computer science

Trigram search

Trigram search 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 Trigram search rather than just read about it. In short: Trigram search is a method of searching for text when the exact syntax or spelling of the target object is not precisely known or when queries may be regular expressions. It finds objects which have the most matches against three-character substrings (i.e. trigrams) of the search terms, which are generally near matches.

Key takeaways

Trigram search 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 Trigram search to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Trigram search from memory before moving on to harder problems.

Reference excerpt

Trigram search is a method of searching for text when the exact syntax or spelling of the target object is not precisely known or when queries may be regular expressions. It finds objects which have the most matches against three-character substrings (i.e. trigrams) of the search terms, which are generally near matches. Two strings with many shared trigrams can be expected to be very similar. Trigrams also allow for efficiently creating search engine indexes for searches that are regular expressions or match the text inexactly. Indexes can significantly accelerate searches. A threshold for number of trigram matches can be specified as a cutoff point, after a result is unmatched. Using trigrams for accelerating searches is a technique used in some systems for code searching, in situations in which queries that are regular expressions may be useful, in search engines such as Elasticsearch, as well as in databases such as PostgreSQL.

Examples Consider the string "alice". The trigrams of the string would be "ali", "lic", and "ice", not including spaces. Searching for this string in a database with a trigram-based index would involve finding which objects contain as many of the three trigrams as possible. As a concrete example of using trigram search to search for a regular expression query, consider searching for the string ab[cd]e, where the brackets denote that the third character in the string being searched for could be c or d. In this situation, one could query the index for objects that have the two trigrams abc and bce or the two trigrams abd and bde. Thus, finding this query would involve no string matching, and could just query the index directly, which can be faster in practice.

See also Approximate string matching Google Code Search N-gram

References

Worked examples

Example 1 — a first encounter with Trigram search

Start with the simplest possible case. Write down what Trigram search 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 Trigram search 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 Trigram search 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 Trigram search

In research: Trigram search 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 Trigram search 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: Trigram search is common in secondary-school and first-year university syllabi. It links to neighbouring topics Search algorithms, String matching algorithms, so understanding it makes those chapters shorter.
In everyday life: Look for Trigram search 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 Trigram search in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Trigram search 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 Trigram search out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Trigram search in simple terms?

Why does Trigram search 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 Trigram search?

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 Trigram search.