19. Sequential Access Files

Data is read or written in order, from beginning to end — you cannot jump directly to the middle without reading through everything before it.

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

int main()
{
    ofstream fout("data.txt");
    fout << "Hello CSS aspirant" << endl;
    fout.close();

    ifstream fin("data.txt");
    string line;
    getline(fin, line);
    cout << "Read back: " << line << endl;
    fin.close();
    return 0;
}

Best suited for logs, simple text files, or any case where records are always processed start-to-end.

The core constraint: order matters

Sequential access means you always start from the beginning and read forward — there's no way to jump to record 500 without passing through records 1–499 first.

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

int main()
{
    // Writing multiple lines sequentially
    ofstream fout("students.txt");
    fout << "Ali,85" << endl;
    fout << "Sara,90" << endl;
    fout << "Bilal,78" << endl;
    fout.close();

    // Reading them back, line by line, IN ORDER
    ifstream fin("students.txt");
    string line;
    while (getline(fin, line))       // reads one line at a time, sequentially
    {
        cout << line << endl;
    }
    fin.close();

    return 0;
}

Output: all three lines, in the exact order they were written — you cannot ask for just "Bilal's line" without scanning through Ali's and Sara's first.

When sequential access is the right choice

• Log files (always appended, always read start-to-end)
• Simple configuration files
• Any case where you process every record anyway, so jumping around wouldn't help

Contrast, briefly

If a program needs to update just record #47 out of 10,000 without touching the rest, sequential access would force reading through 46 records first — that's when Random Access Files (with seekg/seekp) become necessary instead.

Important MCQs

1. In sequential file access, to read record #50 you must:
   • a) Jump directly using seekg() b) Read through records 1–49 first, in order c) Use binary mode only d) Skip to it randomly
2. Sequential access is best suited for:
   • a) Frequent random updates b) Logs and files always processed start-to-end c) Databases needing instant lookup d) Only binary files
3. Which function is commonly used to read a sequential text file line by line?
   • a) seekg() b) getline() c) write() d) seekp()

Self-Test Questions

1. Why can't you jump directly to the 10th line of a sequential file without reading the first 9?
2. Give one real-world example where sequential access is clearly the right choice.
3. What C++ function is commonly used to read a sequential text file one line at a time?

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📌 Sample & Repeated FPSC Questions (2016–2026)

No FPSC Computer Science question in 2016–2026 isolates "sequential file access" in C++ as its own question. The closest past-paper coverage is the same 2020 Paper-I Q5(b) file-based system on the Files & Streams and Random Access Files pages — its "Find a student by CGPA" operation scans a file sequentially via fin.read() in a loop, though the question itself doesn't name or contrast the "sequential access" concept explicitly.