Input and output using cin and cout in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C++ code?

Consider the following code snippet. What will be printed when the input is 5 10?

C++

#include <iostream>
using namespace std;

int main() {
    int a, b;
    cin >> a >> b;
    cout << a + b << endl;
    return 0;
}

A 5 10

B 510

C 15

D Compilation error

💡 Hint

Remember that operator >> reads integers and skips spaces.

The code reads two integers 5 and 10, then outputs their sum, which is 15.

❓ Predict Output

intermediate

What does this code print given input "Hello World"?

Look at this code. What will it output if the user types Hello World?

C++

#include <iostream>
#include <string>
using namespace std;

int main() {
    string s;
    cin >> s;
    cout << s << endl;
    return 0;
}

A Hello World

B Hello

C World

D Compilation error

💡 Hint

cin >> string reads only until the first space.

cin >> s reads only the first word 'Hello' and stops at the space.

❓ Predict Output

advanced

What is the output of this code reading multiple inputs?

Given the input 3 4 5, what will this program print?

C++

#include <iostream>
using namespace std;

int main() {
    int x, y, z;
    cin >> x >> y >> z;
    cout << x << y << z << endl;
    return 0;
}

A 345

B 3 4 5

C 12

D Compilation error

💡 Hint

cout with multiple << outputs values without spaces.

The program prints the three integers concatenated without spaces: 345.

❓ Predict Output

advanced

What error does this code cause?

What error will this code produce when run?

C++

#include <iostream>
using namespace std;

int main() {
    int a;
    cout << "Enter a number: ";
    cin >> a;
    cout << "You entered: " << b << endl;
    return 0;
}

A Compilation error: 'b' was not declared

B Runtime error: segmentation fault

C Outputs the entered number

D No output

💡 Hint

Check variable names carefully.

Variable 'b' is used but never declared, causing a compilation error.

🧠 Conceptual

expert

What is the output of this code with mixed input types?

Given the input 42 hello 3.14, what will this program output?

C++

#include <iostream>
#include <string>
using namespace std;

int main() {
    int i;
    string s;
    double d;
    cin >> i >> s >> d;
    cout << s << " " << i << " " << d << endl;
    return 0;
}

A Compilation error

B 42 hello 3.14

C 3.14 42 hello

D hello 42 3.14

💡 Hint

cin reads inputs in order and cout prints in the order given.

cin reads 42 into i, 'hello' into s, and 3.14 into d. cout prints s, i, d in that order.

Practice

1. What does cin do in a C++ program?

easy

A. It declares a variable.

B. It prints output to the screen.

C. It reads input from the user.

D. It ends the program.

Solution

1. Step 1: Understand the role of cin

   cin is used to get input from the user during program execution.

2. Step 2: Differentiate from cout

   cout is used to show output, not input.

3. Final Answer:

   It reads input from the user. -> Option C

4. Quick Check:

   cin = input [OK]

Hint: Remember: cin reads input, cout shows output [OK]

Common Mistakes:

• Confusing cin with cout
• Thinking cin prints output
• Mixing input and output operators

2. Which of the following is the correct syntax to read an integer variable num from the user?

easy

A. cout >> num;

B. cout << num;

C. cin << num;

D. cin >> num;

Solution

1. Step 1: Identify input operator usage

   To read input, use cin with the extraction operator >>.

2. Step 2: Check each option

   cin >> num; is the correct syntax for input. Options using cout are for output, not input. The option with cin << num; uses the wrong operator <<.

3. Final Answer:

   cin >> num; -> Option D

4. Quick Check:

   Input syntax = cin >> variable [OK]

Hint: Use >> with cin to get input [OK]

Common Mistakes:

• Using << with cin instead of >>
• Using cout instead of cin for input
• Forgetting semicolon

3. What will be the output of this code if the user inputs 5?

int num;
std::cin >> num;
std::cout << num + 3;

medium

A. 5

B. 8

C. 3

D. Error

Solution

1. Step 1: Read input value

   The user inputs 5, so num becomes 5.

2. Step 2: Calculate output expression

   The program outputs num + 3 which is 5 + 3 = 8.

3. Final Answer:

   8 -> Option B

4. Quick Check:

   5 + 3 = 8 [OK]

Hint: Add 3 to input number before printing [OK]

Common Mistakes:

• Printing input without adding 3
• Confusing input and output
• Expecting error due to input

4. What is wrong with this code snippet?

int age;
std::cin << age;
std::cout >> age;

medium

A. The operators with cin and cout are reversed.

B. Variables are not declared.

C. Missing semicolons.

D. No errors.

Solution

1. Step 1: Check operators used with cin and cout

   cin should use >> to get input, but code uses <<. Similarly, cout should use << to output, but code uses >>.

2. Step 2: Confirm variable declaration and semicolons

   Variable age is declared and semicolons are present, so no issues there.

3. Final Answer:

   The operators with cin and cout are reversed. -> Option A

4. Quick Check:

   cin >> input, cout << output [OK]

Hint: cin uses >>, cout uses << operators [OK]

Common Mistakes:

• Using << with cin
• Using >> with cout
• Ignoring operator direction

5. You want to read two integers from the user and print their sum. Which code correctly does this?

hard

A. int a, b;
cin >> a >> b;
cout << a + b;

B. int a, b;
cin << a << b;
cout >> a + b;

C. int a, b;
cin >> a;
cin << b;
cout << a + b;

D. int a, b;
cout >> a >> b;
cin << a + b;

Solution

1. Step 1: Check input statements

   int a, b; cin >> a >> b; cout << a + b; uses cin >> a >> b; which correctly reads two integers.

2. Step 2: Check output statement

   int a, b; cin >> a >> b; cout << a + b; uses cout << a + b; which correctly prints the sum.

3. Final Answer:

   int a, b; cin >> a >> b; cout << a + b; -> Option A

4. Quick Check:

   Use >> with cin and << with cout [OK]

Hint: Chain inputs with >> and output with << [OK]

Common Mistakes:

• Using << with cin or >> with cout
• Separating input into wrong operators
• Mixing input and output streams