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Re: Re: What does & mean?

  • To: mathgroup at
  • Subject: [mg107131] Re: [mg107059] Re: What does & mean?
  • From: Canopus56 <canopus56 at>
  • Date: Wed, 3 Feb 2010 06:12:45 -0500 (EST)
  • References: <hk6d22$m61$> <>

On Feb 1, 4:14 am, Michael Knudsen <micknud... at> wrote:
> However, there is no explanation of how & works . . . 

I'm a beginner too.  This is how I interpreted the ampersand syntax. I also could not find an acceptable definition in the software documentation.  I got it from a separate dummies 

The ampersand is a way to force evaluation of a function dynamically on the fly.  It is used to execute one - time dynamic functions on-the-fly. 

Let' s look at a sine function applied to these data points. (Copy and paste each formula into your copy of Mathematica and evaluate.) 

data = Table[x, {x, (\[Pi]/8), \[Pi], (\[Pi]/8)}]

A conventional method would be to declare a traditional static called function. The := creates - what you might term in a scripting language like Javascript or VBA - a static called function.  This function is stored in memory and reused each time it is called.  It returns a computed result each time it is called. 

g[x_] := x*Sin[x]

A conventional method to call this static function stored in memory and to apply it to a set of points would be -

Table[g[data[[k]]], {k, 1, Length[data]}]

The Map function is another conventional way to apply the static function to the data points - 

Map[g, data]

Instead of a static call, the ampersand can be used to invoke evaluation of a function dynamically - the so called pure function - 

Map[g[#] &, data]

We also can evaluate the desired root function dynamically using ampersand, thus dispensing with the need to create a separate static called function in the first place - 

Map[#*Sin[#] &, data]

The above syntax can be condensed further into a highly symbolic form by using the symbolic invocation of Map function /@ -

#*Sin[#] & /@ data 

The above says - 

1) Take the dynamic function #*Sin[#] that I just defined on-the-fly;
2) Apply that dynamic function by mapping to each element in the list "data"; and,
3) When we are done, trash dynamic function #*Sin[#] because we do not need it anymore. 

The difference is the static stored function g[x_] := x*Sin[x] sits in memory taking up space. 

The dynamic pure function #*Sin[#] & is created dynamically and is automatically destroyed after it is used. 
 This frees up some memory for other tasks. 

Hope that helps. 

- Kurt 

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