computer science

Indexed file

Indexed file is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Indexed file rather than just read about it. In short: An indexed file is a computer file with an index that allows random access to any record given its key, data which identifies a record within a file. Many implementations of indexed files allow a file to have multiple indices.

Key takeaways

Indexed file belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Indexed file to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Indexed file from memory before moving on to harder problems.

Reference excerpt

An indexed file is a computer file with an index that allows random access to any record given its key, data which identifies a record within a file. Many implementations of indexed files allow a file to have multiple indices. The first or only index, the primary index, contains the primary key. If other secondary indices are present, they contain alternate keys. The primary key must uniquely identify a record. If more than one index is present the other ones are called alternate indexes. The indexes are created with the file and maintained by the system. IBM supports indexed files with the Indexed Sequential Access Method (ISAM) on OS/360 and successors. IBM virtual storage operating systems added VSAM, which supports indexed files as Key Sequenced Data Sets (KSDS), with more options. Support for indexed files is built into COBOL and PL/I. Other languages with more limited I/O facilities such as C support indexed files through add-on packages in a runtime library such as C-ISAM. Some of Digital's operating systems, such as OpenVMS, support indexed file I/O using the Record Management Services. In recent systems, relational databases are often used in place of indexed files.

Language support The COBOL languages supports indexed files with the following command in the FILE CONTROL section

ORGANIZATION IS INDEXED

IBM PL/I uses the file attribute ENVIRONMENT(INDEXED) or ENVIRONMENT(VSAM) to declare an indexed file.

See also B-trees Hash table Data set (IBM mainframe) Legacy system dbm also X/Open ndbm and GNU gdbm Berkeley DB

Inline citations

Worked examples

Example 1 — a first encounter with Indexed file

Start with the simplest possible case. Write down what Indexed file claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Indexed file before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Indexed file ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Indexed file

In research: Indexed file appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Indexed file in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Indexed file is common in secondary-school and first-year university syllabi. It links to neighbouring topics COBOL, Database index techniques, Database stubs, so understanding it makes those chapters shorter.
In everyday life: Look for Indexed file outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Indexed file in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Indexed file means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Indexed file out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Indexed file in simple terms?

Why does Indexed file matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Indexed file?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Indexed file.