File input using ifstream in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a file named "data.txt" for reading.

C++

#include <fstream>
#include <iostream>

int main() {
    std::ifstream file;
    file.[1]("data.txt");
    if (!file) {
        std::cout << "Failed to open file." << std::endl;
    }
    return 0;
}

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

Awrite

Bopen

Cread

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

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

Trying to use 'read' or 'write' directly to open the file.

The open function is used to open a file with ifstream.

2fill in blank

medium

Complete the code to read a single word from the file into the variable 'word'.

C++

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

int main() {
    std::ifstream file("data.txt");
    std::string word;
    if (file >> [1]) {
        std::cout << "Read word: " << word << std::endl;
    }
    return 0;
}

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

Adata

Binput

Cfile

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to read into 'input' or 'file' which are not variables for storing the word.

Using 'data' which is not declared.

We read into the variable word using the extraction operator.

3fill in blank

hard

Fix the error in the code to properly check if the file was opened successfully.

C++

#include <fstream>
#include <iostream>

int main() {
    std::ifstream file("data.txt");
    if ([1]) {
        std::cout << "File opened successfully." << std::endl;
    } else {
        std::cout << "Failed to open file." << std::endl;
    }
    return 0;
}

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

Afile.is_open()

Bfile

C!file

D!file.is_open()

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!file' which checks if the file failed to open, but logic is reversed.

Using '!file.is_open()' which is true when file is not open.

The is_open() method returns true if the file is open.

4fill in blank

hard

Fill both blanks to read all words from the file and print them.

C++

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

int main() {
    std::ifstream file("data.txt");
    std::string [1];
    while (file >> [2]) {
        std::cout << [2] << std::endl;
    }
    return 0;
}

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

Aword

Bfile

Cinput

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for declaration and reading.

Using 'file' as a variable to store words.

The variable word is used to store each word read from the file.

5fill in blank

hard

Fill all three blanks to open a file, read integers, and sum them.

C++

#include <fstream>
#include <iostream>

int main() {
    std::ifstream [1]("numbers.txt");
    int [2], sum = 0;
    while ([1] >> [2]) {
        sum += [2];
    }
    std::cout << "Sum: " << sum << std::endl;
    return 0;
}

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

Afile

Bnum

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the file stream in declaration and reading.

Using 'number' instead of 'num' for the integer variable.

The ifstream object is named file, and the integer variable is num. The same file is used to read integers.

Practice

1. What is the primary purpose of 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 and is used to read data from files.

2. Step 2: Differentiate from other file streams

   ofstream is used for writing, not reading. Creating or deleting files is not handled by ifstream.

3. Final Answer:

   To read data from a file -> Option B

4. Quick Check:

   ifstream = read file [OK]

Hint: Remember: ifstream = input (read) file stream [OK]

Common Mistakes:

• Confusing ifstream with ofstream
• Thinking ifstream writes to files
• Assuming ifstream creates files

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

easy

A. ifstream file = open("data.txt");

B. std::ifstream file.open("data.txt");

C. std::ifstream file("data.txt");

D. ifstream file("data.txt").open();

Solution

1. Step 1: Recall correct syntax for opening files with ifstream

   The correct way is to create an ifstream object and pass the filename as a constructor argument: std::ifstream file("data.txt");

2. Step 2: Identify incorrect syntax in other options

   std::ifstream file.open("data.txt"); incorrectly uses dot notation with constructor. ifstream file = open("data.txt"); uses a non-existent open function. ifstream file("data.txt").open(); tries to call open() after constructor incorrectly.

3. Final Answer:

   std::ifstream file("data.txt"); -> Option C

4. Quick Check:

   Constructor with filename = correct open [OK]

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

Common Mistakes:

• Calling open() on constructor line
• Using open() as a free function
• Mixing syntax of ifstream and other file functions

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 20

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. The file contains 10 and 20 on separate lines, so a=10 and b=20.

2. Step 2: Calculate the output of the sum

   The code outputs a + b, which is 10 + 20 = 30.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   10 + 20 = 30 [OK]

Hint: >> reads values; sum them for output [OK]

Common Mistakes:

• Thinking >> concatenates numbers as strings
• Expecting output as '10 20' instead of sum
• Forgetting to close the file

4. Identify the error in the following code snippet:

std::ifstream file;
file.open("data.txt");
int x;
file >> x;
file.close();

medium

A. File is not opened before reading

B. File should not be closed manually

C. Incorrect syntax for reading integer

D. Missing check if file opened successfully

Solution

1. Step 1: Check file opening and reading steps

   The code opens the file and reads an integer correctly. Syntax is valid.

2. Step 2: Identify missing safety check

   There is no check to confirm if the file opened successfully before reading. This can cause runtime errors if the file doesn't exist.

3. Final Answer:

   Missing check if file opened successfully -> Option D

4. Quick Check:

   Always check file.open() success before reading [OK]

Hint: Always verify file opened before reading [OK]

Common Mistakes:

• Ignoring file open failure
• Assuming file always exists
• Not closing the file (though here it is closed)

5. You want to read all integers from a file values.txt until the end of the file. Which code snippet correctly does this using ifstream?

hard

A. std::ifstream file("values.txt");
int num;
while (file >> num) {
std::cout << num << std::endl;
}
file.close();

B. std::ifstream file("values.txt");
int num;
while (!file.eof()) {
file >> num;
std::cout << num << std::endl;
}
file.close();

C. std::ifstream file("values.txt");
int num;
do {
file >> num;
std::cout << num << std::endl;
} while (!file.fail());
file.close();

D. std::ifstream file("values.txt");
int num;
while (file.good()) {
file >> num;
std::cout << num << std::endl;
}
file.close();

Solution

1. Step 1: Understand correct loop condition for reading

   Using while (file >> num) reads integers and stops when reading fails (end of file or error).

2. Step 2: Analyze why other options are incorrect

   std::ifstream file("values.txt"); int num; while (!file.eof()) { file >> num; std::cout << num << std::endl; } file.close(); uses while (!file.eof()) which can cause reading invalid data after EOF. std::ifstream file("values.txt"); int num; do { file >> num; std::cout << num << std::endl; } while (!file.fail()); file.close(); uses do-while with fail check but prints invalid data after failure. std::ifstream file("values.txt"); int num; while (file.good()) { file >> num; std::cout << num << std::endl; } file.close(); uses file.good() which may not detect EOF correctly before reading.

3. Final Answer:

   std::ifstream file("values.txt"); int num; while (file >> num) { std::cout << num << std::endl; } file.close(); -> Option A

4. Quick Check:

   Use while(file >> var) to read until EOF [OK]

Hint: Use while(file >> var) to read safely till EOF [OK]

Common Mistakes:

• Using while(!file.eof()) causing extra reads
• Printing data after failed read
• Using file.good() incorrectly for loop condition