Then use scanf with "%f" to read the float into height.

C

#include 

int main() {
    int age;
    float height;
    printf("Enter your age: ");
    scanf("%d", &age);
    // Ask for height and read it
    // Your code here
    return 0;
}

Hint

Use %f to read a float value with scanf.

4

Print the age and height

Use printf to display the message: Your age is followed by age, and on the next line, Your height is followed by height with 2 decimal places.

C

#include 

int main() {
    int age;
    float height;
    printf("Enter your age: ");
    scanf("%d", &age);
    printf("Enter your height in meters: ");
    scanf("%f", &height);
    // Print age and height
    // Your code here
    return 0;
}

Hint

Use %d for integers and %.2f to show float with two decimals in printf.

Practice

1. What is the purpose of the scanf function in C?

easy

A. To read input from the user during program execution

B. To print output to the screen

C. To declare variables

D. To perform mathematical calculations

Solution

1. Step 1: Understand the role of scanf

   scanf is used to get input from the user while the program runs.

2. Step 2: Differentiate from other functions

   Functions like printf print output, but scanf reads input.

3. Final Answer:

   To read input from the user during program execution -> Option A

4. Quick Check:

   scanf reads input [OK]

Hint: scanf reads input, printf prints output [OK]

Common Mistakes:

• Confusing scanf with printf
• Thinking scanf declares variables
• Assuming scanf performs calculations

2. Which of the following is the correct way to read an integer variable num using scanf?

easy

A. scanf("%s", &num);

B. scanf("%d", num);

C. scanf("%f", &num);

D. scanf("%d", &num);

Solution

1. Step 1: Identify the correct format specifier for integer

   Use %d to read an integer.

2. Step 2: Use the address operator for variables

   Use &num to pass the address of the variable to scanf.

3. Final Answer:

   scanf("%d", &num); -> Option D

4. Quick Check:

   Integer input needs %d and &variable [OK]

Hint: Use & before variable except arrays [OK]

Common Mistakes:

• Omitting & before variable
• Using wrong format specifier like %f or %s
• Passing variable instead of its address

3. What will be the output of the following code if the user inputs 25?

int age;
scanf("%d", &age);
printf("Age: %d", age);

medium

A. Age: 0

B. Age: 25

C. Age: %d

D. Compilation error

Solution

1. Step 1: Understand input reading

   scanf("%d", &age); reads the integer 25 and stores it in age.

2. Step 2: Print the stored value

   printf("Age: %d", age); prints the string with the value of age, which is 25.

3. Final Answer:

   Age: 25 -> Option B

4. Quick Check:

   scanf stores input, printf prints it [OK]

Hint: scanf stores input, printf prints variable [OK]

Common Mistakes:

• Forgetting & in scanf causing runtime error
• Using wrong format specifier in printf
• Expecting scanf to print output

4. Identify the error in the following code snippet:

int x;
printf("Enter a number: ");
scanf("%d", x);
printf("You entered %d", x);

medium

A. Missing & before variable x in scanf

B. Wrong format specifier in scanf

C. Missing semicolon after printf

D. Variable x not declared

Solution

1. Step 1: Check scanf usage

   scanf("%d", x); is incorrect because it should use &x to pass the address.

2. Step 2: Confirm other parts are correct

   Format specifier is correct, semicolons are present, and x is declared.

3. Final Answer:

   Missing & before variable x in scanf -> Option A

4. Quick Check:

   scanf needs &variable for input [OK]

Hint: Always use & before variables in scanf except arrays [OK]

Common Mistakes:

• Omitting & causing segmentation fault
• Using wrong format specifier
• Assuming scanf prints output

5. You want to read a string (word) from the user into a character array name[20]. Which of the following scanf statements is correct and safe to avoid buffer overflow?

hard

A. scanf("%s", name);

B. scanf("%s", &name);

C. scanf("%19s", name);

D. scanf("%20s", &name);

Solution

1. Step 1: Understand string input with scanf

   To read a string into name, use %s without & because arrays decay to pointers.

2. Step 2: Prevent buffer overflow

   Limit input length to 19 characters using %19s to leave space for the null terminator.

3. Final Answer:

   scanf("%19s", name); -> Option C

4. Quick Check:

   Limit string input length with %19s [OK]

Hint: Use %19s for 20-char array to avoid overflow [OK]

Common Mistakes:

• Using & with arrays in scanf
• Not limiting input length causing overflow
• Using wrong format specifier for strings