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 repeatedrepeat 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 repeatedrepeat 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="pythonruby">

apples = 4

print(puts "I have $#{apples} apples.") # string interpolation

print(puts "I have " + String(apples) + " apples.") # string concatenation

print(puts "I have %sd apples.", % apples) # format string

</syntaxhighlight>

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Console.WriteLine($"I have {apples} apples");

Console.WriteLine($"I have {apples + bananas} fruits");

</syntaxhighlight><ref>{{Cite web|url=https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/strings/#string-interpolation|title = Strings - C# Programming Guide| date=15 March 2024 }}</ref>

The output will be:

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=== Dart ===

{{Main|Dart (programming language)}}

<syntaxhighlight lang="ecmascriptdart">

int apples = 4, bananas = 3;

print('I have $apples apples.');

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=== Go ===

{{Main|Go (programming language)}}

{{as of|20222025}}, Go does not have string interpolation. There have been some proposals for string interpolation, inwhich thehave nextbeen version of the language, Go 2rejected.<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 Instead,web Go|title=proposal: usesspec: [[printfadd formatsimple string]]s ininterpolation thesimilar <code>fmt.Sprintf</code>to function,Swift string· [[concatenation]],Issue or#57616 template· librariesgolang/go like|url=https://github.com/golang/go/issues/57616 |access-date=2025-05-19 |website=GitHub |language=en}}<code>text/template</coderef>.

=== Groovy ===

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=== Java ===

{{Main article|Java (programming language)}}

Java had interpolated strings as a preview feature in Java 21 and Java 22. You could use the constant STR of [https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/lang/StringTemplate.html java.lang.StringTemplate] directly.<syntaxhighlight lang="java" line="1">

{{as of|2022}}, Java does not have interpolated strings, and instead uses format functions, notably the <code>MessageFormat</code> class (Java version 1.1 and above) and the static method <code>String.format</code> (Java version 5 and above). [https://openjdk.org/jeps/430 There is a proposal] to add string interpolation to the Java language. This proposal aims at doing safe string interpolation, making it easier to prevent injection attacks.

enum Stage{test,qa,prod}

record Deploy(UUID image, Stage stage){}

var deploy=new Deploy(UUID.randomUUID(), Stage.test)

STR."Installing \{deploy.image()} on Stage \{deploy.stage()} ..."

var deploy=new Deploy(UUID.randomUUID(), Stage.prod)

STR."Installing \{deploy.image()} on Stage \{deploy.stage()} ..."

</syntaxhighlight lang="ecmascript">

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 ===

{{Main article|JavaScript}}

[[JavaScript]], as of the [[ECMAScript]] 2015 (ES6) standard, supports string interpolation using backticks <code>``</code>. This feature is called ''template literals''.<ref>{{Cite web|url=https://developer.mozilla.org/docs/Web/JavaScript/Reference/Template_literals|title = Template literals (Template strings) - JavaScript &#124; MDN| date=31 May 2024 }}</ref> Here is an example:

<syntaxhighlight lang="javascript">

[[JavaScript]], as of the [[ECMAScript]] 2015 (ES6) standard, supports string interpolation using backticks <code>``</code>. This feature is called ''template literals''.<ref>{{Cite web|url=https://developer.mozilla.org/docs/Web/JavaScript/Reference/Template_literals|title = Template literals (Template strings) - JavaScript &#124; MDN}}</ref> Here is an example:

<syntaxhighlight lang="ecmascript">

const apples = 4;

const bananas = 3;

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Template literals can also be used for multi-line strings:

<syntaxhighlight lang="ecmascriptjavascript">

console.log(`This is the first line of text.

This is the second line of text.`);

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$apples = 5;

$bananas = 3;

echo "There are $apples apples and $bananas bananas.\n";

echo "\n";

echo "I have {$apples} apples and {$bananas} bananas.";

</syntaxhighlight>The output will be:

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{{Main|Python (programming language)}}

Python supports string interpolation as of version 3.6, referred to as

"formatted string literals" or "f-strings".<ref>{{cite web |url=https://docs.python.org/3/tutorial/inputoutput.html#tut-f-strings

|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:

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</syntaxhighlight>

The output will be:

<syntaxhighlight lang="output">

There are 4 apples and 3 bananas.

</syntaxhighlight>

=== Scala ===

{{Main article|Scala (programming language)}}

[[Scala (programming language)|Scala]] 2.10+ hasprovides implementeda thegeneral followingfacility to allow arbitrary processing of a string interpolators:literal, and supports string interpolation using the included <code>s,</code> and <code>f</code> andstring rawinterpolators. It is also possible to write custom ones or override the standard ones.

The <code>f</code> interpolator is a compiler macro that rewrites a format string with embedded expressions as an invocation of String.format. It verifies that the format string is well-formed and well-typed.

==== The standard interpolators ====

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<syntaxhighlight lang="output">I have 4 apples.</syntaxhighlight>

In order to actually format -– and not simply replace -– the values, there is a formatting function.

<syntaxhighlight lang="tcl">

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As of version 1.4, [[TypeScript]] supports string interpolation using backticks <code>``</code>. Here is an example:

<syntaxhighlight lang="ecmascripttypescript">

var apples: number = 4;

console.log(`I have ${apples} apples`);

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<syntaxhighlight lang="output">I have 4 apples</syntaxhighlight>

The <code>console.log</code> function can be used as a <code>printf</code> function. The above example can be rewritten, thusly:

<syntaxhighlight lang="ecmascripttypescript">

var apples: number = 4;

console.log("I have %d apples", apples);

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The output remains the same.

=== Visual Basic .NET ===

As of Visual Basic 14, string interpolation is supported in Visual Basic.<ref>{{Cite web|last=KathleenDollard|title=Interpolated Strings - Visual Basic|url=https://docs.microsoft.com/en-us/dotnet/visual-basic/programming-guide/language-features/strings/interpolated-strings|access-date=2021-06-20|website=docs.microsoft.com|language=en-us}}</ref>

<syntaxhighlight lang="ecmascriptvbnet">

name = "Tom"

Console.WriteLine($"Hello, {name}")

</syntaxhighlight>

The output will be:

will print "Hello, Tom".

{{sxhl|Hello, Tom|output}}

== See also ==