Finding the distance between two points

[Lord Uber Dowzen]

Hi, I was just wondering (to help in the learning process of Lua) if someone could look over this code I spent 10 minutes making to find the distance between two sets of x,y coordinates:

Code: Select all

function find_distance(xorigin, yorigin, xdestination, ydestination)
  if xorigin > xdestination then
   xdistance = xorigin - xdestination
  elseif xorigin < xdestination then
   xdistance = xdestination - xorigin
  end
  
  if yorigin > ydestination then
   ydistance = yorigin - ydestination
  elseif yorigin < ydestination then
   ydistance = ydestination - yorigin
  end
  
  return distance = sqrt(xdistance ^ 2 + ydistance ^ 2)
  end

I know it's terrible, but in which respects? Thanks

[ivan]

How about just:

Code: Select all

function distance ( x1, y1, x2, y2 )
  local dx = x1 - x2
  local dy = y1 - y2
  return math.sqrt ( dx * dx + dy * dy )
end

Negative * a negative = positive so the first part of your code is not necessary.
Also, ^2 is considered slower to execute than x * x
Lastly, xdistance in your function is global - probably a better idea to make it a local variable.

[Jasoco]

This is what I use. One line.

Code: Select all

function distanceFrom(x1,y1,x2,y2) return math.sqrt((x2 - x1) ^ 2 + (y2 - y1) ^ 2) end

[Lord Uber Dowzen]

Ah, yes, those both make a lot more sense than mine. I think my problem might have been that I came up with the initial concept for the function while away from my computer and did all the test calculations in my head, then when I was at my computer spent 10 minutes typing it all out so that probably accounts for my solution being overly complicated and not working. Thanks for those.

Expect a lot more dumb questions over the coming weeks...

[zac352]

Code: Select all

(p1-p2).magnitude

Look how easy it is with vectors.

[ivan]

Code: Select all

(p1-p2).magnitude

Look how easy it is with vectors.

How would that work? Lua doesn't have operator overloading although might be libs for it I think.

[vrld]

How would that work?

With metatables. You can define a function that gets called whenever you try to access a table-field that is not there (for example "magnitude").

Lua doesn't have operator overloading

Not entirely true. You can overload some arithmetic (i.e. +, -, *, /) and relational (i.e. <, <=, ==).

in zac352's example, (p1-p2) invokes the __sub-metamethod, which returns a table with the (self-defined) vector-metatable. magnitude may be a field of the table, which gets set at invoking the __sub metamethod or be an example of the __index metamethod.

Metatables are a very powerful mechanism in Lua. If you are interested go ahead and read the section about methamethods in the Programming in Lua book.
For an example of how to use them in a vector class, either search the forum for zac's vector class (which he posted multiple times), or have a look at mine at github.

[zac352]

That's an awesome explanation.
But yeah, my vectors can be indexed several ways:
r,g,b,a,x,y,z,w,1,2,3,4,5,6,7...
magnitude,<insert lots of functions here>...

[arquivista]

Can't A-Star pathfinding Love's version help you?
http://love2d.org/forums/viewtopic.php?f=5&t=1797

[mawg]

Sorry to sound dumb, back what units are these functions returning? Km? Miles? Nautical miles? Metres, or what?

And does anyoen know of a LUA version of Vincenty's formula? See http://www.delphiforfun.org/Programs/Ma ... stance.htm