       Re: plotting fourier amplitude versus frequency

• To: mathgroup at smc.vnet.net
• Subject: [mg80214] Re: [mg80198] plotting fourier amplitude versus frequency
• From: Sseziwa Mukasa <mukasa at jeol.com>
• Date: Wed, 15 Aug 2007 04:10:33 -0400 (EDT)
• References: <200708141115.HAA10101@smc.vnet.net>

```On Aug 14, 2007, at 7:15 AM, Indrani Sinha wrote:

> Hi,
>
> I am trying to extract frequencies from a data set of
> length versus time.
>
> Dataset:
> time   bondlength
>
> I am using the following script:
> plain=N[Transpose[data][]];
> f=Fourier[plain];
> s=OpenWrite["fourier_tmp.dat"];
> WriteString[s,"#"];
> Do[Write[s,Abs[f[[i]]]],{i,1,Length[f]}];
> Close[s];
> !sed 's/*\^/e/g' fourier_tmp.dat > cfourier.dat
> Quit
>
> This gives me an output file cfourier.dat with the
> fourier amplitude values, but how do I get the
> corresponding frequencies?

The times are the first elements of data, presumably your data is
sampled equispaced or the fourier analysis would make no sense so you
can get the period as:

period = data[[2,1]] - data[[1,1]]

0..(Length[f]/2-1) / (Length[f] period)

for the first Length[f] / 2 points and

-1/(2 period)..-1/(Length[f] period)

for the last Length[f]/2 poiints.

>
> Also the first line in the output has a number; what
> does that signify?

I have no idea, your script outputs a # on the first line of the
output file with the command WriteString[s,"#"].

Regards,

Ssezi

```

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