Re: lazy variables
On 10/18/18 10:08 AM, aliak wrote:
On Wednesday, 17 October 2018 at 23:34:55 UTC, Paul Backus wrote:
On Wednesday, 17 October 2018 at 07:32:37 UTC, aliak wrote:
lazy S x = () {
// do some heavy stuff
}();
if (condition) {
func(x.y); // heavy stuff evaluated here
}
auto x = () {
// do some heavy stuff
};
if (condition) {
func(x().y); // heavy stuff evaluated here
}
That would do heavy stuff everytime i wanted to get y though right?
Yes, but that's what lazy variables do.
-Steve
Re: lazy variables
On Thursday, 18 October 2018 at 16:10:04 UTC, aliak wrote:
On Thursday, 18 October 2018 at 14:16:56 UTC, Simen Kjærås
wrote:
On Wednesday, 17 October 2018 at 07:32:37 UTC, aliak wrote:
Hi,
Is there any notion of lazy vars in D (i see that there're
parameters)?
What the language doesn't provide, it generally provides the
tools to make:
struct Lazy(T) {
T delegate() _payload;
this(lazy T t) {
_payload = () => t;
}
T get() {
return _payload();
}
alias get this;
}
int fun() {
n++;
return 2;
}
int n;
unittest {
Lazy!int a = 1 + fun();
assert(n == 0); // Ensure fun hasn't been called.
auto b = a + 2;
assert(b == 5); // Ensure calculation is correct.
assert(n == 1); // Ensure fun has been called.
}
--
Simen
yes! perfect! Thank you
With single eval:
struct Lazy(T) {
private T delegate() _payload;
private T _value;
private bool set = false;
this(lazy T t) {
_payload = () {
writeln("evaled");
return t;
};
}
@property T value() {
if (!set)
_value = _payload();
set = true;
return _value;
}
@property void value(T newValue) {
if (!set)
_value = _payload();
set = true;
_value = newValue;
}
alias value this;
}
Re: lazy variables
On Wednesday, 17 October 2018 at 07:32:37 UTC, aliak wrote:
Hi,
Is there any notion of lazy vars in D (i see that there're
parameters)?
What the language doesn't provide, it generally provides the
tools to make:
struct Lazy(T) {
T delegate() _payload;
this(lazy T t) {
_payload = () => t;
}
T get() {
return _payload();
}
alias get this;
}
int fun() {
n++;
return 2;
}
int n;
unittest {
Lazy!int a = 1 + fun();
assert(n == 0); // Ensure fun hasn't been called.
auto b = a + 2;
assert(b == 5); // Ensure calculation is correct.
assert(n == 1); // Ensure fun has been called.
}
--
Simen
Re: lazy variables
On Wednesday, 17 October 2018 at 20:32:40 UTC, Ali Çehreli wrote:
On 10/17/2018 12:32 AM, aliak wrote:
[...]
Not very clean but something like this:
import std.stdio;
struct LazyVar(alias exp) {
alias T = typeof(exp());
T value() {
static bool initialized = false;
static T val;
if (!initialized) {
val = exp();
initialized = true;
}
return val;
}
alias value this;
}
LazyVar!(() {
writeln("Doing heavy stuff");
return 42;
}) a;
void main() {
auto b = a.value;// Must specify .value or
int c = a; // must specify type of value (int).
// Otherwise, b and c have type
LazyVar!(...)
// Some more usage
b = c = a;
assert(b == 42);
assert(c == 42);
}
Ali
Well I guess that's certainly a way to go about it :) Not very
clean indeed though.
Re: lazy variables
On Thursday, 18 October 2018 at 14:16:56 UTC, Simen Kjærås wrote:
On Wednesday, 17 October 2018 at 07:32:37 UTC, aliak wrote:
Hi,
Is there any notion of lazy vars in D (i see that there're
parameters)?
What the language doesn't provide, it generally provides the
tools to make:
struct Lazy(T) {
T delegate() _payload;
this(lazy T t) {
_payload = () => t;
}
T get() {
return _payload();
}
alias get this;
}
int fun() {
n++;
return 2;
}
int n;
unittest {
Lazy!int a = 1 + fun();
assert(n == 0); // Ensure fun hasn't been called.
auto b = a + 2;
assert(b == 5); // Ensure calculation is correct.
assert(n == 1); // Ensure fun has been called.
}
--
Simen
yes! perfect! Thank you
Re: lazy variables
On Thursday, 18 October 2018 at 14:11:36 UTC, Steven Schveighoffer wrote: Yes, but that's what lazy variables do. -Steve Not in Swift at least...
Re: lazy variables
On Thursday, 18 October 2018 at 14:08:11 UTC, aliak wrote:
On Wednesday, 17 October 2018 at 23:34:55 UTC, Paul Backus
wrote:
auto x = () {
// do some heavy stuff
};
if (condition) {
func(x().y); // heavy stuff evaluated here
}
That would do heavy stuff everytime i wanted to get y though
right?
Yes. If that's a problem, you could use `std.functional.memoize`,
or Ali's solution.
Re: lazy variables
On 10/18/18 12:11 PM, aliak wrote: On Thursday, 18 October 2018 at 14:11:36 UTC, Steven Schveighoffer wrote: Yes, but that's what lazy variables do. Not in Swift at least... Apparently so (I have not used them before), but this is D! So you should be aware that lazy parameters work that way (the expression is evaluated each time the variable is used). In any case, you can certainly create a Swift-like lazy variable and I think the other responses probably show you the way. -Steve
Re: How to store unique values of array in another array
On Thursday, 18 October 2018 at 18:39:18 UTC, Samir wrote: which leads me to believe that the output of `uniq` is not necessarily another integer array. Right, it is actually a "range" - an object that generates the result on-demand, so it doesn't do work you don't actually need. If you don't need an array per se, you can just foreach over it and print them or further work on it without the cost of creating a new array. But, if you need to copy it into a new array, use `.array` at the end.
Why is dynamic array length required here?
I am working my way through the exercises in the "Programming in
D" tutorial (http://ddili.org/ders/d.en/arrays.html). Why is the
line assigning the length of the dynamic array required?
/*
Write a program that asks the user how many values will be
entered and then
reads all of them. Have the program sort the elements using
sort() and then
reverse the sorted elements using reverse().
*/
import std.stdio;
import std.algorithm;
void main() {
int noValues, i;
int[] myArray;
write("how many values would you like to enter? ");
readf(" %s", );
myArray.length = noValues; // I get a run-time error if I
comment this out
while (i < noValues) {
write("enter value #", i+1, " ");
readf(" %s", [i]);
++i;
}
sort(myArray);
writeln(myArray);
reverse(myArray);
writeln(myArray);
}
Without the line:
myArray.length = noValues;
I get the run-time error:
$ ./exArrays1_1
how many values would you like to enter? 5
core.exception.RangeError@exArrays1_1.d(12): Range violation
??:? _d_arrayboundsp [0x461772]
??:? _Dmain [0x44c284]
I would have thought that since this is a dynamic array, I don't
need to pre-assign its length.
Thanks
Re: Why is dynamic array length required here?
On Friday, 19 October 2018 at 02:04:37 UTC, Samir wrote:
I am working my way through the exercises in the "Programming
in D" tutorial (http://ddili.org/ders/d.en/arrays.html). Why
is the line assigning the length of the dynamic array required?
[...]
Without the line:
myArray.length = noValues;
I get the run-time error...
I would have thought that since this is a dynamic array, I
don't need to pre-assign its length.
Even though the array is dynamic, it doesn't have infinite
storage. So you either:
- preallocate required storage, like in that example code (note
that it's not the same as making a fixed-size ('static') array:
in your case you only learn what the required length is at
run-time, as opposed to static arrays where you must know it at
compile time)
- don't preallocate, but instead append new elements like this:
myArray ~= newValue;
The latter isn't a universally "good" advice: it may cause
reallocation every time you do that. That's why it's almost
always best to preallocate in advance if you do know the length,
or use something like std.array.appender that attempts to reduce
memory reallocation. I'm not familiar with Ali's tutorials, maybe
he talks about the appender in some future examples.
Re: Why is dynamic array length required here?
On 10/18/2018 07:04 PM, Samir wrote:
> myArray.length = noValues; // I get a run-time error if I comment
> this out
It's because the expression that reads the elements below is readf,
which reads on top of an existing element.
> while (i < noValues) {
> write("enter value #", i+1, " ");
> readf(" %s", [i]);
Aside: It may be possible to read like the following without taking the
address, as readf has been improved to take references since that code
was written:
readf(" %s", myArray[i]); // (not tested)
So, the answer to your question is, we need to set the length of the
array so that there are elements to read on top of. If D had function
templates like getResponse() that I use in the Templates chapter,
http://ddili.org/ders/d.en/templates.html
then the best thing to do would be to append the elements directly to
the array:
// No need to set .length beforehand
myArray ~= getResponse!int(/* ... */);
Ali
Re: How to store unique values of array in another array
On Thursday, 18 October 2018 at 18:53:06 UTC, Adam D. Ruppe wrote: But, if you need to copy it into a new array, use `.array` at the end. Thanks. That did the trick. But if I may, what is the difference between uniqueArray = uniq(sort(unsortedArray)).array; and uniqueArray = unsortedArray.sort().uniq.array; They both seem to work. Is one preferred over the other?
Re: Error: non-shared method core.sync.condition.Condition.notify is not callable using a shared object
On Thursday, October 18, 2018 4:50:18 AM MDT Paolo Invernizzi via Digitalmars-d-learn wrote: > There's a rational behind the fact that there's not a 'shared' > version of notify/wait method in Condition? > > Thanks, > Paolo The original author of the stuff in core.sync didn't want to update it for shared when shared was first introduced, because he didn't think that it was well enough defined yet (e.g. there was discussion over whether it should have write barriers or not). Some work has been done to improve shared support in there since then but not enough. shared _does_ still need need some rough edges sorted out (it's supposed to guarantee that you can't do any operations on it that isn't thread-safe, but while many operations are illegal, some which should be aren't, and some of the finer details likely need to be cleaner up). The basics of shared have been in place and working fine for nearly a decade, but because we've been waiting for the finer details to be finished, and it's never been a high enough priority for that to happen, stuff like core.sync has suffered. It doesn't help that the vast majority of D programs don't need to do much with shared, and those that D use higher level mechanisms such as std.concurrency or use something like the framework that vibe.d provides. However, as annoying as it is, with some extra casting, the pieces of core.sync that haven't been updated properly _can_ be used with shared. shared in general requires unsafe casting and always will. There really isn't any way around that, but the parts of core.sync that don't use shared properly will require that much more of it, unfortunately. There's no question though that the situation with core.sync needs to be fixed. Some of it has been worked on, but nowhere near enough. - Jonathan M Davis
Path.GetDirectoryName for D?
Are there a function like C#'s Path.GetDirectoryName() (https://docs.microsoft.com/en-us/dotnet/api/system.io.path.getdirectoryname?redirectedfrom=MSDN=netframework-4.7.2#System_IO_Path_GetDirectoryName_System_String_) in D standard library or some dub package? just checking if there's one, so that I don't reinvente the wheel...
Re: Path.GetDirectoryName for D?
On Thursday, 18 October 2018 at 19:54:59 UTC, Dr.No wrote: Are there a function like C#'s Path.GetDirectoryName() Looks the same as "dirName" from "import std.path" http://dpldocs.info/experimental-docs/std.path.dirName.1.html
How to store unique values of array in another array
What is the proper way to find the unique values of an array and
store them in another array? When I try:
import std.stdio: writeln;
import std.conv;
import std.algorithm;
void main() {
int[] unsortedArray = [5, 3, 8, 5, 2, 3, 0, 8];
int[] uniqueArray;
uniqueArray = uniq(sort(unsortedArray));
}
I get the compilation error:
sortSetUnique2.d(9): Error: cannot implicitly convert expression
uniq(sort(unsortedArray)) of type UniqResult!(binaryFun,
SortedRange!(int[], "a < b")) to int[]
which leads me to believe that the output of `uniq` is not
necessarily another integer array.
Thanks
Re: How to store unique values of array in another array
On Thursday, 18 October 2018 at 19:25:26 UTC, Adam D. Ruppe wrote: Which is better simply depends on which one reads better to you. Thanks again, Adam.
Re: How to store unique values of array in another array
On Thursday, 18 October 2018 at 19:19:53 UTC, Samir wrote: They both seem to work. Is one preferred over the other? No difference. The compile will just transform o.f into f(o) if it can. They both do the same thing and it is just a different way of writing it. Which is better simply depends on which one reads better to you.
Re: Why is dynamic array length required here?
On Friday, 19 October 2018 at 02:04:37 UTC, Samir wrote:
I would have thought that since this is a dynamic array, I
don't need to pre-assign its length.
Thanks
Just to expand on the previous answers, a dynamic array
declaration with no initializer is an empty array:
int[] arr;
assert(arr.length == 0);
No memory has been allocated to store any elements. Appending
elements will allocate memory. In fact, the runtime will allocate
more than needed so that it doesn't have to allocate on each
append.
arr ~= 1;
assert(arr.length == 1);
writeln(arr.capacity);
When the capacity is 0, the next append will trigger another
allocation. The reserve function can be used to allocate enough
memory for a number of elements, but the array will still be
empty:
int[] arr;
arr.reserve(100);
assert(arr.length == 0);
writeln(arr.capacity);
Now 100+ elements can be appended without triggering an
allocation.
Setting the length directly as Ali's example does means that not
only is the required memory allocated, but also that the array is
not empty.
int[] arr;
arr.length = 100;
assert(arr.length == 100);
writeln(arr.capacity);
And now the array can elements can be written to directly via the
index operator, and a pointer to each element can be referenced
as it is in the call to readf.
Note that it's still possible to write elements directly to a
dynamic array without the append operator after memory has been
reserved, but it completely bypasses the runtime.
void main() {
int[] myArray;
myArray.reserve(10);
foreach(i; 0 .. 10) {
*(myArray.ptr + i) = i;
}
assert(myArray == []);
assert(myArray.length == 0);
foreach(i; 0 .. 10) {
writeln(*(myArray.ptr + i));
}
}
This is essentially treating myArray.ptr as a C array, with no
bounds checking and no automatic memory management. Here be
dragons.
Re: Documents about ddoc? and markdown in ddoc?
On 18/10/2018 6:38 PM, dangbinghoo wrote: hi, Is there any other documents related about ddoc usage? the only thing I can find is: https://dlang.org/spec/ddoc.html#using_ddoc_to_generate_examples But I found it never mentioned something like $(LI a list item), is there a full ddoc document available? Default for html output: https://github.com/dlang/dmd/blob/master/res/default_ddoc_theme.ddoc And, is there any info about using markdown in ddoc? Not possible.
Re: Documents about ddoc? and markdown in ddoc?
On Thursday, 18 October 2018 at 05:59:36 UTC, rikki cattermole wrote: On 18/10/2018 6:38 PM, dangbinghoo wrote: hi, Is there any other documents related about ddoc usage? the only thing I can find is: https://dlang.org/spec/ddoc.html#using_ddoc_to_generate_examples But I found it never mentioned something like $(LI a list item), is there a full ddoc document available? Default for html output: https://github.com/dlang/dmd/blob/master/res/default_ddoc_theme.ddoc And, is there any info about using markdown in ddoc? Not possible. thanks for your reply. and sorry, I'm too careless for reading the documents https://dlang.org/spec/ddoc.html#using_ddoc_to_generate_examples the $(LI x y) is a macro which was mentioned.
Re: Who can stop it ? Help me,thank you.
On Wednesday, 17 October 2018 at 13:48:04 UTC, FrankLike wrote: Hi,teacher: I like D lang,when I download the soft from http://www.360totalsecurity.com/en/ Contact their support: supp...@360safe.com
Error: non-shared method core.sync.condition.Condition.notify is not callable using a shared object
There's a rational behind the fact that there's not a 'shared' version of notify/wait method in Condition? Thanks, Paolo
Re: Error: non-shared method core.sync.condition.Condition.notify is not callable using a shared object
On Thursday, 18 October 2018 at 10:50:18 UTC, Paolo Invernizzi wrote: There's a rational behind the fact that there's not a 'shared' version of notify/wait method in Condition? They were implemented before complete `shared` spec existed, which is any time between it's conception and now.
Re: Error: non-shared method core.sync.condition.Condition.notify is not callable using a shared object
On Thursday, 18 October 2018 at 10:50:18 UTC, Paolo Invernizzi wrote: There's a rational behind the fact that there's not a 'shared' version of notify/wait method in Condition? core.sync is pretty old, it was written in 2009 before default storage class for global variables became TLS.
Re: lazy variables
On Wednesday, 17 October 2018 at 23:34:55 UTC, Paul Backus wrote:
On Wednesday, 17 October 2018 at 07:32:37 UTC, aliak wrote:
lazy S x = () {
// do some heavy stuff
}();
if (condition) {
func(x.y); // heavy stuff evaluated here
}
auto x = () {
// do some heavy stuff
};
if (condition) {
func(x().y); // heavy stuff evaluated here
}
That would do heavy stuff everytime i wanted to get y though
right?
Re: lazy variables
On 10/17/18 3:32 AM, aliak wrote:
Hi,
Is there any notion of lazy vars in D (i see that there're parameters)?
i.e:
struct S {
//...
int y;
//...
}
/*
lazy S x = () {
// do some heavy stuff
}();
*/
auto x() { // do some heavy stuff
}
if (condition) {
func(x.y); // heavy stuff evaluated here
}
-Steve
