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.

Practice

1. What is the primary purpose of using ifstream in C++?

easy

A. To write data to a file

B. To read data from a file

C. To create a new file

D. To delete a file

Solution

1. Step 1: Understand the role of ifstream

   ifstream stands for input file stream, which is used to read data from files.

2. Step 2: Differentiate from ofstream

   ofstream is used to write data to files, so it is not the correct answer here.

3. Final Answer:

   To read data from a file -> Option B

4. Quick Check:

   ifstream = read file [OK]

Hint: Remember 'i' in ifstream means input (read) [OK]

Common Mistakes:

• Confusing ifstream with ofstream
• Thinking ifstream creates or deletes files
• Mixing input and output streams

2. Which of the following is the correct way to open a file named data.txt for writing using ofstream?

easy

A. ofstream file.open("data.txt");

B. ifstream file("data.txt");

C. file.open(ofstream, "data.txt");

D. ofstream file("data.txt");

Solution

1. Step 1: Recognize correct ofstream syntax

   To open a file for writing, you create an ofstream object and pass the filename to its constructor.

2. Step 2: Check other options for syntax errors

   ifstream file("data.txt"); uses ifstream which is for reading, not writing. ofstream file.open("data.txt"); incorrectly calls open as a constructor. file.open(ofstream, "data.txt"); has wrong parameter order and syntax.

3. Final Answer:

   ofstream file("data.txt"); -> Option D

4. Quick Check:

   Correct ofstream constructor usage = ofstream file("data.txt"); [OK]

Hint: Use constructor with filename to open file for writing [OK]

Common Mistakes:

• Using ifstream instead of ofstream for writing
• Calling open() incorrectly
• Passing wrong arguments to open()

3. What will be the output of the following code if the file numbers.txt contains the numbers 10 and 20 on separate lines?

std::ifstream file("numbers.txt");
int a, b;
file >> a >> b;
std::cout << a + b << std::endl;
file.close();

medium

A. 30

B. 1020

C. 10

D. Compilation error

Solution

1. Step 1: Understand file reading with >> operator

   The code reads two integers from the file into variables a and b. Since the file has 10 and 20 on separate lines, a = 10 and b = 20.

2. Step 2: Calculate the output

   The code outputs the sum of a and b, which is 10 + 20 = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   10 + 20 = 30 [OK]

Hint: >> reads values as numbers, not strings [OK]

Common Mistakes:

• Thinking >> concatenates numbers as strings
• Forgetting to close the file
• Assuming output is the first number only

4. Identify the error in the following code snippet that tries to write "Hello" to a file named output.txt:

std::ofstream file;
file << "Hello";
file.open("output.txt");
file.close();

medium

A. File should be closed before opening

B. Missing semicolon after file.open()

C. File is opened after writing to it

D. ofstream cannot write strings

Solution

1. Step 1: Check order of operations

   The code tries to write to the file before opening it, which is incorrect because the file must be opened first.

2. Step 2: Confirm other options

   Missing semicolon after file.open() is wrong because semicolons are present. ofstream cannot write strings is false; ofstream can write strings. File should be closed before opening is incorrect; you cannot close before opening.

3. Final Answer:

   File is opened after writing to it -> Option C

4. Quick Check:

   Open file before writing [OK]

Hint: Always open file before writing or reading [OK]

Common Mistakes:

• Writing before opening the file
• Confusing order of open and close
• Assuming ofstream can't write strings

5. You want to read integers from a file input.txt and write only the even numbers to even.txt. Which code snippet correctly implements this?

hard

A. std::ifstream inFile("input.txt");
std::ofstream outFile("even.txt");
int num;
while (inFile >> num) {
if (num % 2 == 0) {
outFile << num << '\n';
}
}
inFile.close();
outFile.close();

B. std::ofstream inFile("input.txt");
std::ifstream outFile("even.txt");
int num;
while (inFile >> num) {
if (num % 2 == 0) {
outFile << num << '\n';
}
}
inFile.close();
outFile.close();

C. std::ifstream inFile;
std::ofstream outFile;
inFile.open("input.txt");
outFile.open("even.txt");
int num;
while (inFile >> num) {
if (num % 2 == 1) {
outFile << num << '\n';
}
}
inFile.close();
outFile.close();

D. std::ifstream inFile("input.txt");
std::ofstream outFile("even.txt");
int num;
while (!inFile.eof()) {
inFile >> num;
if (num % 2 == 0) {
outFile << num << '\n';
}
}
inFile.close();
outFile.close();

Solution

1. Step 1: Check correct stream types and file opening

   std::ifstream inFile("input.txt"); std::ofstream outFile("even.txt"); int num; while (inFile >> num) { if (num % 2 == 0) { outFile << num << '\n'; } } inFile.close(); outFile.close(); correctly uses ifstream to read and ofstream to write, opening files in constructors.

2. Step 2: Verify reading loop and condition

   std::ifstream inFile("input.txt"); std::ofstream outFile("even.txt"); int num; while (inFile >> num) { if (num % 2 == 0) { outFile << num << '\n'; } } inFile.close(); outFile.close(); uses while (inFile >> num) which safely reads integers until EOF. It writes only even numbers (num % 2 == 0) to output.

3. Step 3: Identify errors in other options

   std::ofstream inFile("input.txt"); std::ifstream outFile("even.txt"); int num; while (inFile >> num) { if (num % 2 == 0) { outFile << num << '\n'; } } inFile.close(); outFile.close(); swaps stream types incorrectly. std::ifstream inFile; std::ofstream outFile; inFile.open("input.txt"); outFile.open("even.txt"); int num; while (inFile >> num) { if (num % 2 == 1) { outFile << num << '\n'; } } inFile.close(); outFile.close(); writes odd numbers, not even. std::ifstream inFile("input.txt"); std::ofstream outFile("even.txt"); int num; while (!inFile.eof()) { inFile >> num; if (num % 2 == 0) { outFile << num << '\n'; } } inFile.close(); outFile.close(); uses while (!inFile.eof()) which can cause reading errors.

4. Final Answer:

   Option A code snippet -> Option A

5. Quick Check:

   Correct streams + safe read loop + even check = std::ifstream inFile("input.txt"); std::ofstream outFile("even.txt"); int num; while (inFile >> num) { if (num % 2 == 0) { outFile << num << '\n'; } } inFile.close(); outFile.close(); [OK]

Hint: Use while(inFile >> num) and check even with num % 2 == 0 [OK]

Common Mistakes:

• Swapping ifstream and ofstream
• Using eof() in loop condition
• Writing odd numbers instead of even