string interpolation

There are two main types of variable-expanding algorithms for variable interpolation:[https://code.google.com/p/smallest-template-system/wiki/SimplestAlgorithm "smallest-template-system/Simplest algorithms"], an online tutorial for placeholder-template-systems.

1. Replace and expand placeholders: creating a new string from the original one, by find–replace operations. Find variable reference (placeholder), replace it by its variable value. This algorithm offers no cache strategy.
2. Split and join string: splitting the string into an array, merging it with the corresponding array of values, then joining items by concatenation. The split string can be cached for reuse.

String interpolation, like string concatenation, may lead to security problems. If user input data is improperly escaped or filtered, the system will be exposed to SQL injection, script injection, XML external entity (XXE) injection, and cross-site scripting (XSS) attacks.{{cite web |url=http://google-caja.googlecode.com/svn/changes/mikesamuel/string-interpolation-29-Jan-2008/trunk/src/js/com/google/caja/interp/index.html#-autogen-id-1 |title= Secure String Interpolation|website=google-caja.googlecode.com |archive-url=https://web.archive.org/web/20121019065315/http://google-caja.googlecode.com/svn/changes/mikesamuel/string-interpolation-29-Jan-2008/trunk/src/js/com/google/caja/interp/index.html#-autogen-id-1 |archive-date=2012-10-19}}

An SQL injection example:

query = "{{code|2=sql|1=SELECT x, y, z FROM Table WHERE id='$id'}} "

If $id is replaced with "'; {{code|2=sql|1=DELETE FROM Table; SELECT * FROM Table WHERE id='}}", executing this query will wipe out all the data in Table.

{{Main|ABAP}}

DATA(apples) = 4.

WRITE |I have { apples } apples|.

The output will be:

I have 4 apples

{{Main|Bash (Unix shell)}}

apples=4

echo "I have $apples apples"

1. or

echo "I have ${apples} apples"

The output will be:

I have 4 apples

{{Main|Boo (programming language)}}

apples = 4

print("I have $(apples) apples")

1. or

print("I have {0} apples" % apples)

The output will be:

I have 4 apples

{{Main|C Sharp (programming language)}}

var apples = 4;

var bananas = 3;

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

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

{{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 }}

The output will be:

I have 4 apples

I have 7 fruits

{{Main article|ColdFusion Markup Language}}

ColdFusion Markup Language (CFML) script syntax:

apples = 4;

writeOutput("I have #apples# apples");

Tag syntax:

I have #apples# apples

The output will be:

{{samp|I have 4 apples}}

{{Main|CoffeeScript}}

apples = 4

console.log "I have #{apples} apples"

The output will be:

I have 4 apples

{{Main|Dart (programming language)}}

int apples = 4, bananas = 3;

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

print('I have ${apples+bananas} fruits.');

The output will be:

I have 4 apples.

I have 7 fruits.

{{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.{{cite web|url=https://github.com/golang/go/issues/34174 |title=proposal: Go 2: string interpolation #34174|website=GitHub }}{{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 }}{{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}}

{{Main|Groovy (programming language)}}

In groovy, interpolated strings are known as GStrings:{{Cite web|title=The Apache Groovy programming language - Syntax|url=https://groovy-lang.org/syntax.html#_string_interpolation|access-date=2021-06-20|website=groovy-lang.org}}

def quality = "superhero"

final age = 52

def sentence = "A developer is a $quality if he is ${age <= 42 ? 'young' : 'seasoned'}"

println sentence

The output will be:

A developer is a superhero if he is seasoned

{{Main|Haxe}}

var apples = 4;

var bananas = 3;

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

trace('I have ${apples+bananas} fruits.');

The output will be:{{Cite news|url=https://haxe.org/manual/lf-string-interpolation.html|title=Haxe - Manual - String interpolation|work=Haxe - The Cross-platform Toolkit|access-date=2017-09-12}}

I have 4 apples.

I have 7 fruits.

{{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.

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()} ..."

They were removed in Java 23 due to design issues.https://docs.oracle.com/en/java/javase/23/migrate/significant-changes-jdk-release.html

{{Main article|JavaScript}}

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

const apples = 4;

const bananas = 3;

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

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

The output will be:

I have 4 apples

I have 7 fruits

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

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

This is the second line of text.`);

The output will be:

This is the first line of text.

This is the second line of text.

{{Main|Julia (programming language)}}

apples = 4

bananas = 3

print("I have $apples apples and $bananas bananas, making $(apples + bananas) pieces of fruit in total.")

The output will be:

I have 4 apples and 3 bananas, making 7 pieces of fruit in total.

{{Main|Kotlin (programming language)}}

val quality = "superhero"

val apples = 4

val bananas = 3

val sentence = "A developer is a $quality. I have ${apples + bananas} fruits"

println(sentence)

The output will be:

A developer is a superhero. I have 7 fruits

{{Main|Nemerle}}

def apples = 4;

def bananas = 3;

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

Console.WriteLine($"I have $(apples + bananas) fruit.");

It also supports advanced formatting features, such as:

def fruit = ["apple", "banana"];

Console.WriteLine($<#I have ..$(fruit; "\n"; f => f + "s")#>);

The output will be:

apples

bananas

{{Main|Nim (programming language)}}

Nim provides string interpolation via the strutils module.

Formatted string literals inspired by Python F-string are provided via the strformat module,

the strformat macro verifies that the format string is well-formed and well-typed,

and then are expanded into Nim source code at compile-time.

import strutils, strformat

var apples = 4

var bananas = 3

echo "I have $1 apples".format(apples)

echo fmt"I have {apples} apples"

echo fmt"I have {apples + bananas} fruits"

1. Multi-line

echo fmt"""

I have

{apples} apples"""

1. Debug the formatting

echo fmt"I have {apples=} apples"

1. Custom openChar and closeChar characters

echo fmt("I have (apples) {apples}", '(', ')')

1. Backslash inside the formatted string literal

echo fmt"""{ "yep\nope" }"""

The output will be:

I have 4 apples

I have 4 apples

I have 7 fruits

I have

4 apples

I have apples=4 apples

I have 4 {apples}

yep

ope

{{Main|Nix package manager}}

let numberOfApples = "4";

in "I have ${numberOfApples} apples"

The output will be:

I have 4 apples

{{Main|ParaSail (programming language)}}

const Apples := 4

const Bananas := 3

Println ("I have `(Apples) apples.\n")

Println ("I have `(Apples+Bananas) fruits.\n")

The output will be:

I have 4 apples.

I have 7 fruits.

{{Main|Perl}}

my $apples = 4;

my $bananas = 3;

print "I have $apples apples.\n";

print "I have @{[$apples+$bananas]} fruit.\n"; # Uses the Perl array (@) interpolation.

The output will be:

I have 4 apples.

I have 7 fruit.

{{Main|PHP}}

$apples = 5;

$bananas = 3;

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

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

The output will be:

There are 5 apples and 3 bananas.

I have 5 apples and 3 bananas.

{{Main|Python (programming language)}}

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

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

|title= The Python Tutorial: 7.1.1. Formatted String Literals}}{{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}}{{Cite web|url=https://www.python.org/dev/peps/pep-0498/|title = PEP 498 -- Literal String Interpolation}} Such a literal begins with an f or F before the opening quote, and uses braces for placeholders:

num_apples = 4

num_bananas = 3

print(f'I have {num_apples} apples and {num_bananas} bananas')

The output will be:

I have 4 apples and 3 bananas

{{Main|Ruby (programming language)}}

{{Main|Crystal (programming language)}}

apples = 4

puts "I have #{apples} apples"

1. Format string applications for comparison:

puts "I have %s apples" % apples

puts "I have %{a} apples" % {a: apples}

The output will be:

I have 4 apples

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

Rust does not have general string interpolation, but provides similar functionality via macros, referred to as "Captured identifiers in format strings", introduced in version 1.58.0, released 2022-01-13.{{cite web |url=https://blog.rust-lang.org/2022/01/13/Rust-1.58.0.html#captured-identifiers-in-format-strings |title=Announcing Rust 1.58.0: Captured identifiers in format strings |date=2022-01-13}}

Rust provides formatting via the [https://doc.rust-lang.org/stable/std/fmt/ std::fmt] module, which is interfaced with through various macros such as [https://doc.rust-lang.org/stable/std/macro.format.html format!], [https://doc.rust-lang.org/stable/std/macro.write.html write!], and [https://doc.rust-lang.org/stable/std/macro.print.html print!]. These macros are converted into Rust source code at compile-time, whereby each argument interacts with a [https://doc.rust-lang.org/nightly/std/fmt/struct.Formatter.html formatter]. The formatter supports [https://doc.rust-lang.org/stable/std/fmt/#positional-parameters positional parameters], [https://doc.rust-lang.org/stable/std/fmt/#named-parameters named parameters], [https://doc.rust-lang.org/stable/std/fmt/#argument-types argument types], defining various [https://doc.rust-lang.org/stable/std/fmt/#formatting-traits formatting traits], and capturing identifiers from the environment.

let (apples, bananas) = (4, 3);

// println! captures the identifiers when formatting: the string itself isn't interpolated by Rust.

println!("There are {apples} apples and {bananas} bananas.");

The output will be:

There are 4 apples and 3 bananas.

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

Scala 2.10+ provides a general facility to allow arbitrary processing of a string literal, and supports string interpolation using the included s and f string interpolators. It is also possible to write custom ones or override the standard ones.

The f 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.

Scala 2.10+'s string interpolation allows embedding variable references directly in processed string literals. Here is an example:

val apples = 4

val bananas = 3

//before Scala 2.10

printf("I have %d apples\n", apples)

println("I have %d apples" format apples)

//Scala 2.10+

println(s"I have $apples apples")

println(s"I have ${apples + bananas} fruits")

println(f"I have $apples%d apples")

The output will be:I have 4 apples

In Sciter any function with name starting from $ is considered as interpolating function and so interpolation is customizable and context sensitive:

var apples = 4

var bananas = 3

var domElement = ...;

domElement.$content(

I have {apples} apples

domElement.$append(

I have {apples + bananas} fruits

Where domElement.$content(

I have {apples} apples

domElement.html = "

I have " + apples.toHtmlString() + " apples

{{Main article|SNOBOL}}

apples = 4 ; bananas = 3

Output = "I have " apples " apples."

Output = "I have " (apples + bananas) " fruits."

The output will be:

I have 4 apples.

I have 7 fruits.

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

In Swift, a new String value can be created from a mix of constants, variables, literals, and expressions by including their values inside a string literal.{{Cite web|title=Strings and Characters — The Swift Programming Language (Swift 5.5)|url=https://docs.swift.org/swift-book/LanguageGuide/StringsAndCharacters.html|access-date=2021-06-20|website=docs.swift.org}} Each item inserted into the string literal is wrapped in a pair of parentheses, prefixed by a backslash.

let apples = 4

print("I have \(apples) apples")

The output will be:

I have 4 apples

{{Main article|Tcl}}

The Tool Command Language has always supported string interpolation in all quote-delimited strings.

set apples 4

puts "I have $apples apples."

The output will be:

I have 4 apples.

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

set apples 4

puts [format "I have %d apples." $apples]

{{Main article|TypeScript}}

As of version 1.4, TypeScript supports string interpolation using backticks ``. Here is an example:

var apples: number = 4;

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

The output will be:

I have 4 apples

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

var apples: number = 4;

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

The output remains the same.

As of Visual Basic 14, string interpolation is supported in Visual Basic.{{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}}

name = "Tom"

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

The output will be:

{{sxhl|Hello, Tom|output}}

• Concatenation
• Improper input validation
• printf format string
• Quasi-quotation
• String literal
• Substitution

{{Reflist}}

Category:Programming constructs

Interpolation

Category:Variable (computer science)