Reading and writing files in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What header file is needed to read from and write to files in C++?

You need to include the <fstream> header to work with file streams in C++.

beginner

How do you open a file for writing in C++?

Create an std::ofstream object and pass the filename to its constructor or use the open() method. For example:
std::ofstream file("example.txt");

beginner

How do you check if a file was opened successfully in C++?

Use the is_open() method on the file stream object. For example:
if (file.is_open()) { /* file opened */ }

beginner

What is the difference between std::ifstream and std::ofstream?

std::ifstream is used to read from files, while std::ofstream is used to write to files.

beginner

How do you read a line of text from a file in C++?

Use std::getline() with an ifstream object and a string variable. Example:
std::string line;
std::getline(file, line);

Which C++ class is used to write data to a file?

A std::ofstream

B std::ifstream

C std::fstream

D std::iostream

std::ofstream is specifically designed for writing data to files.

How do you check if a file opened successfully in C++?

A Using the open() method

B Using the is_open() method

C Using the close() method

D Using the read() method

The is_open() method returns true if the file stream is successfully opened.

Which function reads a whole line from a file into a string?

A std::getline()

B file.read()

C file.get()

D std::readline()

std::getline() reads a line from the input stream into a string.

What header must be included to use file streams in C++?

A <fstream.h>

B <iostream>

C <fstream>

D <file>

is the standard header for file stream classes.

Which class can be used for both reading and writing files?

A std::iostream

B std::ifstream

C std::ofstream

D std::fstream

std::fstream supports both input and output operations on files.

Explain how to open a file for reading and check if it opened successfully in C++.

Think about the steps from including the right header to checking the file state.

Describe how to write text to a file and then close it properly in C++.

Remember the sequence: open, write, close.

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