[erlang-questions] Trying to learn the Erlang Way

Daniel Goertzen <>
Fri Feb 7 15:49:37 CET 2014


Regarding #2

You are using both "if" and "case" expressions for the same situation:

  case coordinateIsGreaterThan(R, Head) of
    true -> cull(C, R, Culled, Tail);
    false -> cullOnMagnitude(C, R, D, Culled, Head, Tail)
  end
...
  if M > R -> cull(C, R, Culled, Tail);
     true -> cull(C, R, [Head | Culled], Tail)
  end

Many people find "case" easier to understand and less error prone than
"if", and in this situation you can easily use "case" instead:

  case M > R of
     true  -> cull(C, R, Culled, Tail);
     false -> cull(C, R, [Head | Culled], Tail)
  end


"if" does have its uses, but "case" fits my needs 99% of the time.

Other than that I think your code looks fine.

Cheers,
Dan.





On Fri, Feb 7, 2014 at 1:25 AM, kraythe . <> wrote:

> I am a newbie to erlang so please excuse the newbie questions. To learn
> the language I have a use case, simply to take a vector C which defines the
> center of a sphere with radius R and then cull a list of vectors and return
> only the vectors in the sphere. A simple math problem the most efficient
> method is to calculate the difference in C and a vector V and if any
> component is greater than R then the vector can't be in the sphere, if not
> then we have to do the expensive sort magnitude calculation.
>
> To this end I have created a simple set of functions:
> -module(vector3d).
> -author("rsimmons").
>
> %% API
> -export([subtract/2, magnitude/1, scale/2, cull/3]).
>
> %% Subtract the second vector from the first
> subtract({X1, Y1, Z1}, {X2, Y2, Z2}) -> {(X1 - X2), (Y1 - Y2), (Z1 - Z2)}.
>
> %% Compute the magnitude of the vector
> magnitude({X, Y, Z}) -> math:sqrt((X * X) + (Y * Y) + (Z * Z)).
>
> %% Determines if any coordinate in the given vector is bigger than the
> passed in value V.
> coordinateGreaterThan(V, {X, Y, Z}) when X > V; Y > V; Z > V -> true;
> coordinateGreaterThan(_, _) -> false.
>
> %% Culls the list of vectors X to only those that are in the sphere
> devined by vector C as a center and R as a radius.
> cull(C, R, Vectors) when is_number(R), is_tuple(C) -> cull(C, R, Vectors,
> []).
> cull(C, R, Culled, []) -> Culled;
> cull(C, R, Culled, [Head|Tail]) ->
>   D = subtract(C, Head),
>   case coordinateIsGreaterThan(R, Head) of
>     true -> cull(C, R, Culled, Tail);
>     false -> cullOnMagnitude(C, R, D, Culled, Head, Tail)
>   end.
>
> cullOnMagnitude(C, R, D, Culled, Head, Tail) when is_tuple(D),
> is_number(R) ->
>   M = magnitude(D),
>   if M > R -> cull(C, R, Culled, Tail);
>      true -> cull(C, R, [Head | Culled], Tail)
>   end.
>
> When I load these into a file and compile them I get the following result:
>
> 16> c(vector3d).
> vector3d.erl:53: function coordinateIsGreaterThan/2 undefined
> vector3d.erl:45: Warning: function coordinateGreaterThan/2 is unused
> vector3d.erl:50: Warning: variable 'C' is unused
> vector3d.erl:50: Warning: variable 'R' is unused
>
> Note that there are some other methods in this file so the line numbers
> will vary. My questions are:
>
> 1) Why is it saying my coordinateIsGreaterThan/2 method is undefined when
> I can see it here and then to say it is unused in the next line? That seems
> contradictory?
> 2) Is this in the direction of the "erlang way" to do things?
> 3) I understand the variables not being used but it seems odd to me from
> the java language world to not define the names for the variables. Is this
> normal in erlang?
>
> Thanks in advance.
>
> *Robert Simmons Jr. MSc.*
> *LinkedIn: **http://www.linkedin.com/pub/robert-simmons/40/852/a39
> <http://www.linkedin.com/pub/robert-simmons/40/852/a39>*
>
> _______________________________________________
> erlang-questions mailing list
> 
> http://erlang.org/mailman/listinfo/erlang-questions
>
>
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