Ellipsis (computer programming)

In computer programming, ellipsis notation (.. or ...) is used to denote ranges, an unspecified number of arguments, or a parent directory. Most programming languages require the ellipsis to be written as a series of periods; a single (Unicode) ellipsis character cannot be used.

Ranges
In some programming languages (including Ada, Perl, Ruby, Apache Groovy, Kotlin, Haskell, and Pascal), a shortened two-dot ellipsis is used to represent a range of values given two endpoints; for example, to iterate through a list of integers between 1 and 100 inclusive in Perl:



In Ruby the  operator denotes a half-open range, i.e. that includes the start value but not the end value.

In Rust the  operator denotes an inclusive range for cases in matches and the   operator represents a range not including the end value.

Perl and Ruby overload the ".." operator in scalar context as a flip-flop operator - a stateful bistable Boolean test, roughly equivalent to "true while x but not yet y", similarly to the "," operator in sed and AWK.

GNU C compatible compilers have an extension to the C and C++ language to allow case ranges in switch statements: Additionally, GNU C allows a similar range syntax for designated initializers, available in the C language only:

Delphi / Turbo Pascal / Free Pascal:

In the Unified Modeling Language (UML), a two-character ellipsis is used to indicate variable cardinality of an association. For example, a cardinality of 1..* means that the number of elements aggregated in an association can range from 1 to infinity (a usage equivalent to Kleene plus).

Parent directory
On Windows and Unix-like operating systems, ".." is used to access the parent directory in a path.

Incomplete code
In Perl and Raku the 3-character ellipsis is also known as the "yada yada yada" operator and, similarly to its linguistic meaning, serves as a "stand-in" for code to be inserted later.

Python3 also allows the 3-character ellipsis to be used as an expressive place-holder for code to be inserted later.

In Abstract Syntax Notation One (ASN.1), the ellipsis is used as an extension marker to indicate the possibility of type extensions in future revisions of a protocol specification. In a type constraint expression like  an ellipsis is used to separate the extension root from extension additions. The definition of type A in version 1 system of the form  and the definition of type A in version 2 system of the form   constitute an extension series of the same type A in different versions of the same specification. The ellipsis can also be used in compound type definitions to separate the set of fields belonging to the extension root from the set of fields constituting extension additions. Here is an example: B ::= SEQUENCE { a INTEGER, b INTEGER, ..., c INTEGER }

C and C++
In the C programming language, an ellipsis is used to represent a variable number of parameters to a function. For example:


 * int printf( const char* format, ... );

The above function in C could then be called with different types and numbers of parameters such as:


 * printf("numbers %i %i %i", 5, 10, 15);

and
 * printf("input string %s, %f", "another string", 0.5);

C99 introduced macros with a variable number of arguments.

C++11 included the C99 preprocessor, and also introduced templates with a variable number of arguments.

Java
As of version 1.5, Java has adopted this "varargs" functionality. For example:
 * public int func(int num, String... strings)

PHP
PHP 5.6 supports use of ellipsis to define an explicitly variadic function, where  before an argument in a function definition means that arguments from that point on will be collected into an array. For example:

Produces this output:

Multiple dimensions
In Python, the ellipsis is a nullary expression that represents the  singleton.

It's used particularly in NumPy, where an ellipsis is used for slicing an arbitrary number of dimensions for a high-dimensional array:

MATLAB
In MATLAB, a three-character ellipsis is used to indicate line continuation, making the sequence of lines

semantically equivalent to the single line

In Raku an actual Unicode (U+2026) ellipsis (…) character is used to serve as a type of marker in a format string.

PHP
Since PHP 8.1, a nullary ellipsis may be used to create a closure from a callable or an object method:

Python
In Python, the ellipsis can also be used as the first parameter within the  type annotation to denote any number of arguments: