Using cin for input in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code reading two integers?

Consider the following C++ code that reads two integers from the user and prints their sum. What will be the output if the user inputs 3 7?

C++

#include <iostream>
using namespace std;

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

A Error: invalid input

B 37

C 3 7

D 10

💡 Hint

Remember that cin reads values separated by spaces and stores them in variables.

The code reads two integers 3 and 7, then outputs their sum, which is 10.

❓ Predict Output

intermediate

What happens if input is a word instead of a number?

What will be the output or behavior of this code if the user inputs the word hello instead of a number?

C++

#include <iostream>
using namespace std;

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

A Input failure, variable x remains uninitialized or zero

B hello

C 0

D Program crashes

💡 Hint

Think about what happens when cin tries to read an integer but gets a non-numeric string.

When cin tries to read an integer but the input is not a number, it sets a fail state and does not assign a value to x. The variable x remains uninitialized or zero if default initialized.

🔧 Debug

advanced

Why does this code not read the full line after reading an integer?

This code reads an integer and then tries to read a full line of text. Why does the second input read an empty string?

C++

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

int main() {
    int n;
    string line;
    cin >> n;
    getline(cin, line);
    cout << "Number: " << n << endl;
    cout << "Line: " << line << endl;
    return 0;
}

A The newline after the integer input is still in the input buffer, so getline reads it as an empty line.

B The variable 'line' is not initialized before getline, causing it to be empty.

C The integer input fails, so getline reads nothing.

D getline cannot be used after cin >> for any input.

💡 Hint

Think about what remains in the input buffer after reading with cin >>.

After reading the integer, the newline character remains in the input buffer. The getline reads this leftover newline as an empty line.

📝 Syntax

advanced

Which option correctly reads three integers using cin?

Which of the following code snippets correctly reads three integers a, b, c from the user using cin?

A cin >> a, b, c;

B cin << a << b << c;

C cin >> a >> b >> c;

D cin >> (a, b, c);

💡 Hint

Remember the operator used with cin to read multiple values.

The correct syntax to read multiple variables with cin is chaining the extraction operator >> like cin >> a >> b >> c;.

🚀 Application

expert

What is the value of x after this input sequence?

Given the following code, what will be the value of x after the user inputs 42 hello 100?

C++

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

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

A 42

B 100

C hello

D Input failure, x is uninitialized

💡 Hint

The input reads in order: integer, string, integer. The last integer overwrites x.

The first cin >> x reads 42 into x. Then cin >> s reads "hello". Finally, cin >> x reads 100, overwriting the previous value of x. So x is 100.

Practice

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

easy

A. It prints output to the screen.

B. It reads input from the user.

C. It declares a variable.

D. It creates a loop.

Solution

1. Step 1: Understand the role of cin

   cin is used to get input from the user in C++ programs.

2. Step 2: Differentiate from other commands

   Unlike cout which outputs data, cin reads data from the keyboard.

3. Final Answer:

   It reads input from the user. -> Option B

4. Quick Check:

   cin = input [OK]

Hint: Remember: cin means input from user [OK]

Common Mistakes:

• Confusing cin with cout
• Thinking cin outputs data
• Assuming cin declares variables

2. Which of the following is the correct syntax to read an integer variable age using cin?

easy

A. cin >> age;

B. cin >> &age;

C. cin >> "age";

D. cin << age;

Solution

1. Step 1: Recall the operator for input

   cin uses the extraction operator >> to read input into variables.

2. Step 2: Check each option

   cin >> age; uses cin >> age; which is correct syntax to read an integer variable.

3. Final Answer:

   cin >> age; -> Option A

4. Quick Check:

   Use >> with cin [OK]

Hint: Use >> operator with cin to read input [OK]

Common Mistakes:

• Using << instead of >> with cin
• Trying to input variable name as string
• Using address operator & incorrectly

3. What will be the output of this code if the user inputs 42?

int num;
std::cin >> num;
std::cout << num + 10;

medium

A. num + 10

B. 4210

C. 52

D. Syntax error

Solution

1. Step 1: Understand input and variable assignment

   User inputs 42, which is stored in variable num.

2. Step 2: Calculate output expression

   num + 10 equals 42 + 10 = 52, which is printed.

3. Final Answer:

   52 -> Option C

4. Quick Check:

   42 + 10 = 52 [OK]

Hint: Add input value to 10 and print [OK]

Common Mistakes:

• Treating input as string and concatenating
• Expecting variable name printed
• Assuming syntax error without checking code

4. Identify the error in this code snippet:

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

medium

A. Variable x not declared

B. Wrong operator used with cin

C. No error

D. Missing semicolon after std::cin >> x

Solution

1. Step 1: Check syntax line by line

   The line std::cin >> x is missing a semicolon at the end.

2. Step 2: Confirm other parts

   Variable x is declared, and operators are correct.

3. Final Answer:

   Missing semicolon after std::cin >> x -> Option D

4. Quick Check:

   Every statement needs a semicolon [OK]

Hint: Check for missing semicolons after input lines [OK]

Common Mistakes:

• Forgetting semicolon after input statement
• Confusing input operator >> with <<
• Assuming variable not declared

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

hard

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

B. int a, b;
std::cin << a << b;
std::cout << a + b;

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

D. int a, b;
std::cin >> (a + b);
std::cout << a + b;

Solution

1. Step 1: Understand how to read multiple inputs

   Use std::cin >> a >> b; to read two integers from the user.

2. Step 2: Check each option for correctness

   Only the snippet with chained >> operators (std::cin >> a >> b;) is correct. Others use the output operator <<, commas instead of chaining, or try to read into an expression (a + b).

3. Final Answer:

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

4. Quick Check:

   Use chained >> operators for multiple inputs [OK]

Hint: Chain >> operators to read multiple variables [OK]

Common Mistakes:

• Using << instead of >> with cin
• Separating inputs with commas instead of >>
• Trying to input expressions instead of variables