Ellipsis (computer programming)


In computer programming, ellipsis notation 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 ellipsis character cannot be used.

Ranges

In some programming languages, 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.
The GNU Compiler Collection has an extension to the C and C++ language to allow case ranges in switch statements:

switch

Delphi / Turbo Pascal / Free Pascal:

var FilteredChars: set of ;
var CheckedItems: set of ;

In the Unified Modeling Language, 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.

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
var_dump;

Produces this output:

array

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
>>> t.shape # select 1st element from last dimension, copy rest
>>> t.shape # select 1st element from first dimension, copy rest

Other semantics

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 ellipsis character is used to serve as a type of marker in a format string.