Multiple input and output in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with multiple input parameters

What is the output of this C program when calling sum_and_product(3, 4, &sum, &product);?

C

#include <stdio.h>

void sum_and_product(int a, int b, int *sum, int *product) {
    *sum = a + b;
    *product = a * b;
}

int main() {
    int sum = 0, product = 0;
    sum_and_product(3, 4, &sum, &product);
    printf("Sum = %d, Product = %d\n", sum, product);
    return 0;
}

A Sum = 7, Product = 12

B Sum = 12, Product = 7

C Sum = 3, Product = 4

D Sum = 0, Product = 0

💡 Hint

Remember that pointers allow functions to modify variables outside their scope.

The function calculates sum and product and stores them via pointers. So sum = 3 + 4 = 7 and product = 3 * 4 = 12.

❓ Predict Output

intermediate

Output of function returning multiple values via struct

What will be printed by this program?

C

#include <stdio.h>

typedef struct {
    int sum;
    int diff;
} Result;

Result calculate(int x, int y) {
    Result r;
    r.sum = x + y;
    r.diff = x - y;
    return r;
}

int main() {
    Result res = calculate(10, 4);
    printf("Sum: %d, Difference: %d\n", res.sum, res.diff);
    return 0;
}

A Sum: 4, Difference: 10

B Sum: 6, Difference: 14

C Sum: 14, Difference: 6

D Sum: 10, Difference: 4

💡 Hint

Check how sum and difference are calculated inside the struct.

Sum is 10 + 4 = 14 and difference is 10 - 4 = 6, stored in the struct and printed.

❓ Predict Output

advanced

Output of function with multiple pointer parameters and side effects

What is the output of this program?

C

#include <stdio.h>

void update_values(int *a, int *b) {
    *a = *a + *b;
    *b = *a - *b;
}

int main() {
    int x = 5, y = 3;
    update_values(&x, &y);
    printf("x = %d, y = %d\n", x, y);
    return 0;
}

A x = 8, y = 3

B x = 8, y = 5

C x = 5, y = 3

D x = 3, y = 5

💡 Hint

Trace the pointer updates step by step.

Initially x=5, y=3. After *a = *a + *b, x=8. Then *b = *a - *b = 8 - 3 = 5. So x=8, y=5.

❓ Predict Output

advanced

Output of function returning multiple outputs via array parameters

What will this program print?

C

#include <stdio.h>

void min_max(int arr[], int size, int *min, int *max) {
    *min = arr[0];
    *max = arr[0];
    for (int i = 1; i < size; i++) {
        if (arr[i] < *min) *min = arr[i];
        if (arr[i] > *max) *max = arr[i];
    }
}

int main() {
    int numbers[] = {7, 2, 9, 4, 5};
    int minimum, maximum;
    min_max(numbers, 5, &minimum, &maximum);
    printf("Min: %d, Max: %d\n", minimum, maximum);
    return 0;
}

A Min: 7, Max: 5

B Min: 2, Max: 7

C Min: 4, Max: 9

D Min: 2, Max: 9

💡 Hint

Look carefully at how min and max are updated in the loop.

The smallest number is 2 and the largest is 9 in the array.

🧠 Conceptual

expert

Why use pointers for multiple outputs in C functions?

Which is the main reason C functions use pointers to return multiple outputs instead of returning multiple values directly?

A Because C functions can only return one value, pointers allow modifying variables outside the function to simulate multiple outputs.

B Because pointers make the program run faster by avoiding copying values.

C Because pointers automatically allocate memory for multiple return values.

D Because pointers prevent any changes to the original variables passed to the function.

💡 Hint

Think about how many values a C function can return directly.

C functions can return only one value directly. Using pointers allows the function to change variables outside its scope, effectively returning multiple 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