Reading and writing files in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this file reading code?

Consider the following C++ code that reads from a file named data.txt containing the text "Hello World". What will be printed to the console?

C++

#include <iostream>
#include <fstream>
#include <string>

int main() {
    std::ifstream file("data.txt");
    std::string word;
    while (file >> word) {
        std::cout << word << " ";
    }
    return 0;
}

A Hello\nWorld\n

B HelloWorld

C Hello World

D

Hello
World

💡 Hint

Think about how the extraction operator >> reads words separated by whitespace.

The extraction operator >> reads one word at a time, skipping whitespace. The code prints each word followed by a space, so the output is Hello World with a space between words.

❓ Predict Output

intermediate

What does this file writing code produce in the file?

Given the following C++ code, what will be the content of the file output.txt after running it?

C++

#include <fstream>

int main() {
    std::ofstream file("output.txt");
    file << "Line1\n";
    file << "Line2";
    file.close();
    return 0;
}

A Line1\nLine2

B

Line1
Line2

C Line1 Line2

D Line1\n\nLine2

💡 Hint

Remember that \n is a newline character in C++ strings.

The code writes "Line1" followed by a newline, then "Line2" without newline. So the file content is two lines: first line "Line1", second line "Line2".

🔧 Debug

advanced

Why does this file reading code fail to read the entire line?

Look at this C++ code snippet intended to read a full line from a file. Why does it only read the first word?

C++

#include <iostream>
#include <fstream>
#include <string>

int main() {
    std::ifstream file("input.txt");
    std::string line;
    file >> line;
    std::cout << line << std::endl;
    return 0;
}

A Because operator >> reads only until the first whitespace, not the whole line.

B Because the file is not opened in binary mode.

C Because the string variable is not initialized.

D Because the file stream is not flushed before reading.

💡 Hint

Think about the difference between operator>> and getline().

The extraction operator >> reads input until the first whitespace, so it only reads the first word. To read a full line, std::getline(file, line) should be used.

📝 Syntax

advanced

Which option causes a compilation error?

Which of the following code snippets will cause a compilation error when trying to open and write to a file?

A

#include <fstream>
int main() {
    std::ofstream file("test.txt");
    file << "Hello";
    return 0;
}

B

#include <fstream>
int main() {
    std::ofstream file;
    file.open("test.txt");
    file << "Hello";
    return 0;
}

C

#include <fstream>
int main() {
    std::ofstream file("test.txt");
    file.write("Hello", 5);
    return 0;
}

D

#include <fstream>
int main() {
    std::ofstream file;
    file.open();
    file << "Hello";
    return 0;
}

💡 Hint

Check the parameters required by open() method.

The open() method requires a filename as a parameter. Option D calls file.open() without arguments, causing a compilation error.

🚀 Application

expert

How many lines will be in the output file after running this code?

This C++ program reads lines from input.txt and writes only non-empty lines to output.txt. If input.txt contains 5 lines, where 2 lines are empty, how many lines will output.txt have?

C++

#include <fstream>
#include <string>

int main() {
    std::ifstream infile("input.txt");
    std::ofstream outfile("output.txt");
    std::string line;
    while (std::getline(infile, line)) {
        if (!line.empty()) {
            outfile << line << "\n";
        }
    }
    return 0;
}

A 3

B 0

C 2

D 5

💡 Hint

Count only lines that are not empty.

The code writes only lines that are not empty. Since 2 lines are empty out of 5, the output file will have 3 lines.