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Math exercise

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Posted by Gore (207 posts) Bio

Date Sun 22 Apr 2007 06:57 PM (UTC)

Message

<aliases>
  <alias
   script="evaluate"
   match="^eval (.*?)$"
   enabled="y"
   group="z"
   regexp="y"
   send_to="12"
   ignore_case="y"
   keep_evaluating="y"
   sequence="100"
  >
  <send>Note ('%1'..'='..%1)</send>
  </alias>
</aliases>

function evaluate (n,o,wc)
  Note (tostring(wc[1])..'='..wc[1])
end

when the alias eval 2+2 is used, that script performs the following output:


Quote:
2+2=4
2+2=2+2

The in alias script works correctly, 2+2=4, how do I make the function reference equivelent work correctly, and instead of displaying wc[1] as a string, it performs the evaluation?

Thanks in advance

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Posted by Nick Gammon Australia (23,133 posts) Bio Forum Administrator

Date Reply #1 on Sun 22 Apr 2007 09:29 PM (UTC)

Message

Inside the "send to script" the entire contents is sent to the script engine to be compiled and run.

Thus something like "Note (2+2)" will work.

However when you call the function you are simply displaying the string "2+2".

You can rework the function a bit to use loadstring (which is effectively what the "send to script" does). Here is an example:


function evaluate (n,o,wc)

  -- compile it
  local f, e = loadstring (wc [1])
  
  -- error? show it
  if not f then
    ColourNote ("brown", "", e)
    return
  end -- compile error
  
  -- no errors, execute it
  f ()
  
end

The function loadstring calls the Lua compiler, and returns a function, which is nil if an error occurs, and an error message, if there is an error.

Thus we can test for nil and show the error.

Afterwards, if no error, we can execute the function to cause it to run.

A simpler approach is to use assert, and let the built-in error handling kick in:


function evaluate (n,o,wc)
  
  assert (loadstring ( wc [1])) ()
  
end

However in either case we have a problem - you are supposed to supply an entire Lua chunk, not just an expression. For example:


eval 2+2 --> unexpected symbol near '2'

This is because "2+2" is not valid, any more than it would be inside a "send to script". This works however:


eval print (2+2) --> 4

I presume you are just playing here? What you end up doing depends on what you are really trying to do. For example, if you are always planning to evaluate a calculation, you could make it into an assignent:


function evaluate (n,o,wc)

  local calculation = "i = " .. wc [1]
  
  -- compile it
  local f, e = loadstring (calculation)
  
  -- error? show it
  if not f then
    ColourNote ("brown", "", e)
    return
  end -- compile error
  
  -- no errors, execute it
  f ()
  
  -- print results
  print (i)
  
end

Now I have put the results (2+2) into "i", which is then printed at the end.

- Nick Gammon

www.gammon.com.au, www.mushclient.com

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