Multiple input and output in C

Introduction

We use multiple input and output to handle more than one piece of information at the same time. This helps programs interact with users or other programs better.

When you want to ask a user for their name and age together.

When reading several numbers from a file to process them.

When printing multiple results like sum and average at once.

When a program needs to get multiple answers from a user in one go.

When you want to display several pieces of information clearly.

Syntax

C

scanf("%d %d", &var1, &var2);
printf("%d %d", var1, var2);

Use scanf to read multiple inputs by listing format specifiers and variables separated by spaces.

Use printf to print multiple outputs by listing format specifiers and variables in order.

Examples

This reads two integers from the user and stores them in a and b.

C

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

This prints the two integers stored in a and b separated by a space.

C

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

This reads two floating-point numbers and prints them with 2 decimal places.

C

float x, y;
scanf("%f %f", &x, &y);
printf("%.2f %.2f", x, y);

Sample Program

This program asks the user to enter two numbers. It reads both numbers at once using scanf. Then it calculates their sum and product and prints both results separately.

C

#include <stdio.h>

int main() {
    int num1, num2;
    printf("Enter two numbers separated by space: ");
    scanf("%d %d", &num1, &num2);

    int sum = num1 + num2;
    int product = num1 * num2;

    printf("Sum: %d\n", sum);
    printf("Product: %d\n", product);

    return 0;
}

Important Notes

Always provide the address of variables to scanf using &.

Separate format specifiers in scanf and printf with spaces to match input/output format.

Be careful to match the data types in format specifiers with the variables you use.

Summary

Multiple input and output lets you handle several values at once.

Use scanf with multiple format specifiers and variable addresses to read inputs.

Use printf with multiple format specifiers and variables to print outputs.

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