computer science

Identity transform

Identity transform 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 Identity transform rather than just read about it. In short: The identity transform is a data transformation that copies the source data into the destination data without change. The identity transformation is considered an essential process in creating a reusable transformation library.

Key takeaways

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

Reference excerpt

The identity transform is a data transformation that copies the source data into the destination data without change. The identity transformation is considered an essential process in creating a reusable transformation library. By creating a library of variations of the base identity transformation, a variety of data transformation filters can be easily maintained. These filters can be chained together in a format similar to UNIX shell pipes.

Examples of recursive transforms The "copy with recursion" permits, changing little portions of code, produce entire new and different output, filtering or updating the input. Understanding the "identity by recursion" we can understand the filters.

Using XSLT The most frequently cited example of the identity transform (for XSLT version 1.0) is the "copy.xsl" transform as expressed in XSLT. This transformation uses the xsl:copy command to perform the identity transformation:

This template works by matching all attributes (@*) and other nodes (node()), copying each node matched, then applying the identity transformation to all attributes and child nodes of the context node. This recursively descends the element tree and outputs all structures in the same structure they were found in the original file, within the limitations of what information is considered significant in the XPath data model. Since node() matches text, processing instructions, root, and comments, as well as elements, all XML nodes are copied. A more explicit version of the identity transform is:

This version is equivalent to the first, but explicitly enumerates the types of XML nodes that it will copy. Both versions copy data that is unnecessary for most XML usage (e.g., comments).

XSLT 3.0 XSLT 3.0 specifies an on-no-match attribute of the xsl:mode instruction that allows the identity transform to be declared rather than implemented as an explicit template rule. Specifically:

is essentially equivalent to the earlier template rules. See the XSLT 3.0 standard's description of shallow-copy for details. Finally, note that markup details, such as the use of CDATA sections or the order of attributes, are not necessarily preserved in the output, since this information is not part of the XPath data model. To show CDATA markup in the output, the XSLT stylesheet that contains the identity transform template (not the identity transform template itself) should make use of the xsl:output attribute called cdata-section-elements. cdata-section-elements specifies a list of the names of elements whose text node children should be output using CDATA sections.

For example:

Using XQuery XQuery can define recursive functions. The following example XQuery function copies the input directly to the output without modification.

The same function can also be achieved using a typeswitch-style transform.

The typeswitch transform is sometime preferable since it can easily be modified by simply adding a case statement for any element that needs special processing.

Non-recursive transforms Two simple and illustrative "copy all" transforms.

Using XSLT

Using XProc

Here one important note about the XProc identity, is that it can take either one document like this example or a sequence of document as input.

More complex examples Generally the identity transform is used as a base on which one can make local modifications.

Remove named element transform

Using XSLT The identity transformation can be modified to copy everything from an input tree to an output tree except a given node. For example, the following will copy everything from the input to the output except the social security number:

Using XQuery

To call this one would add:

Using XProc

Worked examples

Example 1 — a first encounter with Identity transform

Start with the simplest possible case. Write down what Identity transform 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 Identity transform 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 Identity transform 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 Identity transform

In research: Identity transform 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 Identity transform 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: Identity transform is common in secondary-school and first-year university syllabi. It links to neighbouring topics Computer programming, Transforms, so understanding it makes those chapters shorter.
In everyday life: Look for Identity transform 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.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Identity transform” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Identity transform in 20 minutes

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

Frequently asked questions

What is Identity transform in simple terms?

Why does Identity transform 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 Identity transform?

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 Identity transform.