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Re: Pattern Matching


Jeff wrote:
> Given a list of numbers, t = {0, -1, 0, -1, -2, -3, -4,-3,-4,-5}, I
> would like to extract those numbers that are less than or equal to the
> starting value by, for example, at least 2, but once the first (and
> each subsequent) match is found, I would the starting value to drop by
> 2.  So for the list t, the output would be {-2, -4}.
> 
> I can use Cases or map using If[ ] on the list to extract those
> elements that are less than or equal to -2, but I'm having difficutly
> in figuring out how to change the starting value to be applied to
> subsequent elements of the list.
> 
> Given my example, I could just extract all the multiples of 2 in the
> list, but I would like the starting value to change by a fixed
> percentage or combination of fixed number and percentage.
> 
> Thanks,
> Jeff

One way is to write a function that will alter arguments, and use it in 
the selection predicate.

t = {0, -1, 0, -1, -2, -3, -4,-3,-4,-5}

SetAttributes[f,HoldFirst]
f[s_,x_,diff_:-2] := If [x<=s+diff, s+=diff; True, False]

s = First[t];

In[15]:= Select[Rest[t], f[s,#]&]
Out[15]= {-2, -4}


Daniel Lichtblau
Wolfram Research


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