# [Tutor] Help with a Conversion

S. P. Molnar s.molnar at sbcglobal.net
Thu Jan 5 13:31:06 EST 2017

```On 01/05/2017 01:10 PM, Peter Otten wrote:
> S. P. Molnar wrote:
>
>> I have just started attempting programming in Python and am using Spyder
>> with Python 3.5.2 on a Linux platform. (I first started programing in
>> Fortran II using punched paper tape.  Yes, am a rather elderly . .  .).
>>
>> I have bumbled through, what I foolishly thought was a simple problem, a
>> short program to change frequency to wavelength for a plot of
>> ultraviolet spectra.  I have attached a pdf of the program.
>>
>> During my attempt at programming I have printed results at various
>> stages.  Printing wavelength = [row[0] for row in data] gives me 25000
>> as the first frequency in the wavelength list (the corresponding
>> wavelength is 400).
>>
>> To change the frequency to wave length I did the following:
>>
>>
>> p=1/1e7
>> wave_length = p*np.array(frequency)
>>
>> (The relationship between wavelength and frequency is: wavelength =
>> 1.0e7/frequency, where 1e7 is the speed of light)
>>
>>
>> Apparently whhat I have managed to do is divide each element of the
>> frequency list by 1/1e7.
>>
>> What I want to do is divide 1e7 by each element of the freqquency list.
>>
>> How di I do this?
> Since you are using numpy anyway I'd put the frequencies into a numpy.array
> as soon as possible:
>
>>>> import numpy
>>>> frequencies = numpy.array([25000, 1250, 400])
> Because of numpy's "broadcasting" you can mix skalars and vectors as you
> already tried -- and with the right formula, lamda = c / nu, you get the
> correct result:
>
>>>> speed_of_light = 1e7
>>>> wavelengths = speed_of_light / frequencies
>>>> wavelengths
> array([   400.,   8000.,  25000.])
>
> The equivalent list comprehension in plain Python looks like this:
>
>>>> frequencies = [25000, 1250, 400]
>>>> wavelengths = [speed_of_light/freq for freq in frequencies]
>>>> wavelengths
> [400.0, 8000.0, 25000.0]
>
>> Please keep in mind that many, many hyears ago I learned the ole
>> arithmetic
> That hasn't changed and is honoured by numpy; you were probably confused by
> the new tool ;)
>
>> and an not trying to start a flame war.
>> Thanks in advance for the assistance tha I am sure will be most helpful.
>
>
>
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Somehow I KNEW THIS WAS THE LIWST TO Ask.

Thanks very much.

--
Stephen P. Molnar, Ph.D.	Life is a fuzzy set
www.Molecular-Modeling.net	Stochastic and multivariate
(614)312-7528 (c)
Skype:  smolnar1

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