Ellipsis (programming operator)

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:

:foreach (1..100)

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:

switch(u)

Additionally, GNU C allows a similar range syntax for designated initializers, available in the C language only:

int array 0= ;

Delphi / Turbo Pascal / Free Pascal:

var FilteredChars: set of 0..#32,#127,'a'..'z'
var CheckedItems: set of ,10..38,241,58

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.

Variable number of parameters

C and C++

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

:

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

:
and
:

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:
:

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:

function variadic_function($a, $b, ...$other)

var_dump(variadic_function(1, 2, 3, 4, 5));

Produces this output:

array(3)

Multiple dimensions

In Python, particularly in NumPy, an ellipsis is used for slicing an arbitrary number of dimensions for a high-dimensional array:

>>> import numpy as np
>>> t = np.random.rand(2, 3, 4, 5)
>>> t .., 0shape # select 1st element from last dimension, copy rest
(2, 3, 4)
>>> t , ...shape # select 1st element from first dimension, copy rest
(3, 4, 5)

Other semantics

In MATLAB, a three-character ellipsis is used to indicate line continuation, making the sequence of lines
:x = 1 2 3 ...
4 5 6
semantically equivalent to the single line
:x = 1 2 3 4 5 6

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

References

{{reflist

Operators (programming)