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Computer Science · Class 12 · Binary Files
Binary filesSearching⏱️ 13 min read

Searching a Binary File

A binary file has no index and no way to jump to a record. Searching is therefore the reading loop with an if in it — and the half of the answer students lose marks for is what happens when nothing matches.

1There is no shortcut: you read from the start

The records sit one behind another. Nothing at the front of the file lists what is inside it, and there is no way to ask for “the record with roll number 2”. So a search is: read a record, check it, read the next.

search.py
import pickle

def search(roll):
    f = open('student.dat', 'rb')
    found = False

    try:
        while True:
            record = pickle.load(f)
            if record[0] == roll:
                print('Found:', record[1], 'scored', record[2])
                found = True
                break
    except EOFError:
        pass

    f.close()

    if found == False:
        print('No student with roll number', roll)

search(2)
search(9)
Output
Found: Meera scored 91
No student with roll number 9
found = False

Set BEFORE the loop. It remembers whether anything ever matched — the loop itself cannot tell you that after it has finished.

if record[0] == roll:

The test. record[0] is the roll number, because that is the position it was written in.

found = True

The flag is raised the moment something matches. This one line is what the last if depends on.

break

Roll numbers are unique, so there is no point reading the rest of the file. Leave it out if several records could match.

if found == False:

After the loop, and only now. Put this inside the loop and it prints 'not found' once for every record that is not the one you wanted.

🔍 Searching, one record at a time

Looking for roll number 3. Each load() brings back one record to test.

student.dat
[1, 'Ravi', 78]
[2, 'Meera', 91]
[3, 'Amit', 65]
[4, 'Neha', 88]
end of file
nothing read yet
—
the test, and the flag
record[0] == 3 → —found = False

The file is open and the pointer is at the start. Nothing has been read.

The flag is the whole exam question
“Search a record and display a suitable message if it is not found” is how the paper words it. Without found, a search for a roll number that is not there prints nothing at all — and a program that says nothing looks broken to whoever is using it.

2Searching for everything that matches

When several records could match — every student above 80, every book under 200 rupees — the shape barely changes. Take out the break, and let the loop see the whole file:

search_condition.py
import pickle

f = open('student.dat', 'rb')
found = False

try:
    while True:
        record = pickle.load(f)
        if record[2] > 80:
            print(record[1], 'scored', record[2])
            found = True
except EOFError:
    pass

f.close()

if found == False:
    print('Nobody scored more than 80.')
Output
Meera scored 91
With break — the first match

For a unique key such as a roll number or an admission number. Once you have found it, reading on is wasted work.

Without break — every match

For a condition several records can meet. The loop runs to the end of the file and prints each one as it comes.

3Asking the user what to look for

search_typed.py
import pickle

roll = int(input('Roll number to find: '))

f = open('student.dat', 'rb')
found = False

try:
    while True:
        record = pickle.load(f)
        if record[0] == roll:
            print('Roll   :', record[0])
            print('Name   :', record[1])
            print('Marks  :', record[2])
            found = True
            break
except EOFError:
    pass

f.close()

if found == False:
    print('Record not found.')
Output
Roll number to find: 3
Roll   : 3
Name   : Amit
Marks  : 65
int() on what was typed
input() gives a string, and the roll number in the record is a number. '3' == 3 is False, so without int() the search finds nothing, every time, with no error to explain why.

4Try it

The file is written first so the program is complete. Change the roll number, or search on the name instead:

searching.py

5Recap

A search is the reading loop with an if

No index, no jumping. Read from the start and test each record as it arrives.

found = False, before the loop

The flag that remembers whether anything ever matched.

The message goes after the loop

Inside it, 'not found' would print once per record that is not the one you want.

break for a unique key

Roll numbers do not repeat, so stop as soon as you have it.

No break for a condition

Several records may match; the loop should see all of them.

int() what the user typed

'3' never equals 3, and the search would quietly fail.

✍️ Now write these yourself
  1. 1

    Search for a student by name instead of roll number.

    Hint · record[1] == name — and no int() this time.

  2. 2

    Display every student who scored less than 70.

    Hint · The condition search, with no break.

  3. 3

    Count how many records match, instead of printing them.

    Hint · A counter instead of a flag — and 0 is its own “not found”.

  4. 4

    Take the found flag out and search for a roll number that is not in the file.

    Hint · The program prints nothing and looks broken. That is the point of the flag.

Quick Check

Why does a binary-file search have to read from the start?

Quick Check

Where does 'record not found' belong?

Quick Check

A search for roll number 3 finds nothing, even though record [3, 'Amit', 65] is in the file. What is the likely cause?

Quick Check

When should the search loop NOT use break?

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📖 Reading a Binary File
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Appending Records ➕