Related
So this is more of a theoretical question. C++ and languages (in)directly based on it (Java, C#, PHP) have shortcut operators for assigning the result of most binary operators to the first operand, such as
a += 3; // for a = a + 3
a *= 3; // for a = a * 3;
a <<= 3; // for a = a << 3;
but when I want to toggle a boolean expression I always find myself writing something like
a = !a;
which gets annoying when a is a long expression like.
this.dataSource.trackedObject.currentValue.booleanFlag =
!this.dataSource.trackedObject.currentValue.booleanFlag;
(yeah, Demeter's Law, I know).
So I was wondering, is there any language with a unary boolean toggle operator that would allow me to abbreviate a = !a without repeating the expression for a, for example
!=a;
// or
a!!;
Let's assume that our language has a proper boolean type (like bool in C++) and that a is of that type (so no C-style int a = TRUE).
If you can find a documented source, I'd also be interested to learn whether e.g. the C++ designers have considered adding an operator like that when bool became a built-in type and if so, why they decided against it.
(Note: I know that some people are of the opinion that assignment should not use
= and that ++ and += are not useful operators but design flaws; let's just assume I'm happy with them and focus on why they would not extend to bools).
Toggling the boolean bit
... that would allow me to abbreviate a = !a without repeating the
expression for a ...
This approach is not really a pure "mutating flip" operator, but does fulfill your criteria above; the right hand side of the expression does not involve the variable itself.
Any language with a boolean XOR assignment (e.g. ^=) would allow flipping the current value of a variable, say a, by means of XOR assignment to true:
// type of a is bool
a ^= true; // if a was false, it is now true,
// if a was true, it is now false
As pointed out by #cmaster in the comments below, the above assumes a is of type bool, and not e.g. an integer or a pointer. If a is in fact something else (e.g. something non-bool evaluating to a "truthy" or "falsy" value, with a bit representation that is not 0b1 or 0b0, respectively), the above does not hold.
For a concrete example, Java is a language where this is well-defined and not subject to any silent conversions. Quoting #Boann's comment from below:
In Java, ^ and ^= have explicitly defined behavior for booleans
and for integers
(15.22.2.
Boolean Logical Operators &, ^, and | ), where either both sides
of the operator must be booleans, or both sides must be integers.
There's no silent conversion between those types. So it's not going to
silently malfunction if a is declared as an integer, but rather,
give a compile error. So a ^= true; is safe and well-defined in
Java.
Swift: toggle()
As of Swift 4.2, the following evolution proposal has been accepted and implemented:
SE-0199: Adding toggle to Bool
This adds a native toggle() function to the Bool type in Swift.
toggle()
Toggles the Boolean variable’s value.
Declaration
mutating func toggle()
Discussion
Use this method to toggle a Boolean value from true to false or
from false to true.
var bools = [true, false]
bools[0].toggle() // bools == [false, false]
This is not an operator, per se, but does allow a language native approach for boolean toggling.
In C++ it is possible to commit the Cardinal Sin of redefining the meaning of operators. With this in mind, and a little bit of ADL, all we need to do in order to unleash mayhem on our user base is this:
#include <iostream>
namespace notstd
{
// define a flag type
struct invert_flag { };
// make it available in all translation units at zero cost
static constexpr auto invert = invert_flag{};
// for any T, (T << invert) ~= (T = !T)
template<class T>
constexpr T& operator<<(T& x, invert_flag)
{
x = !x;
return x;
}
}
int main()
{
// unleash Hell
using notstd::invert;
int a = 6;
std::cout << a << std::endl;
// let confusion reign amongst our hapless maintainers
a << invert;
std::cout << a << std::endl;
a << invert;
std::cout << a << std::endl;
auto b = false;
std::cout << b << std::endl;
b << invert;
std::cout << b << std::endl;
}
expected output:
6
0
1
0
1
As long as we include assembly language...
FORTH
INVERT for a bitwise complement.
0= for a logical (true/false) complement.
Decrementing a C99 bool will have the desired effect, as will incrementing or decrementing the bit types supported in some tiny-microcontroller dialects (which from what I've observed treat bits as single-bit wide bitfields, so all even numbers get truncated to 0 and all odd numbers to 1). I wouldn't particularly recommend such usage, in part because I'm not a big fan of the bool type semantics [IMHO, the type should have specified that a bool to which any value other than 0 or 1 is stored may behave when read as though it holds an Unspecified (not necessarily consistent) integer value; if a program is trying to store an integer value that isn't known to be 0 or 1, it should use !! on it first].
Assembly language
NOT eax
See https://www.tutorialspoint.com/assembly_programming/assembly_logical_instructions.htm
I'm assuming you're not going to be choosing a language based solely upon this :-) In any case, you can do this in C++ with something like:
inline void makenot(bool &b) { b = !b; }
See the following complete program for example:
#include <iostream>
inline void makenot(bool &b) { b = !b; }
inline void outBool(bool b) { std::cout << (b ? "true" : "false") << '\n'; }
int main() {
bool this_dataSource_trackedObject_currentValue_booleanFlag = false;
outBool(this_dataSource_trackedObject_currentValue_booleanFlag);
makenot(this_dataSource_trackedObject_currentValue_booleanFlag);
outBool(this_dataSource_trackedObject_currentValue_booleanFlag);
makenot(this_dataSource_trackedObject_currentValue_booleanFlag);
outBool(this_dataSource_trackedObject_currentValue_booleanFlag);
}
This outputs, as expected:
false
true
false
PostScript, being a concatenative, stack-oriented language like Forth, has a unary toggle, not. The not operator toggles the value on top of the stack. For example,
true % push true onto the stack
not % invert the top of stack
% the top of stack is now false
See the PostScript Language Reference Manual (pdf), p. 458.
Visual Basic.Net supports this via an extension method.
Define the extension method like so:
<Extension>
Public Sub Flip(ByRef someBool As Boolean)
someBool = Not someBool
End Sub
And then call it like this:
Dim someVariable As Boolean
someVariable = True
someVariable.Flip
So, your original example would look something like:
me.DataSource.TrackedObject.CurrentValue.BooleanFlag.Flip
This question is indeed interesting from a purely theoretical standpoint. Setting aside whether or not a unary, mutating boolean toggle operator would be useful, or why many languages have opted to not provide one, I ventured on a quest to see whether or not it indeed exists.
TL;DR apparently no, but Swift lets you implement one. If you'd only like to see how it's done, you can scroll to the bottom of this answer.
After a (quick) search to features of various languages, I'd feel safe to say that no language has implemented this operator as a strict mutating in-place operation (do correct me if you find one). So the next thing would be to see if there are languages that let you build one. What this would require is two things:
being able to implement (unary) operators with functions
allowing said functions to have pass-by-reference arguments (so that they may mutate their arguments directly)
Many languages will immediately get ruled out for not supporting either or both of these requirements. Java for one does not allow operator overloading (or custom operators) and in addition, all primitive types are passed by value. Go has no support for operator overloading (except by hacks) whatsoever. Rust only allows operator overloading for custom types. You could almost achieve this in Scala, which let's you use very creatively named functions and also omit parentheses, but sadly there's no pass-by-reference. Fortran gets very close in that it allows for custom operators, but specifically forbids them from having inout parameters (which are allowed in normal functions and subroutines).
There is however at least one language that ticks all the necessary boxes: Swift. While some people have linked to the upcoming .toggle() member function, you can also write your own operator, which indeed supports inout arguments. Lo and behold:
prefix operator ^
prefix func ^ (b: inout Bool) {
b = !b
}
var foo = true
print(foo)
// true
^foo
print(foo)
// false
In Rust, you can create your own trait to extend the types that implement the Not trait:
use std::ops::Not;
use std::mem::replace;
trait Flip {
fn flip(&mut self);
}
impl<T> Flip for T
where
T: Not<Output = T> + Default,
{
fn flip(&mut self) {
*self = replace(self, Default::default()).not();
}
}
#[test]
fn it_works() {
let mut b = true;
b.flip();
assert_eq!(b, false);
}
You can also use ^= true as suggested, and in the case of Rust, there is no possible issue to do this because false is not a "disguised" integer like in C or C++:
fn main() {
let mut b = true;
b ^= true;
assert_eq!(b, false);
let mut b = false;
b ^= true;
assert_eq!(b, true);
}
In Python
Python supports such functionality, if the variable has bool type (which is True or False) with the exclusive or (^=) operator:
a = False
a ^= True
print(a) # --> True
a ^= True
print(a) # --> False
In C#:
boolean.variable.down.here ^= true;
The boolean ^ operator is XOR, and XORing with true is the same as inverting.
Is it possible, that a function can return a function in Ada? I am trying to get currying to work.
type Integer_Func_Type is access function (Y : Integer) return Integer;
function Add (X : Integer) return Integer_Func_Type is
function Inner (Y : Integer) return Integer is
begin
return X + Y;
end Inner;
begin
return Inner'Access;
end;
At the end, I do not want to provide all arguments of a function one at a time. For example: if x is a ternary function and y is curry(x), then I can use following function calls: y(a,b,c), y(a,b)(c), y(a)(b,c), y(a)(b)(c).
EDIT
I implemented 'Jacob Sparre Andersen' suggestions. But it does not look like currying will be easy to implement. I must implement every possible variant of any type I want to use in advance. Is this correct?
with Ada.Text_IO;
with R;
procedure Hello is
Add_Two : R.Test2 := (X => 2);
begin
Ada.Text_IO.Put_Line(Add_Two.Add(3)'Img);
end Hello;
r.adb
package body R is
function Add(A : Test2; Y : Integer) return Integer is
begin
return A.X + Y;
end Add;
end R;
r.ads
package R is
type Test is abstract tagged null record;
function Add(A : Test; Y : Integer) return Integer is abstract;
type Test2 is new Test with
record
X : Integer;
end record;
overriding
function Add(A : Test2; Y : Integer) return Integer;
end R;
This is how to do it with generics:
with Ada.Text_IO;
procedure Test is
-- shorthand Ada 2012 syntax; can also use full body
function Add (X, Y : Integer) return Integer is (X + Y);
generic
type A_Type (<>) is limited private;
type B_Type (<>) is limited private;
type Return_Type (<>) is limited private;
with function Orig (A : A_Type; B : B_Type) return Return_Type;
A : A_Type;
function Curry_2_to_1 (B : B_Type) return Return_Type;
function Curry_2_to_1 (B : B_Type) return Return_Type is
(Orig (A, B));
function Curried_Add is new Curry_2_to_1
(Integer, Integer, Integer, Add, 3);
begin
Ada.Text_IO.Put_Line (Integer'Image (Curried_Add (39)));
end Test;
As you see, it is quite verbose. Also, you need to provide a currying implementation for every count X of parameters of the original function and every number Y of parameters of the generated function, so you'd have a lot of Curry_X_to_Y functions. This is necessary because Ada does not have variadic generics.
A lot of the verbosity also comes from Ada not doing type inference: You need to explicitly specifiy A_Type, B_Type and Return_Type even though theoretically, they could be inferred from the given original function (this is what some functional programming languages do).
Finally, you need a named instance from the currying function because Ada does not support anonymous instances of generic functions.
So, in principle, currying does work, but it is not anything as elegant as in a language like Haskell. If you only want currying for a specific type, the code gets significantly shorter, but you also lose flexibility.
You can't do quite what you're trying to do, since Inner stops to exist as soon as Add returns.
You could do something with the effect you describe using tagged types.
One abstract tagged type with a primitive operation matching your function type.
And then a derived tagged type with X as an attribute and an implementation of the function matching Inner.
Many of the answers seem to deal with ways to have subprograms that deal with variable numbers of parameters. One way to deal with this is with a sequence of values. For example,
type Integer_List is array (Positive range <>) of Integer;
function Add (List : Integer_List) return Integer;
can be considered a function that takes an arbitrary number of parameters of type Integer. This is simple if all your parameters have the same type. It's more complicated, but still possible, if you deal with a finite set of possible parameter types:
type Number_ID is (Int, Flt, Dur);
type Number (ID : Number_ID) is record
case ID is
when Int =>
Int_Value : Integer;
when Flt =>
Flt_Value : Float;
when Dur =>
Dur_Value : Duration;
end case;
end record;
type Number_List is array (Positive range <>) of Number;
function Add (List : Number_List) return Number;
If you have to be able to handle types not known in advance, this technique is not suitable.
Let's say I have 1000 functions defined as follows
void func dummy1(int a);
void func dummy2(int a, int aa);
void func dummy3(int a, int aa, int aaa);
.
.
.
void func dummy1000(int a, int aa, int aaa, ...);
I want to write a function that takes an integer, n (n < 1000) and calls nth dummy function (in case of 10, dummy10) with exactly n arguments(arguments can be any integer, let's say 0) as required. I know this can be achieved by writing a switch case statement with 1000 cases which is not plausible.
In my opinion, this cannot be achieved without recompilation at run time so languages like java, c, c++ will never let such a thing happen.
Hopefully, there is a way to do this. If so I am curious.
Note: This is not something that I will ever use, I asked question just because of my curiosity.
In modern functional languages, you can make a list of functions which take a list as an argument. This will arguably solve your problem, but it is also arguably cheating, as it is not quite the statically-typed implementation your question seems to imply. However, it is pretty much what dynamic languages such as Python, Ruby, or Perl do when using "manual" argument handling...
Anyway, the following is in Haskell: it supplies the nth function (from its first argument fs) a list of n copies of the second argument (x), and returns the result. Of course, you will need to put together the list of functions somehow, but unlike a switch statement this list will be reusable as a first-class argument.
selectApplyFunction :: [ [Int] -> a ] -> Int -> Int -> a
selectApplyFunction fs x n = (fs !! (n-1)) (replicate n x)
dummy1 [a] = 5 * a
dummy2 [a, b] = (a + 3) * b
dummy3 [a, b, c] = (a*b*c) / (a*b + b*c + c*a)
...
myFunctionList = [ dummy1, dummy2, dummy3, ... ]
-- (myfunction n) provides n copies of the number 42 to the n'th function
myFunction = selectApplyFunction myFunctionList 42
-- call the 666'th function with 666 copies of 42
result = myFunction 666
Of course, you will get an exception if n is greater than the number of functions, or if the function can't handle the list it is given. Note, too, that it is poor Haskell style -- mainly because of the way it abuses lists to (abusively) solve your problem...
No, you are incorrect. Most modern languages support some form of Reflection that will allow you to call a function by name and pass params to it.
You can create an array of functions in most of modern languages.
In pseudo code,
var dummy = new Array();
dummy[1] = function(int a);
dummy[2] = function(int a, int aa);
...
var result = dummy[whateveryoucall](1,2,3,...,whateveryoucall);
In functional languages you could do something like this, in strongly typed ones, like Haskell, the functions must have the same type, though:
funs = [reverse, tail, init] -- 3 functions of type [a]->[a]
run fn arg = (funs !! fn) $ args -- applies function at index fn to args
In object oriented languages, you can use function objects and reflection together to achieve exactly what you want. The problem of the variable number of arguments is solved by passing appropriate POJOs (recalling C stucts) to the function object.
interface Functor<A,B> {
public B compute(A input);
}
class SumInput {
private int x, y;
// getters and setters
}
class Sum implements Functor<SumInput, Integer> {
#Override
public Integer compute(SumInput input) {
return input.getX() + input.getY();
}
}
Now imagine you have a large number of these "functors". You gather them in a configuration file (maybe an XML file with metadata about each functor, usage scenarios, instructions, etc...) and return the list to the user.
The user picks one of them. By using reflection, you can see what is the required input and the expected output. The user fills in the input, and by using reflection you instantiate the functor class (newInstance()), call the compute() function and get the output.
When you add a new functor, you just have to change the list of the functors in the config file.
I'm trying to define a new type and have not had much luck finding any information about using lists within them. Basically my new type will contain two lists, lets say x and y of type SqlSingle (the user defined type is written in C#) is this even possible?
If not how are you supposed to go about simulating a two lists of an arbitary length in an SQL Server 2008 column?
I'm possibly going about this the wrong way but it is the best approach I can think of at the moment. Any help is very much appreciated.
You can use a List<T> in a CLR UDT - although CLR types are structs, which should be immutable, so a ReadOnlyCollection<T> would be a better choice if you don't have a very compelling reason for the mutability. What you need to know in either case is that SQL won't know how to use the list itself; you can't simply expose the list type as a public IList<T> or IEnumerable<T> and be on your merry way, like you would be able to do in pure .NET.
Typically the way to get around this would be to expose a Count property and some methods to get at the individual list items.
Also, in this case, instead of maintaining two separate lists of SqlSingle instances, I would create an additional type to represent a single point, so you can manage it independently and pass it around in SQL if you need to:
[Serializable]
[SqlUserDefinedType(Format.Native)]
public struct MyPoint
{
private SqlSingle x;
private SqlSingle y;
public MyPoint()
{
}
public MyPoint(SqlSingle x, SqlSingle y) : this()
{
this.x = x;
this.y = y;
}
// You need this method because SQL can't use the ctors
[SqlFunction(Name = "CreateMyPoint")]
public static MyPoint Create(SqlSingle x, SqlSingle y)
{
return new MyPoint(x, y);
}
// Snip Parse method, Null property, etc.
}
The main type would look something like this:
[Serializable]
[SqlUserDefinedType(Format.UserDefined, IsByteOrdered = true, MaxByteSize = ...)]
public struct MyUdt
{
// Make sure to initialize this in any constructors/builders
private IList<MyPoint> points;
[SqlMethod(OnNullCall = false, IsDeterministic = true, IsPrecise = true)]
public MyPoint GetPoint(int index)
{
if ((index >= 0) && (index < points.Count))
{
return points[index];
}
return MyPoint.Null;
}
public int Count
{
get { return points.Count; }
}
}
If you need SQL to be able to get a sequence of all the points, then you can add an enumerable method to the sequence type as well:
[SqlFunction(FillRowMethodName = "FillPointRow",
TableDefinition = "[X] real, [Y] real")]
public static IEnumerable GetPoints(MyUdt obj)
{
return obj.Points;
}
public static void FillPointRow(object obj, out SqlSingle x, out SqlSingle y)
{
MyPoint point = (MyPoint)obj;
x = point.X;
y = point.Y;
}
You might think that it's possible to use an IEnumerable<T> and/or use an instance method instead of a static one, but don't even bother trying, it doesn't work.
So the way you can use the resulting type in SQL Server is:
DECLARE #UDT MyUdt
SET #UDT = <whatever>
-- Will show the number of points
SELECT #UDT.Count
-- Will show the binary representation of the second point
SELECT #UDT.GetPoint(1) AS [Point]
-- Will show the X and Y values for the second point
SELECT #UDT.GetPoint(1).X AS [X], #UDT.GetPoint(1).Y AS [Y]
-- Will show all the points
SELECT * FROM dbo.GetPoints(#UDT)
Hope this helps get you on the right track. UDTs can get pretty complicated to manage when they're dealing with list/sequence data.
Also note that you'll obviously need to add serialization methods, builder methods, aggregate methods, and so on. It can be quite an ordeal; make sure that this is actually the direction you want to go in, because once you start adding UDT columns it can be very difficult to make changes if you realize that you made the wrong choice.
Lists as you describe are usually normalized - that is, stored in separate tables with one row per item - rather than trying to cram them into a single column. If you can share more info on what you are trying to accomplish, maybe we can offer more assistance.
Edit - suggested table structure:
-- route table--
route_id int (PK)
route_length int (or whatever)
route_info <other fields as needed>
-- waypoint table --
route_id int (PK)
sequence tinyint (PK)
lat decimal(9,6)
lon decimal(9,6)
waypoint_info <other fields as needed>
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Closed 10 years ago.
When learning a new programming language, one of the possible roadblocks you might encounter is the question whether the language is, by default, pass-by-value or pass-by-reference.
So here is my question to all of you, in your favorite language, how is it actually done? And what are the possible pitfalls?
Your favorite language can, of course, be anything you have ever played with: popular, obscure, esoteric, new, old...
Here is my own contribution for the Java programming language.
first some code:
public void swap(int x, int y)
{
int tmp = x;
x = y;
y = tmp;
}
calling this method will result in this:
int pi = 3;
int everything = 42;
swap(pi, everything);
System.out.println("pi: " + pi);
System.out.println("everything: " + everything);
"Output:
pi: 3
everything: 42"
even using 'real' objects will show a similar result:
public class MyObj {
private String msg;
private int number;
//getters and setters
public String getMsg() {
return this.msg;
}
public void setMsg(String msg) {
this.msg = msg;
}
public int getNumber() {
return this.number;
}
public void setNumber(int number) {
this.number = number;
}
//constructor
public MyObj(String msg, int number) {
setMsg(msg);
setNumber(number);
}
}
public static void swap(MyObj x, MyObj y)
{
MyObj tmp = x;
x = y;
y = tmp;
}
public static void main(String args[]) {
MyObj x = new MyObj("Hello world", 1);
MyObj y = new MyObj("Goodbye Cruel World", -1);
swap(x, y);
System.out.println(x.getMsg() + " -- "+ x.getNumber());
System.out.println(y.getMsg() + " -- "+ y.getNumber());
}
"Output:
Hello world -- 1
Goodbye Cruel World -- -1"
thus it is clear that Java passes its parameters by value, as the value for pi and everything and the MyObj objects aren't swapped.
be aware that "by value" is the only way in java to pass parameters to a method. (for example a language like c++ allows the developer to pass a parameter by reference using '&' after the parameter's type)
now the tricky part, or at least the part that will confuse most of the new java developers: (borrowed from javaworld)
Original author: Tony Sintes
public void tricky(Point arg1, Point arg2)
{
arg1.x = 100;
arg1.y = 100;
Point temp = arg1;
arg1 = arg2;
arg2 = temp;
}
public static void main(String [] args)
{
Point pnt1 = new Point(0,0);
Point pnt2 = new Point(0,0);
System.out.println("X: " + pnt1.x + " Y: " +pnt1.y);
System.out.println("X: " + pnt2.x + " Y: " +pnt2.y);
System.out.println(" ");
tricky(pnt1,pnt2);
System.out.println("X: " + pnt1.x + " Y:" + pnt1.y);
System.out.println("X: " + pnt2.x + " Y: " +pnt2.y);
}
"Output
X: 0 Y: 0
X: 0 Y: 0
X: 100 Y: 100
X: 0 Y: 0"
tricky successfully changes the value of pnt1!
This would imply that Objects are passed by reference, this is not the case!
A correct statement would be: the Object references are passed by value.
more from Tony Sintes:
The method successfully alters the
value of pnt1, even though it is
passed by value; however, a swap of
pnt1 and pnt2 fails! This is the major
source of confusion. In the main()
method, pnt1 and pnt2 are nothing more
than object references. When you pass
pnt1 and pnt2 to the tricky() method,
Java passes the references by value
just like any other parameter. This
means the references passed to the
method are actually copies of the
original references. Figure 1 below
shows two references pointing to the
same object after Java passes an
object to a method.
(source: javaworld.com)
Conclusion or a long story short:
Java passes it parameters by value
"by value" is the only way in java to pass a parameter to a method
using methods from the object given as parameter will alter the object as the references point to the original objects. (if that method itself alters some values)
useful links:
http://www.javaworld.com/javaworld/javaqa/2000-05/03-qa-0526-pass.html
http://www.ibm.com/developerworks/java/library/j-passbyval/
http://www.ibm.com/developerworks/library/j-praxis/pr1.html
http://javadude.com/articles/passbyvalue.htm
Here is another article for the c# programming language
c# passes its arguments by value (by default)
private void swap(string a, string b) {
string tmp = a;
a = b;
b = tmp;
}
calling this version of swap will thus have no result:
string x = "foo";
string y = "bar";
swap(x, y);
"output:
x: foo
y: bar"
however, unlike java c# does give the developer the opportunity to pass parameters by reference, this is done by using the 'ref' keyword before the type of the parameter:
private void swap(ref string a, ref string b) {
string tmp = a;
a = b;
b = tmp;
}
this swap will change the value of the referenced parameter:
string x = "foo";
string y = "bar";
swap(x, y);
"output:
x: bar
y: foo"
c# also has a out keyword, and the difference between ref and out is a subtle one.
from msdn:
The caller of a method which takes an
out parameter is not required to
assign to the variable passed as the
out parameter prior to the call;
however, the callee is required to
assign to the out parameter before
returning.
and
In contrast ref parameters are
considered initially assigned by the
callee. As such, the callee is not
required to assign to the ref
parameter before use. Ref parameters
are passed both into and out of a
method.
a small pitfall is, like in java, that objects passed by value can still be changed using their inner methods
conclusion:
c# passes its parameters, by default, by value
but when needed parameters can also be passed by reference using the ref keyword
inner methods from a parameter passed by value will alter the object (if that method itself alters some values)
useful links:
http://msdn.microsoft.com/en-us/vcsharp/aa336814.aspx
http://www.c-sharpcorner.com/UploadFile/saragana/Willswapwork11162005012542AM/Willswapwork.aspx
http://en.csharp-online.net/Value_vs_Reference
Python uses pass-by-value, but since all such values are object references, the net effect is something akin to pass-by-reference. However, Python programmers think more about whether an object type is mutable or immutable. Mutable objects can be changed in-place (e.g., dictionaries, lists, user-defined objects), whereas immutable objects can't (e.g., integers, strings, tuples).
The following example shows a function that is passed two arguments, an immutable string, and a mutable list.
>>> def do_something(a, b):
... a = "Red"
... b.append("Blue")
...
>>> a = "Yellow"
>>> b = ["Black", "Burgundy"]
>>> do_something(a, b)
>>> print a, b
Yellow ['Black', 'Burgundy', 'Blue']
The line a = "Red" merely creates a local name, a, for the string value "Red" and has no effect on the passed-in argument (which is now hidden, as a must refer to the local name from then on). Assignment is not an in-place operation, regardless of whether the argument is mutable or immutable.
The b parameter is a reference to a mutable list object, and the .append() method performs an in-place extension of the list, tacking on the new "Blue" string value.
(Because string objects are immutable, they don't have any methods that support in-place modifications.)
Once the function returns, the re-assignment of a has had no effect, while the extension of b clearly shows pass-by-reference style call semantics.
As mentioned before, even if the argument for a is a mutable type, the re-assignment within the function is not an in-place operation, and so there would be no change to the passed argument's value:
>>> a = ["Purple", "Violet"]
>>> do_something(a, b)
>>> print a, b
['Purple', 'Violet'] ['Black', 'Burgundy', 'Blue', 'Blue']
If you didn't want your list modified by the called function, you would instead use the immutable tuple type (identified by the parentheses in the literal form, rather than square brackets), which does not support the in-place .append() method:
>>> a = "Yellow"
>>> b = ("Black", "Burgundy")
>>> do_something(a, b)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in do_something
AttributeError: 'tuple' object has no attribute 'append'
Since I haven't seen a Perl answer yet, I thought I'd write one.
Under the hood, Perl works effectively as pass-by-reference. Variables as function call arguments are passed referentially, constants are passed as read-only values, and results of expressions are passed as temporaries. The usual idioms to construct argument lists by list assignment from #_, or by shift tend to hide this from the user, giving the appearance of pass-by-value:
sub incr {
my ( $x ) = #_;
$x++;
}
my $value = 1;
incr($value);
say "Value is now $value";
This will print Value is now 1 because the $x++ has incremented the lexical variable declared within the incr() function, rather than the variable passed in. This pass-by-value style is usually what is wanted most of the time, as functions that modify their arguments are rare in Perl, and the style should be avoided.
However, if for some reason this behaviour is specifically desired, it can be achieved by operating directly on elements of the #_ array, because they will be aliases for variables passed into the function.
sub incr {
$_[0]++;
}
my $value = 1;
incr($value);
say "Value is now $value";
This time it will print Value is now 2, because the $_[0]++ expression incremented the actual $value variable. The way this works is that under the hood #_ is not a real array like most other arrays (such as would be obtained by my #array), but instead its elements are built directly out of the arguments passed to a function call. This allows you to construct pass-by-reference semantics if that would be required. Function call arguments that are plain variables are inserted as-is into this array, and constants or results of more complex expressions are inserted as read-only temporaries.
It is however exceedingly rare to do this in practice, because Perl supports reference values; that is, values that refer to other variables. Normally it is far clearer to construct a function that has an obvious side-effect on a variable, by passing in a reference to that variable. This is a clear indication to the reader at the callsite, that pass-by-reference semantics are in effect.
sub incr_ref {
my ( $ref ) = #_;
$$ref++;
}
my $value = 1;
incr(\$value);
say "Value is now $value";
Here the \ operator yields a reference in much the same way as the & address-of operator in C.
There's a good explanation here for .NET.
A lot of people are surprise that reference objects are actually passed by value (in both C# and Java). It's a copy of a stack address. This prevents a method from changing where the object actually points to, but still allows a method to change the values of the object. In C# its possible to pass a reference by reference, which means you can change where an actual object points to.
Don't forget there is also pass by name, and pass by value-result.
Pass by value-result is similar to pass by value, with the added aspect that the value is set in the original variable that was passed as the parameter. It can, to some extent, avoid interference with global variables. It is apparently better in partitioned memory, where a pass by reference could cause a page fault (Reference).
Pass by name means that the values are only calculated when they are actually used, rather than at the start of the procedure. Algol used pass-by-name, but an interesting side effect is that is it very difficult to write a swap procedure (Reference). Also, the expression passed by name is re-evaluated each time it is accessed, which can also have side effects.
Whatever you say as pass-by-value or pass-by-reference must be consistent across languages. The most common and consistent definition used across languages is that with pass-by-reference, you can pass a variable to a function "normally" (i.e. without explicitly taking address or anything like that), and the function can assign to (not mutate the contents of) the parameter inside the function and it will have the same effect as assigning to the variable in the calling scope.
From this view, the languages are grouped as follows; each group having the same passing semantics. If you think that two languages should not be put in the same group, I challenge you to come up with an example that distinguishes them.
The vast majority of languages including C, Java, Python, Ruby, JavaScript, Scheme, OCaml, Standard ML, Go, Objective-C, Smalltalk, etc. are all pass-by-value only. Passing a pointer value (some languages call it a "reference") does not count as pass by reference; we are only concerned about the thing passed, the pointer, not the thing pointed to.
Languages such as C++, C#, PHP are by default pass-by-value like the languages above, but functions can explicitly declare parameters to be pass-by-reference, using & or ref.
Perl is always pass-by-reference; however, in practice people almost always copy the values after getting it, thus using it in a pass-by-value way.
by value
is slower than by reference since the system has to copy the parameter
used for input only
by reference
faster since only a pointer is passed
used for input and output
can be very dangerous if used in conjunction with global variables
Concerning J, while there is only, AFAIK, passing by value, there is a form of passing by reference which enables moving a lot of data. You simply pass something known as a locale to a verb (or function). It can be an instance of a class or just a generic container.
spaceused=: [: 7!:5 <
exectime =: 6!:2
big_chunk_of_data =. i. 1000 1000 100
passbyvalue =: 3 : 0
$ y
''
)
locale =. cocreate''
big_chunk_of_data__locale =. big_chunk_of_data
passbyreference =: 3 : 0
l =. y
$ big_chunk_of_data__l
''
)
exectime 'passbyvalue big_chunk_of_data'
0.00205586720663967
exectime 'passbyreference locale'
8.57957102144893e_6
The obvious disadvantage is that you need to know the name of your variable in some way in the called function. But this technique can move a lot of data painlessly. That's why, while technically not pass by reference, I call it "pretty much that".
PHP is also pass by value.
<?php
class Holder {
private $value;
public function __construct($value) {
$this->value = $value;
}
public function getValue() {
return $this->value;
}
}
function swap($x, $y) {
$tmp = $x;
$x = $y;
$y = $tmp;
}
$a = new Holder('a');
$b = new Holder('b');
swap($a, $b);
echo $a->getValue() . ", " . $b->getValue() . "\n";
Outputs:
a b
However in PHP4 objects were treated like primitives. Which means:
<?php
$myData = new Holder('this should be replaced');
function replaceWithGreeting($holder) {
$myData->setValue('hello');
}
replaceWithGreeting($myData);
echo $myData->getValue(); // Prints out "this should be replaced"
By default, ANSI/ISO C uses either--it depends on how you declare your function and its parameters.
If you declare your function parameters as pointers then the function will be pass-by-reference, and if you declare your function parameters as not-pointer variables then the function will be pass-by-value.
void swap(int *x, int *y); //< Declared as pass-by-reference.
void swap(int x, int y); //< Declared as pass-by-value (and probably doesn't do anything useful.)
You can run into problems if you create a function that returns a pointer to a non-static variable that was created within that function. The returned value of the following code would be undefined--there is no way to know if the memory space allocated to the temporary variable created in the function was overwritten or not.
float *FtoC(float temp)
{
float c;
c = (temp-32)*9/5;
return &c;
}
You could, however, return a reference to a static variable or a pointer that was passed in the parameter list.
float *FtoC(float *temp)
{
*temp = (*temp-32)*9/5;
return temp;
}