# [Tutor] Help with a Conversion

Alan Gauld alan.gauld at yahoo.co.uk
Thu Jan 5 11:41:24 EST 2017

```On 05/01/17 13:29, S. P. Molnar wrote:

> Fortran II using punched paper tape.  Yes, am a rather elderly . .  .).

You are not the only one, there are at least 2 more of us on
this list that started in that era...

> short program to change frequency to wavelength for a plot of
> ultraviolet spectra.  I have attached a pdf of the program.

This is a text list so attachments usually get stripped off.
Please post the code in the body of the email using plain text
formatting.

> To change the frequency to wave length I did the following:
>
> p=1/1e7

p = 1e-7

> wave_length = p*np.array(frequency)

I don't really use numpy so don't know what that line does.
But assuming it applies the multiplication to each array element
I'd probably use:

wave_lengths = [p*f for f in frequencies]

or possibly

wave_lengths = map(lambda f: p*f, frequencies)

where frequencies was a tuple/list of frequency values.

However...

> (The relationship between wavelength and frequency is: wavelength =
> 1.0e7/frequency, where 1e7 is the speed of light)

That formula doesn't look like the one you use above
if my guess is correct. That would look like:

wave_length = [1e7/f for f in frequencies]

ie positive exponent and division instead of multiplication

> Apparently what 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?

Rearrange the equation and use division instead of multiplication
I think that by calculating 1/p you have made things much more
complicated - unless there is some subtle arithmetic magic going
on that I'm missing?

--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
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http://www.flickr.com/photos/alangauldphotos

```