String interpolation

In computer programming, string interpolation or variable interpolation (also variable substitution or variable expansion) is the process of evaluating a string literal containing one or more placeholders, yielding a result in which the placeholders are replaced with their corresponding values. It is a form of simple template processing.^[1] or, in formal terms, a form of Quasi-quotation (or logic substitution interpretation). String interpolation allows for easier and more intuitive string formatting and content-specification compared with string concatenation.^[2]

String interpolation is common in many programming languages which make heavy use of string representations of data, such as C, Perl, PHP, Python, Ruby, Groovy, Scala and Swift, and most Unix shells. Two modes of literal expression are usually offered: one with interpolation enabled, the other without ("raw string"). Placeholders are usually represented by a bare or a named sigil, (typically $ or %), e.g. $placeholder or %123. Expansion of the string usually occurs at run time.

Variations

Some languages do not offer string interpolation, instead offering a standard function where one parameter is the printf format string, and other(s) provide the values for each placeholder.

Ruby uses the "#" symbol for interpolation, and allows one to interpolate any expression, not just variables. Other languages may support more advanced interpolation with a special formatting function, such as printf, in which the first argument, the format, specifies the pattern in which the remaining arguments are substituted.

Algorithms

There are two main types of expand variable algorithms for variable interpolation:^[3]

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, and merging it with the corresponding array of values; then join items by concatenation. The split string can be cached to reuse.

Security issues

String Interpolation, like string concatenation, may lead to security problems. If the programmer failed to properly escape or filter user input data, the system will be exposed to SQL injection, script injection, XML External Entity Injection (XXE), and cross-site scripting (XSS) attacks.^[4]

An example of SQL injection will be like this:

query = "SELECT x, y, z FROM Table WHERE id='$id' "

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

Examples

The following Perl code works identically in PHP:

$name = "Alice";
print "${name} said Hello World to the crowd of people.";

produces the output: Alice said Hello World to the crowd of people.

Boo

apples = 4
print("I have $(apples) apples")
# or
print("I have {0} apples" % apples)

The output will be:

I have 4 apples

C#.NET

var apples = 4;
// Before C# 6.0
System.Console.WriteLine(String.Format("I have {0} apples", apples));
// C# 6.0
System.Console.WriteLine($"I have {apples} apples");

^[5]

The output will be:

I have 4 apples

CFML

Script syntax:

apples = 4;
writeOutput("I have #apples# apples");

Tag syntax:

<cfset apples = 4>
<cfoutput>I have #apples# apples</cfoutput>

The output will be:

I have 4 apples

CoffeeScript

apples = 4
console.log "I have #{apples} apples"

The output will be:

I have 4 apples

Dart

int apples = 4, bananas = 3;
print('I have $apples apples.');
print('I have ${apples+bananas} fruit.');

The output will be:

I have 4 apples.
I have 7 fruit.

Groovy

def quality = 'superhero'
def sentence = "A developer is a ${quality}"
print sentence

The output will be:

A developer is a superhero

Haxe ^[6]

var apples = 4;
var bananas = 3;
trace('I have $apples apples.');
trace('I have ${apples+bananas} fruit.');

The output will be:

I have 4 apples.
I have 7 fruit.

Nemerle

def apples = 4;
def bananas = 3;
Console.WriteLine($"I have $apples apples.");
Console.WriteLine($"I have $(apples + bananas) fruit.");

You can also use advanced formatting features like this:

def fruit = ["apple", "banana"];
Console.WriteLine($<#I have ..$(fruit; "\n"; f => f + "s")#>);

The output will be:

apples
bananas

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.

PHP

<?php
class foo {
    var $foo;
    var $bar;
    function foo() {
        $this->foo = 'Foo';
        $this->bar = array('Bar1', 'Bar2', 'Bar3');
    }
}
$foo = new foo();
$name = 'Jason';
echo <<<EOT
My name is "$name". I am printing some $foo->foo.
Now, I am printing some {$foo->bar[1]}.
This should print a capital 'A': \x41
EOT;
?>

The output will be:

My name is "Jason". I am printing some Foo.
Now, I am printing some Bar2.
This should print a capital 'A': A

Python

# in Python 2
apples = 4
print "I have %d apples" % apples
print "I have %(apples)d apples" % locals()
# or in Python 2.6
print "I have {} apples".format(apples)
print "I have {a} apples".format(a=apples)
# or in Python 3
print("I have {apples} apples".format(**locals()))
print("I have",apples,"apples",sep=" ")
# or with Python 3.6
print(f"I have {apples} apples")

^{[7] [8]}

The output will be:

I have 4 apples

Ruby

apples = 4
puts "I have #{apples} apples"
# or
puts "I have %s apples" % apples
# or
puts "I have %{a} apples" % {a: apples}

The output will be:

I have 4 apples

Scala

Scala 2.10+ has implemented the following string interpolators: s, f and raw. It is even possible to write your own or override the standard interpolators.

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.

The standard interpolators

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

val apples = 4
//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(f"I have $apples%d apples")

^[9]

The output will be:

I have 4 apples

Swift

In Swift you can create a new String value from a combination of constants, variables, literals, and expressions by including their values inside a string literal. Each item that you insert 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

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 like this :

var apples: number = 4;
console.log("I have %d apples", apples);

The output remains the same.

See also

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

Notes