Re: Exercise at end of Ch. 56 of "Programming in D"
On Monday, 15 August 2022 at 02:59:59 UTC, Ali Çehreli wrote:
class Point {
int x;
int y;
Color color;
// ...
override size_t toHash() const {
return x + y;
}
}
The book is simply forgetting to show that function.
(Otherwise, I never put any code without testing.)
WARNING: Hash functions can be very tricky. I have a strong
feeling that adding two integers is not an efficient one.
Fortunately there is no need to invent our own hash function
here, because we can use the generic hashOf [1] from druntime:
override size_t toHash() const {
return hashOf(y, hashOf(x));
}
The second argument is optional, and is used when you want to
"accumulate" a single hash value from multiple input values.
[1] https://druntime.dpldocs.info/object.hashOf.1.html
Re: Exercise at end of Ch. 56 of "Programming in D"
On Monday, 15 August 2022 at 03:19:43 UTC, johntp wrote: Your solution worked. I guess it is a little unnatural to ignore the color. I tried overriding the toHash() of Point, but I don't know enough D to get it to work. I wonder if that could be a solution. Depends on what you're trying to do. Metadata unrelated to the value of the object i would ignore and not be part of hashing or comparisons. I've also done things for strings that held information like what original position in the array the data was (*for visually sorting testing*) and could yield me information while not interfering with the object/data in question. Though x+y as a hash seems terrible. I'd probably do ((x+1000)**2)+y (assuming x and y are both going to be generally small ensuring the hash for location is unique. Then point(2,1) and point(1,2) have different hashes. But I'm not familiar with the exercise in question.
Re: Recommendation for parallelism with nested for loops?
On 20.08.22 12:28, Christian Köstlin wrote:
On 19.08.22 03:49, Shriramana Sharma wrote:
Hello. I want to parallelize a computation which has two for loops,
one nested within another. All inner-loop-param+outer-loop-param
combinations can be computed independent of one another.
As I suspected,
[https://forum.dlang.org/post/xysyidbkjdinclmrx...@forum.dlang.org](this forum post) says that only one loop can be parallelized. Will it be an error or inefficient or useless if I try to do both?
Also, what is the best way to do parallelism in such a situation?
You could also do a custom range that makes a one-dimensional range (aka
iota out of your nested loops) and then process this with parallel.
Another way (more similar to Ali's solution) would be to write the
nested loops as you have them, but collect the parameters for the work
in an array or something and then process this array in parallel.
Kind regards,
Christian
Actually phobos brings already all that is required (especially the
cartesian product):
```d
import std;
void doSomething(int i, string s)
{
writeln("%s-%s".format(i, s));
}
void main()
{
foreach (t; cartesianProduct(iota(1, 4), ["abc", "def",
"ghi"]).parallel) {
doSomething(t.expand);
}
}
```
kind regards,
Christian
Re: Recommendation for parallelism with nested for loops?
On 19.08.22 03:49, Shriramana Sharma wrote: Hello. I want to parallelize a computation which has two for loops, one nested within another. All inner-loop-param+outer-loop-param combinations can be computed independent of one another. As I suspected, [https://forum.dlang.org/post/xysyidbkjdinclmrx...@forum.dlang.org](this forum post) says that only one loop can be parallelized. Will it be an error or inefficient or useless if I try to do both? Also, what is the best way to do parallelism in such a situation? You could also do a custom range that makes a one-dimensional range (aka iota out of your nested loops) and then process this with parallel. Another way (more similar to Ali's solution) would be to write the nested loops as you have them, but collect the parameters for the work in an array or something and then process this array in parallel. Kind regards, Christian
