String interpolation is a form of Quasi-quotation, common in many programming languages which make heavy use of string representations of data, such as Python, Ruby, PHP, Perl, Scala, Nemerle, Swift, etc. It means to insert a string or replace a variable with its value. It makes string formatting and specifying contents more intuitive.^[1]

Examples

Boo

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

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} fruits');

The output will be:

I have 4 apples
I have 7 fruits

Java

int apples = 4;
System.out.println("Caspar has " + apples + " apples");

The output will be:

Caspar has 4 apples

Lisp

Using strings:

(print (format t "I have ~D apples" 4))

The output will be:

I have 4 apples

We can also generalise this to arbitrary (non-string) LISP expressions, known as s-expressions. The equivalent of string interpolation for s-expressions is quasi-quotation, for example:

(let ((num 4))
     (quasiquote (I have (unquote num) apples)))

This results in the s-expression (I have 4 apples), where "I", "have", "4" and "apples" are symbols (i.e. identifiers), rather than strings.

Nemerle

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

You can also use advanced formatting features like this:

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

The output will be:

apples
bananas

Perl

my $apples = 4;
print "I have $apples apples\n";

The output will be:

I have 4 apples

PHP

<?php
$str = <<<EOD
Example of string
spanning multiple lines
using heredoc syntax.
EOD;
 
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

apples = 4
print "I have %d apples" % apples
# or in newer versions:
print "I have {} apples".format(apples)
print "I have {a} apples".format(a=apples)

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

The s interpolator

Scala's string Interpolation allows users to embed variable references directly in processed string literals. It's done by the Scala Macro feature. Macros are functions that are called by the compiler during compilation. Here is an example:

val apples = 4
println(s"I have $apples apples")

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
println("I have \(apples) apples")

The output will be:

I have 4 apples

Security issues

String Interpolation, like string concatenation, may lead to security problems. When failed to properly escape or filter user input data, system will expose to SQL injection, script injection, XML External Entity Injection (XXE), and cross-site scripting (XSS) attacks.^[2]

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.

See also

• String literal
• Concatenation
• Improper input validation
• Quasi-quotation

Notes