Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
...
...
Is there a better way (since 2017)?
Each programming language has their pros and cons. They all have
their way of solving the problem. A higher level language,
generally, allows you to write less code compared to a lower
level one.
ie - A Python program thats 20 LOC could be 50 LOC in C99 or much
much more.
The downside to (something like) python is that writing less code
has its drawbacks. What does it do in background? Can there be a
huge performance penalty once inside a loop?
D provides you the power of being low level like C, or high level
that can be comparable in a number of ways to C#. I have a choice
to turn off the GC, if I want to!
My response to this is, without sounding rude, who cares about
your example?
- Could D be able to achieve this? Maybe.
- If not, could it in the future? Maybe.
- Would you still have to write an extra line or two compared to
the Python example? Maybe.
You are just splitting a string to 3 variables as ints.
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
"What if" s has more than 3 numbers?
"What if" s includes characters?
Python will spit out an error - "not enough values" or "invalid
literal"
To build reliable code in your example add more LOC.
I can do this in D. May not be the best example. Is stores them
in int arrays and
handles potentials characters:-
```d
void main()
{
import std.stdio;
import std.array;
import std.string;
import std.algorithm;
import std.conv;
auto s = "1 2 a 3";
auto abc = s.split(' ').map!(s => isNumeric(s) ? s.to!int :
-1);
writeln(abc); // [1, 2, -1, 3]
}
```
I will leave it there.
Re: how to assign multiple variables at once by unpacking array?
On Sunday, 8 October 2023 at 06:02:14 UTC, Jonathan M Davis wrote:
The problem is that the compiler needs to be able to verify
that the types match, and when the return type of a function is
a dynamic array such as int[], it has no way of knowing how
many elements the array has and therefore can't verify at
compile time that the assignment will work. add a runtime check
to verify that the number of elements match, but that's not the
sort of thing that you typically do with a statically typed
language.
I agree.
And solutions like "algebraic types" used by typescript are not
applicable to D (You cant declare int|undefined)
```typescript
const sorted=([first,...xs]:T[]):T[] => first===undefined ?
[] : [
...sorted(xs.filter(x=> x<=first )), first, ...sorted(
xs.filter(x=> x > first))
];
console.log( sorted([1,2,3,4,-1,-2,-3]) );
```
But it should with pattern matching
```D
auto sorted(T)(T[] entries) =>
match(entries) {
case [item, ...others] => ...
otherwise => []
}
```
Or with overloading (patter matching expressions on function
signature... similar to haskell)
```D
auto sorted(T)(T[] [item, ...others]) => ...;
auto sorted(T)(T[] []) => [];
```
Well, not really: because compiler can't predict witch "sorted"
version will be called on compile time. (May be it should be a
syntax suggar of an unique sorted method with match in the body)
However, while there are some benefits to being able to do
this, the response by many programmers from statically typed
languages is that it's cleaner to create a struct for this sort
of thing
I completly agree
, since then the values are contained together, and they have
names associated with them (since they'll be member variables
of the struct). So, while some programmers definitely want
tuple types to be built into D, a number of others don't like
the idea. As such, it's an open question whether we'll ever
have such tuples in D.
Destructuring is only a **Syntax sugar** applicable to well known
types (i.e. Structs). It is not related (exclusively) with
tuples:
The type of a function call (or a parameter) is perfectly known.
Typescript applies destructuring based on type matching (type
members names):
```typescript
type ICounter = {
inc: ()=>void
dec: ()=>void
value: ()=>number
}
function Counter(n:number=0):ICounter {
return {
inc: ()=>{ n++ };
dec: ()=>{ n-- }
value: ()=> n;
}
}
const {inc,value} = Counter();
inc();
inc();
console.assert( value() === 2 );
inc();
console.assert( value() === 3 );
```
It could be portable with Structs in D.
Personally, I found it is specially useful with parameters for
"dependencies injection":
```typescript
function CarsDAO( {db:{ withTransaction }, logger:{ info }}:
IContext ):ICarsDao {
return {
createCar,
updateCar
}
function createCar(data:CarDto):Promise {
info("Lets create a car");
return withTransaction( trx=> trx.executeQuery("insert .
returning key") );
}
}
const context:IContext = {
logger: Singleton(logger),
db: Singleton(Db),
carsDao: Singleton(CarsDao),
}
(async ({ carsDAO:{ createCar }, logger }:IContext)=>{
data:CarDTO = { registration: "ASD1232", model: "Citroën XS" };
const key = await createCar( data )
logger.info( `Car ${key} has been registered`);
})(context);
```
This kind of destructuring is semantically rich.
Note: The "hard" part for this kind of solutions (dependency
injection not based on classes) is the clousures management, and
D solves it magnificently...
Either way, the unpacking of dynamic arrays likely stands no
chance whatsoever of ever being added, > because it would
require runtime checks to determine whether the unpacking was
valid.
This is what pattern matching, some day, at runtime, should solve
:-)
-- Antonio
Re: how to assign multiple variables at once by unpacking array?
On Monday, 9 October 2023 at 01:15:21 UTC, Salih Dincer wrote: the staticMapN() template implemented by Ali Çehreli, which is not in the standard library, is needed: It would be great if the unlimited version was added to std.meta. This template seeded/sprouted in here: https://forum.dlang.org/thread/vwxilrqgjqtvjrnjj...@forum.dlang.org SDB@79
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 16:12:47 UTC, mw wrote:
Interesting: in terms of easy of coding, clarity and future
maintenance, which one is superior?
The one liner in Python, or your "solution" with dozen lines of
code? BTW, is that a solution at all? Did it achieved what the
original goal asked in the OP question?
So, who should learn from whom?
If you don't expect to do a single line of coding, there are many
methods in D that can do this kind of thing (but at compile time).
**Your snippet with struct and tupple:**
```d
import std;
struct MyVariables
{
int age, phone, country;
}
void main()
{
enum sep = ", ";
enum str = "21, 3149474, 90";
enum arr = str.split(sep)
.map!(x => x.to!int)
.array//*/
;
alias MrSmith = AliasSeq!(arr[0],
arr[1],
arr[2]);
auto mv = MyVariables(MrSmith);
assert(mv == MyVariables(21, 3149474, 90));
}
```
and **worksheet example:**
```d
import std;
struct DATA(string str, T, size_t s)
{
enum title = str;
T[s] data;
}
void main()
{
alias Columns = AliasSeq!("Stock Name", "PN Codes", "P.Prices");
alias T = AliasSeq!(string, int, double);
alias Items = AliasSeq!(4, 8, 8);
staticMapN!(3, DATA, Columns, T, Items) worksheet;
// inputs first column:
worksheet[0].data = ["capacitor", "transistor", "resistor",
"varistor"];
// prints column titles:
foreach(s; worksheet)
s.title.writef!"%14s";
"=".repeat(42).writefln!"\n%-(%s%)";
// prints first column:
foreach(name; worksheet[0].data)
name.writefln!"%14s";
//...
"=".repeat(42).writefln!"%-(%s%)";
}/* Prints:
Stock Name PN Codes P.Prices
==
capacitor
transistor
resistor
varistor
==
*/
```
By the way, in order for the above code snippet to work, the
staticMapN() template implemented by Ali Çehreli, which is not in
the standard library, is needed:
```d
template staticMapN(size_t N, alias fun, args...)
if(args.length % N == 0)
{
alias staticMapN = AliasSeq!();
static foreach (i; 0..args.length / N)
{
static if (N == 1)
{
staticMapN = AliasSeq!(staticMapN, fun!(
args[i]
));}
else static if (N == 2)
{
staticMapN = AliasSeq!(staticMapN, fun!(
args[0..$ / N][i],
args[$ / N..($ / N) * 2][i]
));}
else static if (N == 3)
{
staticMapN = AliasSeq!(staticMapN, fun!(
args[0..$ / N][i],
args[$ / N..($ / N) * 2][i],
args[($ / N) * 2..($ / N) * 3][i]
));}
}
}
```
SDB@79
Re: how to assign multiple variables at once by unpacking array?
On Sunday, 8 October 2023 at 07:45:56 UTC, Andrea Fontana wrote:
On Sunday, 8 October 2023 at 07:44:04 UTC, Andrea Fontana wrote:
```
int a,b,c;
"1,2,3"
.splitter(',')
.zip(only(, , ))
.each!(x => *x[1] = x[0].to!int);
writeln(a, b, c);
```
or:
```
int a,b,c;
only(, , )
.zip("1,2,3".splitter(','))
.each!(x => *x[0] = x[1].to!int);
writeln(a, b, c);
```
Nice.
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 17:23:40 UTC, ryuukk_ wrote: there was a DIP for tuple/deconstruction prior to that question, sadly nothing came out of it, I don't think it was formally submitted. and now the language is frozen... The DIP process is temporarily suspended, it may be modified. The priority for (most of) this year is fixing bugs. However language changes do happen, e.g. just this weekend switch can now declare a variable in the condition. Atila is working on a DIP for editions.
Re: how to assign multiple variables at once by unpacking array?
On Sunday, 8 October 2023 at 07:44:04 UTC, Andrea Fontana wrote:
```
int a,b,c;
"1,2,3"
.splitter(',')
.zip(only(, , ))
.each!(x => *x[1] = x[0].to!int);
writeln(a, b, c);
```
or:
```
int a,b,c;
only(, , )
.zip("1,2,3".splitter(','))
.each!(x => *x[0] = x[1].to!int);
writeln(a, b, c);
```
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
Is there a better way (since 2017)?
Ranges for the win!
```
int a,b,c;
"1,2,3"
.splitter(',')
.zip(only(, , ))
.each!(x => *x[1] = x[0].to!int);
writeln(a, b, c);
```
Re: how to assign multiple variables at once by unpacking array?
On Saturday, October 7, 2023 1:31:45 AM MDT mw via Digitalmars-d-learn wrote:
> https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-mult
> iple-variable-at-once-with-dlang
>
> How to do this Python code in D:
>
> ```
>
> >>> s = "1 2 3"
> >>> A,B,C = map(int, s.split(" "))
> >>> A,B,C
>
> (1, 2, 3)
>
> ```
>
> Is there a better way (since 2017)?
This is the sort of feature that you're much more likely to see in a
dynamically typed language than a statically typed one, and you will almost
certainly never see it in D.
The problem is that the compiler needs to be able to verify that the types
match, and when the return type of a function is a dynamic array such as
int[], it has no way of knowing how many elements the array has and
therefore can't verify at compile time that the assignment will work. At
best, it could add a runtime check to verify that the number of elements
match, but that's not the sort of thing that you typically do with a
statically typed language.
This is in stark contrast to a dynamically typed language where such things
are often done, because everything is already being checked at runtime
anyway, and checking whether the number of elements match then isn't really
any different from checking that the actual types of the variables match
what you're trying to do with them. But statically typed languages expect to
be able to do all of those kinds of checks at compile time, which means that
they're typically not going to do something like convert an array of
arbitrary length to a set of variables like that.
Now, what D might gain the ability to do at some point is to return
language-defined tuples, meaning that you'd be able to do something like
(int, string, float) foo(string bar, int baz)
{
...
return (i * 2, "hello", 2.7 * x);
}
and
(a, b, c) = foo("whatever", 42);
This would work with a statically typed language, because the types are all
known at compile time.
However, while there are some benefits to being able to do this, the
response by many programmers from statically typed languages is that it's
cleaner to create a struct for this sort of thing, since then the values are
contained together, and they have names associated with them (since they'll
be member variables of the struct). So, while some programmers definitely
want tuple types to be built into D, a number of others don't like the idea.
As such, it's an open question whether we'll ever have such tuples in D.
What we do currently have is Tuple and tuple in std.typecons. Tuple allows
you to create a struct with a given set of fields without explicitly
declaring it. e.g.
alias Foo = Tuple!(string, "name", int, "x", int, "y");
foo = Foo("Bob", 12, 22);
assert(foo.name == "Bob");
assert(foo.x == 12);
assert(foo.y == 22);
assert(foo[0] == "Bob");
assert(foo[1] == 12);
assert(foo[2] == 22);
and tuple allows you to create such a struct (without the field names)
simply by calling a function. e.g.
auto foo = tuple("Bob", 12, 22);
assert(foo[0] == "Bob");
assert(foo[1] == 12);
assert(foo[2] == 22);
So, it becomes possible to create a new struct type to return from a
function simply by calling tuple. e.g.
auto doStuff(string str, float f)
{
...
return tuple(x, str[i .. $]);
}
And thanks to some magic in Tuple, you even get unpacking of a sort by using
AliasSeq. E.G.
AliasSeq!(a, b) = doStuff(bar, 2.7);
So, for many programmers, Tuple and tuple from std.typecons are good enough,
and whether we ever get tuples added to the language will largely depend on
whether anyone can come up with a proposal for them that convinces Walter
and Atila that they're worth adding.
Either way, the unpacking of dynamic arrays likely stands no chance
whatsoever of ever being added, because it would require runtime checks to
determine whether the unpacking was valid.
- Jonathan M Davis
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 17:23:40 UTC, ryuukk_ wrote:
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
Is there a better way (since 2017)?
You can't, there was a DIP for tuple/deconstruction prior to
that question, sadly nothing came out of it, and now the
language is frozen...
Priorities are sadly outside of the language these days..
Tuple DIP in question:
https://github.com/tgehr/DIPs/blob/tuple-syntax/DIPs/DIP1xxx-tg.md
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
Is there a better way (since 2017)?
You can't, there was a DIP for tuple/deconstruction prior to that
question, sadly nothing came out of it, and now the language is
frozen...
Priorities are sadly outside of the language these days..
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 16:12:47 UTC, mw wrote: Interesting: in terms of easy of coding, clarity and future maintenance, which one is superior? There is no superior languages. They can successfully co-exist and play in different areas. The one liner in Python, or your "solution" with dozen lines of code? BTW, is that a solution at all? Did it achieved what the original goal asked in the OP question? AFAIK No, D doesn’t have this feature (still). But you can write some kind of workaround code that will do similar thing, but not exactly what you want. Probably solution in SO is the closest one.
Re: how to assign multiple variables at once by unpacking array?
Interesting: in terms of easy of coding, clarity and future
maintenance, which one is superior?
The one liner in Python, or your "solution" with dozen lines of
code? BTW, is that a solution at all? Did it achieved what the
original goal asked in the OP question?
So, who should learn from whom?
On Saturday, 7 October 2023 at 12:01:07 UTC, Salih Dincer wrote:
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
Is there a better way (since 2017)?
My words to those who come from Python:
If you are making money from Python, please stay there, but if
you want to learn new things and a modern language, "Welcome to
D"
and please use Tuples :)
```d
import std.typecons, std.stdio;
struct DICT(C, F, S)
{
S[C] dict;
C[F] freq;
void opAssign(Tuple!(C, F, S) chr) {
dict[chr[0]] = chr[2];
freq[chr[1]] = chr[0];
}
string toString() const
{
import std.array : appender;
import std.algorithm : sort;
import std.format : formattedWrite;
auto r = appender!string;
foreach(f; freq.keys.sort!"a>b") {
auto key = freq[f];
r.formattedWrite("(%c) %s, %.1f\n",
key, dict[key], f);
}
return r.data;
}
}
void main()
{
alias index = char;
alias rank = float;
alias name = string;
alias Dict = DICT!(index, rank, name);
alias chr = Tuple!(index, rank, name);
auto chrs = [ chr(44, 61.3, "Comma"),
chr(34, 26.7, "Doublequote"),
chr(39, 24.3, "Apostrophe"),
chr(45, 15.3, "Hyphen"),
chr(63, 5.6, "Question"),
chr(58, 3.4, "Colon"),
chr(33, 3.3, "Exclamation"),
chr(59, 3.2, "Semicolon")
];
Dict enDict;
foreach(tup; chrs) //multiple insertion
enDict = tup;
writeln("Frequency distributions of punctuation marks used in
English: ");
enDict = chr(46, 65.3, "Dot"); // single insertion
enDict.writeln;
}
```
SDB@79
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
Is there a better way (since 2017)?
My words to those who come from Python:
If you are making money from Python, please stay there, but if
you want to learn new things and a modern language, "Welcome to D"
and please use Tuples :)
```d
import std.typecons, std.stdio;
struct DICT(C, F, S)
{
S[C] dict;
C[F] freq;
void opAssign(Tuple!(C, F, S) chr) {
dict[chr[0]] = chr[2];
freq[chr[1]] = chr[0];
}
string toString() const
{
import std.array : appender;
import std.algorithm : sort;
import std.format : formattedWrite;
auto r = appender!string;
foreach(f; freq.keys.sort!"a>b") {
auto key = freq[f];
r.formattedWrite("(%c) %s, %.1f\n",
key, dict[key], f);
}
return r.data;
}
}
void main()
{
alias index = char;
alias rank = float;
alias name = string;
alias Dict = DICT!(index, rank, name);
alias chr = Tuple!(index, rank, name);
auto chrs = [ chr(44, 61.3, "Comma"),
chr(34, 26.7, "Doublequote"),
chr(39, 24.3, "Apostrophe"),
chr(45, 15.3, "Hyphen"),
chr(63, 5.6, "Question"),
chr(58, 3.4, "Colon"),
chr(33, 3.3, "Exclamation"),
chr(59, 3.2, "Semicolon")
];
Dict enDict;
foreach(tup; chrs) //multiple insertion
enDict = tup;
writeln("Frequency distributions of punctuation marks used in
English: ");
enDict = chr(46, 65.3, "Dot"); // single insertion
enDict.writeln;
}
```
SDB@79
Re: how to assign multiple variables at once by unpacking array?
On Saturday, 7 October 2023 at 07:31:45 UTC, mw wrote:
https://stackoverflow.com/questions/47046850/is-there-any-way-to-assign-multiple-variable-at-once-with-dlang
How to do this Python code in D:
```
s = "1 2 3"
A,B,C = map(int, s.split(" "))
A,B,C
(1, 2, 3)
```
Is there a better way (since 2017)?
That functionality exists, you just have to put things in
different places, but there are no more keystrokes:
```
import std;
void main() {
int x;
double y;
string z;
foo(x, y, z);
writeln(x);
writeln(y);
writeln(z);
}
void foo(ref int x, ref double y, ref string z) {
x = 4;
y = 2.6;
z = "hello world";
}
```
Maybe there is an argument for `x, y, z = foo();` but it's not
that it's easier to read or write. If the goal is to not have to
specify the types of the variables, it's hard for me to see the
advantage of
```
auto x, y, z = foo();
```
over returning a struct.
