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digitalmars.D.learn - Obsecure problem 2

reply pascal111 <judas.the.messiah.111 gmail.com> writes:
I tried to make a template that receive lambda expression to 
apply it on a given range the user specifies, but I found 
non-understood problem:


'''D

module main;

import std.stdio;
import std.functional;


     template foo(alias predicate)
     if (is(typeof(unaryFun!predicate)))
     {
         alias notfunny=unaryFun!predicate;

auto foo(Range)(Range range) if (isInputRange!(Unqual!Range)){

         foreach(x; range)
             notfunny(x);

         return range;}


}


int main()
{

         int[] lolo = [12, 66, 654, -98, 54];

         lolo.foo(a=>a+2);

         lolo.writeln;

         return 0;
     }
     '''
Aug 03 2022
parent reply jfondren <julian.fondren gmail.com> writes:
On Wednesday, 3 August 2022 at 16:59:53 UTC, pascal111 wrote:

 I tried to make a template that receive lambda expression to 
 apply it on a given range the user specifies, but I found 
 non-understood problem:


Compare with:

```D
auto foo(Range)(Range range) { // remove isInputRange!T test
...
          lolo.foo!(a=>a+2); // call foo!(f)() rather than 
foo!()(f)
```
Aug 03 2022
parent reply pascal111 <judas.the.messiah.111 gmail.com> writes:
On Wednesday, 3 August 2022 at 17:09:11 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 16:59:53 UTC, pascal111 wrote:

 I tried to make a template that receive lambda expression to 
 apply it on a given range the user specifies, but I found 
 non-understood problem:


 Compare with:

 ```D
 auto foo(Range)(Range range) { // remove isInputRange!T test
 ...
          lolo.foo!(a=>a+2); // call foo!(f)() rather than 
 foo!()(f)
 ```


Ok! the program ran and gives "[12, 66, 654, -98, 54]" without 
adding "2" as I requested!! why?!
Aug 03 2022
parent reply jfondren <julian.fondren gmail.com> writes:
On Wednesday, 3 August 2022 at 17:33:40 UTC, pascal111 wrote:

 On Wednesday, 3 August 2022 at 17:09:11 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 16:59:53 UTC, pascal111 wrote:

 I tried to make a template that receive lambda expression to 
 apply it on a given range the user specifies, but I found 
 non-understood problem:


 Compare with:

 ```D
 auto foo(Range)(Range range) { // remove isInputRange!T test
 ...
          lolo.foo!(a=>a+2); // call foo!(f)() rather than 
 foo!()(f)
 ```


 Ok! the program ran and gives "[12, 66, 654, -98, 54]" without 
 adding "2" as I requested!! why?!


You call notfunny(x) in a loop, discarding the result of the 
addition.
Aug 03 2022
parent reply pascal111 <judas.the.messiah.111 gmail.com> writes:
On Wednesday, 3 August 2022 at 18:25:50 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 17:33:40 UTC, pascal111 wrote:

 On Wednesday, 3 August 2022 at 17:09:11 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 16:59:53 UTC, pascal111 wrote:

 I tried to make a template that receive lambda expression to 
 apply it on a given range the user specifies, but I found 
 non-understood problem:


 Compare with:

 ```D
 auto foo(Range)(Range range) { // remove isInputRange!T test
 ...
          lolo.foo!(a=>a+2); // call foo!(f)() rather than 
 foo!()(f)
 ```


 Ok! the program ran and gives "[12, 66, 654, -98, 54]" without 
 adding "2" as I requested!! why?!


 You call notfunny(x) in a loop, discarding the result of the 
 addition.


I changed it to "x=notfunny(x);" and has the same result.
Aug 03 2022
parent reply jfondren <julian.fondren gmail.com> writes:
On Wednesday, 3 August 2022 at 18:33:37 UTC, pascal111 wrote:

 I changed it to "x=notfunny(x);" and has the same result.


Now you are changing the value of the temporary loop variable 
that is still immediately discarded afterwards.

You should return a new range that has the values you want, not 
try to mutate the range you're given.
Aug 03 2022
parent reply pascal111 <judas.the.messiah.111 gmail.com> writes:
On Wednesday, 3 August 2022 at 18:53:35 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 18:33:37 UTC, pascal111 wrote:

 I changed it to "x=notfunny(x);" and has the same result.


 Now you are changing the value of the temporary loop variable 
 that is still immediately discarded afterwards.

 You should return a new range that has the values you want, not 
 try to mutate the range you're given.


You should provide a code instead of describing my mistakes!!
Aug 03 2022
next sibling parent reply Ruby The Roobster <rubytheroobster yandex.com> writes:
On Wednesday, 3 August 2022 at 19:11:51 UTC, pascal111 wrote:

 On Wednesday, 3 August 2022 at 18:53:35 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 18:33:37 UTC, pascal111 wrote:

 I changed it to "x=notfunny(x);" and has the same result.


 Now you are changing the value of the temporary loop variable 
 that is still immediately discarded afterwards.

 You should return a new range that has the values you want, 
 not try to mutate the range you're given.


 You should provide a code instead of describing my mistakes!!


No need for a code.  See, there is a keyword called `ref`, that 
can be used both in function parameters and in foreach loops, and 
it is the equivalent of a pointer to the type specified, but it 
is automatically dereferenced. As arrays are themselves reference 
types, you don't need to change the parameter from `Range range` 
to `ref Range range`.

On the other hand, `int` in it of itself isn't a reference type, 
meaning that to modify the actual value, you need to change the 
foreach loop to be:

```d
foreach(ref x; range)
     x = notfunny(x);
```

Proof:  I tested it, and it works.
Aug 03 2022
parent pascal111 <judas.the.messiah.111 gmail.com> writes:
On Wednesday, 3 August 2022 at 21:35:29 UTC, Ruby The Roobster 
wrote:

 On Wednesday, 3 August 2022 at 19:11:51 UTC, pascal111 wrote:

 [...]


 No need for a code.  See, there is a keyword called `ref`, that 
 can be used both in function parameters and in foreach loops, 
 and it is the equivalent of a pointer to the type specified, 
 but it is automatically dereferenced. As arrays are themselves 
 reference types, you don't need to change the parameter from 
 `Range range` to `ref Range range`.

 On the other hand, `int` in it of itself isn't a reference 
 type, meaning that to modify the actual value, you need to 
 change the foreach loop to be:

 ```d
 foreach(ref x; range)
     x = notfunny(x);
 ```

 Proof:  I tested it, and it works.


It works!
Aug 04 2022
prev sibling parent reply jfondren <julian.fondren gmail.com> writes:
On Wednesday, 3 August 2022 at 19:11:51 UTC, pascal111 wrote:

 On Wednesday, 3 August 2022 at 18:53:35 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 18:33:37 UTC, pascal111 wrote:

 I changed it to "x=notfunny(x);" and has the same result.


 Now you are changing the value of the temporary loop variable 
 that is still immediately discarded afterwards.

 You should return a new range that has the values you want, 
 not try to mutate the range you're given.


 You should provide a code instead of describing my mistakes!!


I agree to the extent that I wanted to do so, and might still 
have if a delivery hadn't interrupted by post. But one way to 
encourage others to put in more effort is put in effort yourself. 
Here you have your first post, where you didn't use 'preview' to 
check that your formatting worked, or review when linked Markdown 
guide to see what the problem was when it didn't work. In that 
post you also didn't describe what the problem was or ask any 
question, or say what you wanted to accomplish. It wasn't until 
your second post that it was even clear to me that you wanted to 
reimplement `map`. I thought the addition was a placeholder. The 
thread title is also a placeholder title for a thread.

In your second and third posts, it didn't come across at all that 
you'd put much thought into your questions before asking them. 
The second post--just look at your code. There's a function call 
and data going nowhere. That should be pretty easy to see. For 
the third post, if you thought that 'foreach' aliased the members 
of a range to the degree that simple assignment would update the 
range, you could test that thought in an instant:

```
$ rdmd --eval 'auto a = [1, 2, 3]; foreach (n; a) n = 0; 
writeln(a)'
[1, 2, 3]
```

It seemed to me that with these quickly asked questions that 
you're treating the thread like an ongoing interpersonal dialogue 
where questions are cheap. That's fine, really. I didn't mind 
that you didn't put in a lot of effort because it also didn't 
require a lot of effort to help you. But if you're going to 
complain about it, at least put the effort into asking for what 
you want at the outset.

The last thing that discouraged me from writing a full example is 
that I wasn't sure that I'd really be helping you. If you're 
trying to do this, don't you have a range tutorial open? That 
should already be pretty well-written already. Now that I've 
helped you with some incidental problems you should get back to 
the tutorial and see where you got things mixed up. My guess was 
that you didn't realize you should be returning a new range, and 
not trying to modify the range you were given.

If you don't have a tutorial open, the first link at 
https://dlang.org/phobos/std_range.html is pretty good: 
http://ddili.org/ders/d.en/ranges.html You won't find the exact 
thing you want to do, but if you put that aside and thoroughly go 
through that chapter, the only way you won't be able to do what 
you want to do is if you get caught up again by some other more 
fundamental issue with D. The worst likely outcome is that you'll 
get something that works but that isn't a great way to do it. You 
should post that code here for feedback.
Aug 03 2022
parent reply pascal111 <judas.the.messiah.111 gmail.com> writes:
On Wednesday, 3 August 2022 at 21:37:50 UTC, jfondren wrote:

 On Wednesday, 3 August 2022 at 19:11:51 UTC, pascal111 wrote:

 [...]


 I agree to the extent that I wanted to do so, and might still 
 have if a delivery hadn't interrupted by post. But one way to 
 encourage others to put in more effort is put in effort 
 yourself. Here you have your first post, where you didn't use 
 'preview' to check that your formatting worked, or review when 
 linked Markdown guide to see what the problem was when it 
 didn't work. In that post you also didn't describe what the 
 problem was or ask any question, or say what you wanted to 
 accomplish. It wasn't until your second post that it was even 
 clear to me that you wanted to reimplement `map`. I thought the 
 addition was a placeholder. The thread title is also a 
 placeholder title for a thread.

 [...]


I don't see my post.
Aug 04 2022
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 8/4/22 00:57, pascal111 wrote:


 I don't see my post.


Some posts are blocked by the spam filter. (Apparently, your message did 
not have a sender name and cannot be passed because of that.)

Ali
Aug 04 2022