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<Null> while building lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112170] <Null> while building lists
  • From: Alessandro <alexxx.magni at gmail.com>
  • Date: Fri, 3 Sep 2010 06:09:07 -0400 (EDT)

Hi group,
I'm sure I'll bang me on my head as soon as I will Press <Post
Message>,
but it's since yesterday that I'm struggling with this...

I fast defined this awkward function, to collect the {index,value} of
the maxima of a list of reals (a Fourier transform):

maxima[f_] := Block[{old = f[[1]], m = {}},
  For[i = 2, i < Length[f]/2, i++,
    If[f[[i]] > old && f[[i + 1]] < f[[i]],
     AppendTo[m, {i, f[[i]]}]];
    old = f[[i]]
    ]
   m
  ]

Problem is, what I get is e.g. this:

> maxima[f]
> {{14 Null, 10.8853 Null}, {21 Null, 3.34261 Null}, {28 Null, 1.47219 Null}}

I'm sure I do not have spaces anywhere (it happened something similar
due to this error, time ago),
so what's wrong?

thanks for any help...


alessandro





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