Reading and writing files in C++ - Mini Project: Build & Apply

Reading and writing files

📖 Scenario: You are working on a simple program that saves and reads a list of favorite fruits to and from a file. This is useful when you want to keep your data even after the program stops running.

🎯 Goal: Build a C++ program that writes a list of fruits to a file and then reads the fruits back from the file to display them.

📋 What You'll Learn

Create a vector of strings with exact fruit names

Create a string variable for the filename

Write the fruits to the file using ofstream

Read the fruits from the file using ifstream

Print each fruit read from the file

💡 Why This Matters

🌍 Real World

Reading and writing files is a basic skill used in many programs to save user data, settings, or logs so the information is not lost when the program closes.

💼 Career

Many software jobs require handling files for data storage, configuration, or communication between programs. Knowing how to read and write files is essential.

1

Create the list of fruits

Create a std::vector<std::string> called fruits with these exact entries: "Apple", "Banana", "Cherry", "Date", "Elderberry".

C++

#include 
#include 
#include 
#include 

int main() {

    // Create the vector fruits with the given fruit names
    // Your code here

    return 0;
}

Hint

Use curly braces {} to list the fruit names inside the vector.

2

Set the filename

Create a std::string variable called filename and set it to "fruits.txt".

C++

#include 
#include 
#include 
#include 

int main() {

    std::vector fruits = {"Apple", "Banana", "Cherry", "Date", "Elderberry"};

    // Create the filename string
    // Your code here

    return 0;
}

Hint

Use double quotes "" to set the filename string.

3

Write fruits to the file

Use std::ofstream to open the file with the name filename. Then use a for loop with variable fruit to write each fruit from fruits to the file, each on its own line. Close the file after writing.

C++

#include 
#include 
#include 
#include 

int main() {

    std::vector fruits = {"Apple", "Banana", "Cherry", "Date", "Elderberry"};

    std::string filename = "fruits.txt";

    // Open the file for writing and write each fruit
    // Your code here

    return 0;
}

Hint

Use std::ofstream to open the file and the insertion operator << to write each fruit followed by a newline.

4

Read fruits from the file and print

Use std::ifstream to open the file with the name filename. Use a std::string variable called line. Use a while loop with std::getline to read each line from the file into line. Inside the loop, print the line using std::cout. Close the file after reading.

C++

#include 
#include 
#include 
#include 

int main() {

    std::vector fruits = {"Apple", "Banana", "Cherry", "Date", "Elderberry"};

    std::string filename = "fruits.txt";

    std::ofstream outFile(filename);
    for (const std::string& fruit : fruits) {
        outFile << fruit << "\n";
    }
    outFile.close();

    // Open the file for reading and print each line
    // Your code here

    return 0;
}

Hint

Use std::getline inside a while loop to read each line and print 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