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digitalmars.D.learn - newbie question: Can D do this?

reply clk <clk clksoft.com> writes:
Hello,
I'm new to this mailing list.  I'm trying to learn D  to eventually use 
it in production code.
I'm a little bit intimidated by the fact that the topics in the d-learn 
list look rather advanced to a newbie like me.
I have 3 fairly simple questions:

1) Does D support something like the javascript 1.8 destructuring 
assigment (multiple assigment in python):

[a, b] = [b, a];

2) D doesn't  seem to support the list comprehension syntax available in 
python and javascript.  Is this correct?

[f(x) for x in list if condition]

3) D's slice operator apparently doesn't allow the use of a stride other 
than unity as is allowed with fortran and matlab.  Is there a way to 
implement this feature so that

[1, 2, 3, 4, 5][0..$:2] would refer to [1, 3, 5], etc..., where 2 is the 
non unit stride.  Or is the find function from std.algorithm the only 
option to achieve the same behavior.

I find the 3 features above extremely convenient in every day coding.
Thanks,
-clk
Dec 19 2011
next sibling parent simendsjo <simendsjo gmail.com> writes:
On 19.12.2011 17:17, clk wrote:

 1) Does D support something like the javascript 1.8 destructuring
 assigment (multiple assigment in python):

 [a, b] = [b, a];


I don't think so, but you can do something like this with templates:
void swap(alias a, alias b)() {
     auto t = a;
     a = b;
     b = t;
}

     int a = 1, b = 2;
     swap!(a, b);
     assert(a == 2);
     assert(b == 1);



 2) D doesn't seem to support the list comprehension syntax available in
 python and javascript. Is this correct?

 [f(x) for x in list if condition]


Don't think so. You can use std.algorithm, but it's a bit harder to read:
     auto arr = [1,2,3,4,5,6];
     auto res = array(pipe!(filter!"a>3", map!"a*2")(arr));
     assert(res == [8,10,12]);
     // or
     auto res2 = array(map!"a*2"(filter!"a>3"(arr)));
     assert(res2 == [8,10,12]);


But I'm a newbie myself.
Dec 19 2011
prev sibling next sibling parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
On 12/19/2011 08:17 AM, clk wrote:


 I'm a little bit intimidated by the fact that the topics in the d-learn
 list look rather advanced to a newbie like me.


We need more newbie topics here! :)


 1) Does D support something like the javascript 1.8 destructuring
 assigment (multiple assigment in python):

 [a, b] = [b, a];


No multiple assignment like that. But useful approarches exist for most 
needs, like the swap that simendsjo has shown.


 2) D doesn't seem to support the list comprehension syntax available in
 python and javascript. Is this correct?

 [f(x) for x in list if condition]


List comprehension is not part of the language.

import std.algorithm;

void f(int x)
{}

bool condition(int x)
{
     return true;
}

void main()
{
     auto list = [ 0, 1, 2 ];
     map!f(filter!condition(list));
}

You can define f and condition within the body of main().

It is possible to use function literals as well:

import std.algorithm;

void main()
{
     auto list = [ 0, 1, 2 ];
     map!((x){
             /* ... this is f(x) ...*/
         })(filter!((x) {
                     return true; /* ... condition ... */
                 })(list));
}


 3) D's slice operator apparently doesn't allow the use of a stride other
 than unity as is allowed with fortran and matlab. Is there a way to
 implement this feature so that

 [1, 2, 3, 4, 5][0..$:2] would refer to [1, 3, 5], etc..., where 2 is the
 non unit stride. Or is the find function from std.algorithm the only
 option to achieve the same behavior.


std.range.stride does that:

import std.range;
// ...
stride([1, 2, 3, 4, 5], 2)


 I find the 3 features above extremely convenient in every day coding.
 Thanks,
 -clk


Ali
Dec 19 2011
prev sibling next sibling parent Kai Meyer <kai unixlords.com> writes:
On 12/19/2011 09:17 AM, clk wrote:

 Hello,
 I'm new to this mailing list. I'm trying to learn D to eventually use it
 in production code.
 I'm a little bit intimidated by the fact that the topics in the d-learn
 list look rather advanced to a newbie like me.
 I have 3 fairly simple questions:

 1) Does D support something like the javascript 1.8 destructuring
 assigment (multiple assigment in python):

 [a, b] = [b, a];


I would love multiple assignment like this, but it's tricky. But your 
usage isn't really multiple assignment as much as it is a swap. What I'd 
love is something like this:

[a, b, c] = [get_a(), get_b(), get_c()];

Or

[a, b, c] = [to!(int)(argv[1]), some_other_value, argv[4]);




 2) D doesn't seem to support the list comprehension syntax available in
 python and javascript. Is this correct?

 [f(x) for x in list if condition]


No, D's syntax is very C-ish. I don't expect syntax like this to ever 
show up (though what you are doing is possible with things like 
std.algorithm)


 3) D's slice operator apparently doesn't allow the use of a stride other
 than unity as is allowed with fortran and matlab. Is there a way to
 implement this feature so that

 [1, 2, 3, 4, 5][0..$:2] would refer to [1, 3, 5], etc..., where 2 is the
 non unit stride. Or is the find function from std.algorithm the only
 option to achieve the same behavior.


Ya, std.range, like Ali said.


 I find the 3 features above extremely convenient in every day coding.
 Thanks,
 -clk
Dec 19 2011
prev sibling parent reply =?utf-8?Q?Simen_Kj=C3=A6r=C3=A5s?= <simen.kjaras gmail.com> writes:
On Mon, 19 Dec 2011 17:17:43 +0100, clk <clk clksoft.com> wrote:


 Hello,
 I'm new to this mailing list.  I'm trying to learn D  to eventually use
 it in production code.
 I'm a little bit intimidated by the fact that the topics in the d-learn
 list look rather advanced to a newbie like me.
 I have 3 fairly simple questions:

 1) Does D support something like the javascript 1.8 destructuring
 assigment (multiple assigment in python):

 [a, b] = [b, a];



This, or something quite like it, was covered on Saturday in the thread
"Alias/Ref Tuples ?". This works (but is hardly as elegant as Python's
syntax:

import std.typetuple : TypeTuple;
import std.typecons : tuple;

TypeTuple!(a, b) = tuple(b,a);
Dec 19 2011
parent "Jakob Ovrum" <jakobovrum gmail.com> writes:
On Monday, 19 December 2011 at 19:01:10 UTC, Simen Kjærås wrote:

 import std.typetuple : TypeTuple;
 import std.typecons : tuple;

 TypeTuple!(a, b) = tuple(b,a);


There is a pull request implementing multiple variable 
declarations:
https://github.com/D-Programming-Language/dmd/pull/341

However, the right hand side must still be a tuple.
Dec 19 2011