Multiple input and output in C - Mini Project: Build & Apply

Multiple input and output

📖 Scenario: You are creating a simple program to record the names and ages of three friends. You will input their names and ages, then display this information clearly.

🎯 Goal: Build a C program that takes the names and ages of three friends as input and then prints each friend's name with their age.

📋 What You'll Learn

Use arrays to store names and ages

Use a loop to input data for three friends

Use a loop to output the stored data

Use scanf and printf for input and output

💡 Why This Matters

🌍 Real World

This kind of program helps in collecting and displaying simple records like names and ages, useful in small databases or contact lists.

💼 Career

Understanding multiple inputs and outputs is essential for many programming tasks such as data entry, processing user information, and building interactive applications.

1

Create arrays for names and ages

Declare a 2D character array called names with size 3 by 20 to store three names, and an integer array called ages with size 3 to store their ages.

C

# Declare arrays for names and ages
# Your code here

Hint

Use char names[3][20]; to store three names each up to 19 characters plus null terminator.

2

Add a loop to input names and ages

Add a for loop with variable i from 0 to less than 3. Inside the loop, use scanf("%19s", names[i]) to input each name and scanf("%d", &ages[i]) to input each age.

C

#include 

char names[3][20];
int ages[3];

int main() {
    // Input names and ages
    // Your code here
    return 0;
}

Hint

Use a for loop to input data for each friend.

3

Add a loop to output names and ages

Add a for loop with variable i from 0 to less than 3. Inside the loop, use printf("%s is %d years old\n", names[i], ages[i]) to print each friend's name and age.

C

#include 

char names[3][20];
int ages[3];

int main() {
    for (int i = 0; i < 3; i++) {
        scanf("%19s", names[i]);
        scanf("%d", &ages[i]);
    }
    // Output names and ages
    // Your code here
    return 0;
}

Hint

Use a for loop to print each friend's information.

4

Print the final output

Run the program and input these three friends' data exactly in this order:
Alice 25
Bob 30
Charlie 22
Then print the output exactly as:
Alice is 25 years old
Bob is 30 years old
Charlie is 22 years old

C

#include 

char names[3][20];
int ages[3];

int main() {
    for (int i = 0; i < 3; i++) {
        scanf("%19s", names[i]);
        scanf("%d", &ages[i]);
    }
    for (int i = 0; i < 3; i++) {
        printf("%s is %d years old\n", names[i], ages[i]);
    }
    // Run and check output
    // Your code here
    return 0;
}

Hint

Type the names and ages exactly as shown, then check the printed output.

Practice

1. What does the following C code do?
int a, b;
scanf("%d %d", &a, &b);
printf("%d %d", a, b);

easy

A. Prints two integers without reading input.

B. Reads one integer and prints it twice.

C. Reads two integers from input and prints them in the same order.

D. Reads two integers but prints only the first one.

Solution

1. Step 1: Understand scanf with multiple inputs

   The scanf function reads two integers from the user input and stores them in variables a and b.

2. Step 2: Understand printf with multiple outputs

   The printf function prints the values of a and b in the same order they were read.

3. Final Answer:

   Reads two integers from input and prints them in the same order. -> Option C

4. Quick Check:

   scanf and printf with two %d = same two integers [OK]

Hint: Match each %d with a variable and & in scanf [OK]

Common Mistakes:

• Forgetting & before variables in scanf
• Mixing order of variables in printf
• Using wrong format specifiers

2. Which of the following is the correct syntax to read three integers a, b, c in C?

easy

A. scanf("%d %d %d", &a, &b, &c);

B. scanf("%d %d %d", a, b, c);

C. scanf("%d, %d, %d", &a, &b, &c);

D. scanf("%d %d %d", *a, *b, *c);

Solution

1. Step 1: Check scanf format string and arguments

   The format string should have three %d separated by spaces, and variables must be passed by address using &.

2. Step 2: Identify correct option

   scanf("%d %d %d", &a, &b, &c); uses correct format string and & before variables, so it is correct.

3. Final Answer:

   scanf("%d %d %d", &a, &b, &c); -> Option A

4. Quick Check:

   Use & with variables in scanf [OK]

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

Common Mistakes:

• Omitting & before variables
• Using commas inside format string incorrectly
• Using pointers incorrectly with *

3. What is the output of this C program?

int x, y;
scanf("%d %d", &x, &y);
printf("%d %d %d", x, y, x + y);

Input:
3 7

medium

A. 10 10 20

B. 3 7 10

C. 7 3 10

D. 3 7 37

Solution

1. Step 1: Read input values

   scanf reads 3 into x and 7 into y.

2. Step 2: Calculate and print output

   printf prints x (3), y (7), and x + y (3 + 7 = 10).

3. Final Answer:

   3 7 10 -> Option B

4. Quick Check:

   3 + 7 = 10 [OK]

Hint: Add variables inside printf for combined output [OK]

Common Mistakes:

• Mixing order of variables in printf
• Confusing concatenation with addition
• Not using & in scanf

4. Find the error in this code snippet:

int a, b;
scanf("%d %d", a, b);
printf("%d %d", a, b);

medium

A. Missing semicolon after scanf.

B. Wrong format specifier in scanf.

C. printf format specifiers do not match variables.

D. Missing & before variables in scanf.

Solution

1. Step 1: Check scanf arguments

   scanf requires addresses of variables, so &a and &b are needed.

2. Step 2: Identify error

   Code passes variables a and b directly, causing undefined behavior or crash.

3. Final Answer:

   Missing & before variables in scanf. -> Option D

4. Quick Check:

   scanf needs & before variables [OK]

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

Common Mistakes:

• Forgetting & in scanf
• Assuming variables auto-pass by address
• Confusing scanf and printf syntax

5. Write a C program snippet that reads two integers and prints their sum and product separated by a space. Which code correctly does this?

hard

A. int a, b;scanf("%d %d", &a, &b);printf("%d %d", a + b, a * b);

B. int a, b;scanf("%d %d", a, b);printf("%d %d", a + b, a * b);

C. int a, b;scanf("%d %d", &a, &b);printf("%d %d", a - b, a / b);

D. int a, b;scanf("%d %d", &a, &b);printf("%d %d", a * b, a + b);

Solution

1. Step 1: Check input reading correctness

   scanf must use &a and &b to read two integers correctly.

2. Step 2: Check output matches sum and product

   printf should print sum (a + b) first, then product (a * b) separated by space.

3. Final Answer:

   int a, b; scanf("%d %d", &a, &b); printf("%d %d", a + b, a * b); -> Option A

4. Quick Check:

   scanf with & and printf sum then product [OK]

Hint: Print sum first, product second with correct scanf [OK]

Common Mistakes:

• Forgetting & in scanf
• Swapping sum and product order
• Using wrong operators in printf