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Variadic functionIn mathematics and in computer programming, a variadic function is a function of indefinite arity, i.e., one which accepts a variable number of arguments. Support for variadic functions differs widely among programming languages. The term variadic is a neologism, dating back to 1936–1937.[1] The term was not widely used until the 1970s. OverviewThere are many mathematical and logical operations that come across naturally as variadic functions. For instance, the summing of numbers or the concatenation of strings or other sequences are operations that can be thought of as applicable to any number of operands (even though formally in these cases the associative property is applied). Another operation that has been implemented as a variadic function in many languages is output formatting. The C function Variadic functions can expose type-safety problems in some languages. For instance, C's In functional programming languages, variadics can be considered complementary to the apply function, which takes a function and a list/sequence/array as arguments, and calls the function with the arguments supplied in that list, thus passing a variable number of arguments to the function.[citation needed] In the functional language Haskell, variadic functions can be implemented by returning a value of a type class A related subject in term rewriting research is called hedges, or hedge variables.[3] Unlike variadics, which are functions with arguments, hedges are sequences of arguments themselves. They also can have constraints ('take no more than 4 arguments', for example) to the point where they are not variable-length (such as 'take exactly 4 arguments') - thus calling them variadics can be misleading. However they are referring to the same phenomenon, and sometimes the phrasing is mixed, resulting in names such as variadic variable (synonymous to hedge). Note the double meaning of the word variable and the difference between arguments and variables in functional programming and term rewriting. For example, a term (function) can have three variables, one of them a hedge, thus allowing the term to take three or more arguments (or two or more if the hedge is allowed to be empty). ExamplesIn CTo portably implement variadic functions in the C language, the standard #include <stdarg.h>
#include <stdio.h>
double average(int count, ...) {
va_list ap;
int j;
double sum = 0;
va_start(ap, count); /* Before C23: Requires the last fixed parameter (to get the address) */
for (j = 0; j < count; j++) {
sum += va_arg(ap, int); /* Increments ap to the next argument. */
}
va_end(ap);
return sum / count;
}
int main(int argc, char const *argv[]) {
printf("%f\n", average(3, 1, 2, 3));
return 0;
}
This will compute the average of an arbitrary number of arguments. Note that the function does not know the number of arguments or their types. The above function expects that the types will be
In C#C# describes variadic functions using the using System;
class Program
{
static int Foo(int a, int b, params int[] args)
{
// Return the sum of the integers in args, ignoring a and b.
int sum = 0;
foreach (int i in args)
sum += i;
return sum;
}
static void Main(string[] args)
{
Console.WriteLine(Foo(1, 2)); // 0
Console.WriteLine(Foo(1, 2, 3, 10, 20)); // 33
int[] manyValues = new int[] { 13, 14, 15 };
Console.WriteLine(Foo(1, 2, manyValues)); // 42
}
}
In C++The basic variadic facility in C++ is largely identical to that in C. The only difference is in the syntax, where the comma before the ellipsis can be omitted. C++ allows variadic functions without named parameters but provides no way to access those arguments since #include <iostream>
#include <cstdarg>
void simple_printf(const char* fmt...) // C-style "const char* fmt, ..." is also valid
{
va_list args;
va_start(args, fmt);
while (*fmt != '\0') {
if (*fmt == 'd') {
int i = va_arg(args, int);
std::cout << i << '\n';
} else if (*fmt == 'c') {
// note automatic conversion to integral type
int c = va_arg(args, int);
std::cout << static_cast<char>(c) << '\n';
} else if (*fmt == 'f') {
double d = va_arg(args, double);
std::cout << d << '\n';
}
++fmt;
}
va_end(args);
}
int main()
{
simple_printf("dcff", 3, 'a', 1.999, 42.5);
}
Variadic templates (parameter pack) can also be used in C++ with language built-in fold expressions. #include <iostream>
template <typename... Ts>
void foo_print(Ts... args)
{
((std::cout << args << ' '), ...);
}
int main()
{
std::cout << std::boolalpha;
foo_print(1, 3.14f); // 1 3.14
foo_print("Foo", 'b', true, nullptr); // Foo b true nullptr
}
The CERT Coding Standards for C++ strongly prefers the use of variadic templates (parameter pack) in C++ over the C-style variadic function due to a lower risk of misuse.[7] In GoVariadic functions in Go can be called with any number of trailing arguments.[8] package main
import "fmt"
// This variadic function takes an arbitrary number of ints as arguments.
func sum(nums ...int) {
fmt.Print("The sum of ", nums) // Also a variadic function.
total := 0
for _, num := range nums {
total += num
}
fmt.Println(" is", total) // Also a variadic function.
}
func main() {
// Variadic functions can be called in the usual way with individual
// arguments.
sum(1, 2) // "The sum of [1 2] is 3"
sum(1, 2, 3) // "The sum of [1 2 3] is 6"
// If you already have multiple args in a slice, apply them to a variadic
// function using func(slice...) like this.
nums := []int{1, 2, 3, 4}
sum(nums...) // "The sum of [1 2 3 4] is 10"
}
Output: The sum of [1 2] is 3 The sum of [1 2 3] is 6 The sum of [1 2 3 4] is 10 In JavaAs with C#, the public class Program {
// Variadic methods store any additional arguments they receive in an array.
// Consequentially, `printArgs` is actually a method with one parameter: a
// variable-length array of `String`s.
private static void printArgs(String... strings) {
for (String string : strings) {
System.out.println(string);
}
}
public static void main(String[] args) {
printArgs("hello"); // short for printArgs(["hello"])
printArgs("hello", "world"); // short for printArgs(["hello", "world"])
}
}
In JavaScriptJavaScript does not care about types of variadic arguments. function sum(...numbers) {
return numbers.reduce((a, b) => a + b, 0);
}
console.log(sum(1, 2, 3)); // 6
console.log(sum(3, 2)); // 5
console.log(sum()); // 0
It's also possible to create a variadic function using the arguments object, although it is only usable with functions created with the function sum() {
return Array.prototype.reduce.call(arguments, (a, b) => a + b, 0);
}
console.log(sum(1, 2, 3)); // 6
console.log(sum(3, 2)); // 5
console.log(sum()); // 0
In LuaLua functions may pass varargs to other functions the same way as other values using the function sum(...) --... designates varargs
local sum=0
for _,v in pairs({...}) do --creating a table with a varargs is the same as creating one with standard values
sum=sum+v
end
return sum
end
values={1,2,3,4}
sum(5,table.unpack(values)) --returns 15. table.unpack should go after any other arguments, otherwise not all values will be passed into the function.
function add5(...)
return ...+5 --this is incorrect usage of varargs, and will only return the first value provided
end
entries={}
function process_entries()
local processed={}
for i,v in pairs(entries) do
processed[i]=v --placeholder processing code
end
return table.unpack(processed) --returns all entries in a way that can be used as a vararg
end
print(process_entries()) --the print function takes all varargs and writes them to stdout separated by newlines
In PascalPascal is standardized by ISO standards 7185 (“Standard Pascal”) and 10206 (“Extended Pascal”).
Neither standardized form of Pascal supports variadic routines, except for certain built-in routines ( Nonetheless, dialects of Pascal implement mechanisms resembling variadic routines.
Delphi defines an This implementation, however, technically requires a single argument, that is an Both GNU Pascal and FreePascal allow externally declared functions to use a variadic formal parameter specification using an ellipsis ( In PHPPHP does not care about types of variadic arguments unless the argument is typed. function sum(...$nums): int
{
return array_sum($nums);
}
echo sum(1, 2, 3); // 6
And typed variadic arguments: function sum(int ...$nums): int
{
return array_sum($nums);
}
echo sum(1, 'a', 3); // TypeError: Argument 2 passed to sum() must be of the type int (since PHP 7.3)
In PythonPython does not care about types of variadic arguments. def foo(a, b, *args):
print(args) # args is a tuple (immutable sequence).
foo(1, 2) # ()
foo(1, 2, 3) # (3,)
foo(1, 2, 3, "hello") # (3, "hello")
Keyword arguments can be stored in a dictionary, e.g. In RakuIn Raku, the type of parameters that create variadic functions are known as slurpy array parameters and they're classified into three groups: Flattened slurpyThese parameters are declared with a single asterisk ( sub foo($a, $b, *@args) {
say @args.perl;
}
foo(1, 2) # []
foo(1, 2, 3) # [3]
foo(1, 2, 3, "hello") # [3 "hello"]
foo(1, 2, 3, [4, 5], [6]); # [3, 4, 5, 6]
Unflattened slurpyThese parameters are declared with two asterisks ( sub bar($a, $b, **@args) {
say @args.perl;
}
bar(1, 2); # []
bar(1, 2, 3); # [3]
bar(1, 2, 3, "hello"); # [3 "hello"]
bar(1, 2, 3, [4, 5], [6]); # [3, [4, 5], [6]]
Contextual slurpyThese parameters are declared with a plus ( sub zaz($a, $b, +@args) {
say @args.perl;
}
zaz(1, 2); # []
zaz(1, 2, 3); # [3]
zaz(1, 2, 3, "hello"); # [3 "hello"]
zaz(1, 2, [4, 5]); # [4, 5], single argument fills up array
zaz(1, 2, 3, [4, 5]); # [3, [4, 5]], behaving as **@
zaz(1, 2, 3, [4, 5], [6]); # [3, [4, 5], [6]], behaving as **@
In RubyRuby does not care about types of variadic arguments. def foo(*args)
print args
end
foo(1)
# prints `[1]=> nil`
foo(1, 2)
# prints `[1, 2]=> nil`
In RustRust does not support variadic arguments in functions. Instead, it uses macros.[14] macro_rules! calculate {
// The pattern for a single `eval`
(eval $e:expr) => {{
{
let val: usize = $e; // Force types to be integers
println!("{} = {}", stringify!{$e}, val);
}
}};
// Decompose multiple `eval`s recursively
(eval $e:expr, $(eval $es:expr),+) => {{
calculate! { eval $e }
calculate! { $(eval $es),+ }
}};
}
fn main() {
calculate! { // Look ma! Variadic `calculate!`!
eval 1 + 2,
eval 3 + 4,
eval (2 * 3) + 1
}
}
Rust is able to interact with C's variadic system via a In Scalaobject Program {
// Variadic methods store any additional arguments they receive in an array.
// Consequentially, `printArgs` is actually a method with one parameter: a
// variable-length array of `String`s.
private def printArgs(strings: String*): Unit = {
strings.foreach(println)
}
def main(args: Array[String]): Unit = {
printArgs("hello"); // short for printArgs(["hello"])
printArgs("hello", "world"); // short for printArgs(["hello", "world"])
}
}
In SwiftSwift cares about the type of variadic arguments, but the catch-all func greet(timeOfTheDay: String, names: String...) {
// here, names is [String]
print("Looks like we have \(names.count) people")
for name in names {
print("Hello \(name), good \(timeOfTheDay)")
}
}
greet(timeOfTheDay: "morning", names: "Joseph", "Clara", "William", "Maria")
// Output:
// Looks like we have 4 people
// Hello Joseph, good morning
// Hello Clara, good morning
// Hello William, good morning
// Hello Maria, good morning
In TclA Tcl procedure or lambda is variadic when its last argument is proc greet {timeOfTheDay args} {
puts "Looks like we have [llength $args] people"
foreach name $args {
puts "Hello $name, good $timeOfTheDay"
}
}
greet "morning" "Joseph" "Clara" "William" "Maria"
# Output:
# Looks like we have 4 people
# Hello Joseph, good morning
# Hello Clara, good morning
# Hello William, good morning
# Hello Maria, good morning
See also
Notes
References
External links
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