File input using ifstream in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Reading integers from a file

What is the output of this C++ program if the file numbers.txt contains the numbers 10 20 30?

C++

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ifstream file("numbers.txt");
    int sum = 0, x;
    while (file >> x) {
        sum += x;
    }
    cout << sum << endl;
    return 0;
}

A 0

B 102030

C 10 20 30

D 60

💡 Hint

The program reads integers one by one and adds them.

The program reads each integer from the file and adds it to sum. The numbers are 10, 20, and 30, so the sum is 60.

❓ Predict Output

intermediate

Reading lines from a file

What will this program print if data.txt contains:

Hello
World
C++

C++

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main() {
    ifstream file("data.txt");
    string line;
    int count = 0;
    while (getline(file, line)) {
        count++;
    }
    cout << count << endl;
    return 0;
}

A 3

B Hello World C++

C 0

D 1

💡 Hint

Count how many lines the file has.

The program counts lines using getline. The file has 3 lines, so it prints 3.

🔧 Debug

advanced

Why does this file reading loop fail?

This code tries to read integers from input.txt and print them. Why does it print the last number twice?

C++

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ifstream file("input.txt");
    int x;
    while (!file.eof()) {
        file >> x;
        cout << x << endl;
    }
    return 0;
}

A Because cout prints the buffer twice

B Because checking eof() before reading causes the last read to fail but still prints the old value

C Because the variable x is not initialized

D Because the file is empty

💡 Hint

Check how eof() works in file streams.

The eof() flag is set only after a failed read. Checking eof() before reading causes the loop to run one extra time, printing the last value again.

📝 Syntax

advanced

Identify the syntax error in file reading

Which option contains the correct syntax to open a file and read a single integer?

A ifstream file; file.open = "data.txt"; int x; file >> x;

B ifstream file = "data.txt"; int x; file >> x;

C ifstream file("data.txt"); int x; file >> x;

D ifstream file("data.txt"); int x; file.read(x);

💡 Hint

Check how ifstream is constructed and how to read data.

Option C correctly constructs ifstream with filename and uses >> operator to read an integer. Others have syntax errors or wrong methods.

🚀 Application

expert

Count words in a file

Which program correctly counts the number of words in text.txt?

A

#include <fstream>
#include <string>
#include <iostream>
using namespace std;
int main() {
    ifstream file("text.txt");
    string word;
    int count = 0;
    while (file >> word) {
        count++;
    }
    cout << count << endl;
    return 0;
}

B

#include <fstream>
#include <string>
#include <iostream>
using namespace std;
int main() {
    ifstream file("text.txt");
    string line;
    int count = 0;
    while (getline(file, line)) {
        count += line.size();
    }
    cout << count << endl;
    return 0;
}

C

#include <fstream>
#include <string>
#include <iostream>
using namespace std;
int main() {
    ifstream file("text.txt");
    char c;
    int count = 0;
    while (file.get(c)) {
        if (c == ' ') count++;
    }
    cout << count << endl;
    return 0;
}

D

#include <fstream>
#include <string>
#include <iostream>
using namespace std;
int main() {
    ifstream file("text.txt");
    int count = 0;
    while (!file.eof()) {
        string word;
        file >> word;
        count++;
    }
    cout << count << endl;
    return 0;
}

💡 Hint

Words are separated by whitespace and can be read with >> operator.

Option A reads words one by one using >> operator and counts them correctly. Option A counts characters, not words. Option A counts spaces, which may be less accurate. Option A uses eof() incorrectly causing an extra count.