Reading and writing files in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a file for writing.

C++

#include <fstream>

int main() {
    std::ofstream file;
    file.[1]("output.txt");
    file << "Hello, file!";
    file.close();
    return 0;
}

Drag options to blanks, or click blank then click option'

Aclose

Bwrite

Copen

Dread

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'read' instead of 'open' to open the file.

Trying to write before opening the file.

The open function is used to open a file stream in C++.

2fill in blank

medium

Complete the code to read a line from a file.

C++

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

int main() {
    std::ifstream file("input.txt");
    std::string line;
    if (std::getline(file, [1])) {
        std::cout << line << std::endl;
    }
    file.close();
    return 0;
}

Drag options to blanks, or click blank then click option'

Aline

Bfile

Ctext

Dinput

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the file stream instead of the string variable.

Using an undeclared variable.

The std::getline function reads a line into the variable passed as the second argument, here line.

3fill in blank

hard

Fix the error in the code to write numbers to a file.

C++

#include <fstream>

int main() {
    std::ofstream file("numbers.txt");
    for (int i = 1; i <= 5; i++) {
        file << i [1] " ";
    }
    file.close();
    return 0;
}

Drag options to blanks, or click blank then click option'

A<<

B+

C>>

D=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' which tries to add instead of writing.

Using '>>' which is for input streams.

To write to a file stream, use the insertion operator <<.

4fill in blank

hard

Fill both blanks to create a map of word lengths from a file.

C++

#include <fstream>
#include <string>
#include <map>

int main() {
    std::ifstream file("words.txt");
    std::map<std::string, int> wordLengths;
    std::string word;
    while (file >> [1]) {
        wordLengths[[2]] = word.length();
    }
    file.close();
    return 0;
}

Drag options to blanks, or click blank then click option'

Aword

Bfile

Clength

Dline

Attempts:

3 left

💡 Hint

Common Mistakes

Using the file stream as a key instead of the word.

Using an undefined variable.

The variable word is used to read each word and as the key in the map.

5fill in blank

hard

Fill all three blanks to write only words longer than 3 characters to a file.

C++

#include <fstream>
#include <string>
#include <vector>

int main() {
    std::vector<std::string> words = {"cat", "house", "dog", "elephant"};
    std::ofstream file("long_words.txt");
    for (const auto& [1] : words) {
        if ([2].length() [3] 3) {
            file << word << std::endl;
        }
    }
    file.close();
    return 0;
}

Drag options to blanks, or click blank then click option'

Aword

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the loop and condition.

Using '<' instead of '>' in the condition.

The loop variable is word. We check if word.length() is greater than 3 to write it.

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