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Topic: Delete elements from list..
Replies: 17   Last Post: Mar 8, 2011 5:37 AM

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Maarten van der Burgt

Posts: 10
Registered: 5/7/10
Re: Delete elements from list..
Posted: Feb 24, 2011 6:29 AM
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Thanks everybody who replied to my questions.

The real problem I have is just a bit more complex than my simplified
My list is in fact a numerical 2D list like mylist1 == {{x_0, y_0},
{x_1,y_1},... {x_i, y_i}, ...{x_N, y_N}}.
The xi are strictly increasing and the yi should be as well. Due to some
measurement errors it can happen that this is not the case. I simply
want to delete the {xi, yi} pairs where y_i <== y_i-1. That way I end up
with a list, mylist2, where also the y_i are strictly increasing.
(that way I can make an Interpolation[Reverse/@mylist2] in order to have
a function x_i(y_i)).

I have not had the time to study your answers in this view, but from a
first look and the variety of the answers it seems that there is
definitely something which should help.

Thanks for your help.


-----Original Message-----
From: Achilleas Lazarides []
Sent: Wednesday, 23 February, 2011 11:22
Subject: Re: Delete elements from list..

Do you really want the resulting list to only have increasing elements?
then something like

Module[{tmpl == mylist[[1]]},
Scan[(If[# > tmpl, Sow[#]; tmpl == #]) &,
mylist]; // Reap]

should work (except that it eats the first element).

If you actually meant that you want to delete any element that is not
strictly larger than the previous, then

Module[{tmpl == mylist[[1]]},
Scan[(If[# > tmpl, Sow[#]]; tmpl == #) &,
mylist]; // Reap]

(which again eats the first element).

On Feb22, 2011, at 1:30 AM, Maarten van der Burgt wrote:

> Hallo,
> I have a list like:
> mylist == {1, 2, 3, 4, 5, 6, 4, 5, 7, 8, 9}
> I want to delete any element which is not strictly larger than the
> previous element, until my new list has only increasing values.
> This means in mylist above I want to delete the 4 and the 5 at

> 7 and 8.
> Any elegant and fast way for doing this?
> In practice I want to do this for a large amount (1000) of very large
> lists (1000). So speed is important.
> Thanks for your help.
> Maarten

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