String interpolation: Difference between revisions

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{{short description|Replacing placeholders in a string with values}}

In [[computer programming]], '''string interpolation''' (or '''variable interpolation''', '''variable substitution''', or '''variable expansion''') is the process of evaluating a [[string literal]] containing one or more [[Form (document)#Placeholders|placeholders]], yielding a result in which the placeholders are replaced with their corresponding values. It is a form of simple [[Template processor|template processing]]<ref>"[httphttps://www.cs.usfca.edu/~parrt/papers/mvc.templates.pdf Enforcing Strict Model-View Separation in Template Engines]", T. Parr (2004), WWW2004 conference.</ref> or, in formal terms, a form of [[quasi-quotation]] (or logic [[Substitution (logic)|substitution]] interpretation). The placeholder may be a variable name, or in some languages an arbitrary expression, in either case evaluated in the current [[Scope (computer science)|context]].

String interpolation is an alternative to building string via [[concatenation]], which requires repeat quoting and unquoting;<ref>{{Cite web|url=http://perlmeme.org/howtos/using_perl/interpolation.html|title = Interpolation in Perl |date = 12 December 2024 |quote="This is much tidier than repeat uses of the '.' concatenation operator."}}</ref> or substituting into a [[printf format string]], where the variable is far from where it is used. Compare:

<syntaxhighlight lang="ruby">

apples = 4

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=== Go ===

{{Main|Go (programming language)}}

{{as of|2025}}, Go does not have string interpolation. There have been some proposals for string interpolation, which have been rejected.<ref>{{cite web|url=https://github.com/golang/go/issues/34174 |title=proposal: Go 2: string interpolation #34174|website=[[GitHub]] }}</ref><ref>{{cite web|url=https://github.com/golang/go/issues/50554 |title=proposal: Go 2: string interpolation evaluating to string and list of expressions #50554|website=[[GitHub]] }}</ref><ref>{{Cite web |title=proposal: spec: add simple string interpolation similar to Swift · Issue #57616 · golang/go |url=https://github.com/golang/go/issues/57616 |access-date=2025-05-19 |website=GitHub |language=en}}</ref>

=== Groovy ===

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</syntaxhighlight>

They were removed in Java 23 due to design issues.<ref>{{cite web | title=Significant Changes in the JDK | url=https://docs.oracle.com/en/java/javase/23/migrate/significant-changes-jdk-release.html }}</ref>

=== JavaScript ===

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|title= The Python Tutorial: 7.1.1. Formatted String Literals}}</ref><ref>{{Cite web |url=https://docs.python.org/3/reference/lexical_analysis.html#f-strings |title=The Python Language Reference: 2.4.3. Formatted string literals}}</ref><ref>{{Cite web|url=https://www.python.org/dev/peps/pep-0498/|title = PEP 498 -- Literal String Interpolation}}</ref> Such a literal begins with an <code>f</code> or <code>F</code> before the opening quote, and uses braces for placeholders:

<syntaxhighlight lang="python">

applesnum_apples = 4

bananasnum_bananas = 3

print(f'I have {applesnum_apples} apples and {bananasnum_bananas} bananas')

</syntaxhighlight>

The output will be: