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.