Read from File

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

int main() {
    // Open file for reading
    ifstream inFile("data.txt");
    
    if (inFile.is_open()) {
        string line;
        
        cout << "Reading file line by line:" << endl;
        while (getline(inFile, line)) {
            cout << line << endl;
        }
        
        inFile.close();
    } else {
        cout << "Error: Unable to open file for reading." << endl;
    }
    
    // Alternative: Read word by word
    ifstream inFile2("data.txt");
    if (inFile2.is_open()) {
        string word;
        cout << "\nReading file word by word:" << endl;
        while (inFile2 >> word) {
            cout << word << " ";
        }
        cout << endl;
        inFile2.close();
    }
    
    return 0;
}

Reading file line by line:
Hello, World!
This is a C++ file handling program.
Line 3: Learning file operations.
100 200 300

Reading file word by word:
Hello, World! This is a C++ file handling program. Line 3: Learning file operations. 100 200 300

This program teaches you how to Read from a File in C++. Reading from files uses ifstream (input file stream) to read data from existing files. File reading is essential for loading data, processing files, and reading configuration.

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1. What This Program Does

The program demonstrates file reading operations:

• Opening file for reading
• Reading line by line using getline()
• Reading word by word using >> operator
• Checking if file opened successfully
• Closing file after reading

File reading enables data loading and file processing.

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2. Header Files Used

1. #include <iostream>

   • Provides cout and cin for input/output operations.

2. #include <fstream>

   • Provides file stream classes (ofstream, ifstream).

3. #include <string>

   • Provides string class for string operations.

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3. Understanding File Reading

ifstream Concept:

• Input file stream
• Used for reading from files
• Similar to cin but reads from file
• File must exist to read

Reading Methods:

• getline(): reads entire line

• operator: reads word by word

• read(): reads binary data

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4. Opening File for Reading

Creating ifstream:

ifstream inFile("data.txt");

How it works:

• Creates ifstream object
• Opens file "data.txt" for reading
• File must exist
• Returns error if file not found

Checking Success:

if (inFile.is_open()) { // File opened successfully }

---

5. Reading Line by Line

Using getline():

string line; while (getline(inFile, line)) { cout << line << endl; }

How it works:

• getline() reads until newline
• Returns false at end of file
• Preserves line structure
• Reads entire line including spaces

---

6. Reading Word by Word

Using >> Operator:

string word; while (inFile >> word) { cout << word << " "; }

How it works:

• operator reads until whitespace

• Stops at space, tab, newline
• Skips whitespace automatically
• Returns false at end of file

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7. When to Use Each Method

getline():

• Preserve line structure
• Read formatted lines
• Process line by line
• Keep spaces in line

>> Operator:

• Read individual words
• Skip whitespace
• Parse space-separated data
• Process tokens

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8. Important Considerations

File Existence:

• File must exist to read
• Check is_open() before reading
• Handle file not found errors
• Provide user feedback

End of File:

• Loop until end of file
• getline() and >> return false at EOF
• File pointer moves automatically
• Don't read past EOF

File Paths:

• Relative or absolute paths
• Check file permissions
• Handle access errors

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9. return 0;

This ends the program successfully.

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Summary

• File reading: uses ifstream (input file stream) to read from files.
• getline(): reads entire line, preserves line structure and spaces.

• operator: reads word by word, stops at whitespace.

• Always check is_open() before reading, file must exist.
• File pointer moves automatically as you read.
• Understanding file reading enables data loading and file processing.
• Essential for reading configuration, data files, and user input files.

This program is fundamental for learning file operations, understanding data loading, and preparing for file-based data processing in C++ programs.