| Re: Why doesn't this return the correct value? [message #58672] |
Thu, 14 February 2008 03:22  |
Spon
Messages: 178
Registered: September 2007
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Senior Member |
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On Feb 14, 10:59 am, chloesharro...@gmail.com wrote:
> Dear all
>
> I currently have an array called good_index. Some of the elements are>=0 and others are <0. I want to write a code that firstly finds how
>
> many of these elements are greater than zero and use this number to
> produce another array (called good_indexed_precip) with that number of
> elements. Then I want it to search through each element of good_index
> in turn. If upon searching it finds that the element is >=0 I want it
> to find the value of that element in good_index. I then want to find
> the data stored in the index given by that value in another array
> called precip_change. The code I've written is below:
>
> =====
> counter_2 = 0
> number = total(good_index ge 0, /int) ;this counts how many elements
> in good_index are >= zero
> print, number ;this gives the correct answer 6
>
> FOR s=0, (N_ELEMENTS(good_index)-1) DO BEGIN
> good_indexed_precip=fltarr(number)
> IF good_index[s] ge 0 THEN BEGIN
> good_indexed_precip[counter_2] = precip_change[good_index[s]]
> counter_2++
> ENDIF
> ENDFOR
> =====
>
> Unfortunately, it then prints good_indexed_precip with all 6 elements
> zero, even though the values of precip_change[good_index[s]] are non-
> zero.
>
> If I were to put in the data by hand, eg say
> good_indexed_precip[0] = precip_change[good_index[0]]
> good_indexed_precip[1] = precip_change[good_index[13]]
> This works fine, so I can't see why my loop isn't working correctly.
> I know that the 0th &1st element of good_index are both >=zero, yet if
> I run the loop: FOR s=0, 1 DO BEGIN etc and print good_indexed_precip
> it only has a non-zero value in the 1st element and not the 0th
> element!
>
> Thanks for all your help in advance.
> Chloé
> FOR s=0, (N_ELEMENTS(good_index)-1) DO BEGIN
> good_indexed_precip=fltarr(number)
There's the reason it's not working - you're redefining your array
every time. Take the second line out of the loop.
In terms of doing what you want to do, you should really look into
using WHERE.
IndexOfIndices = WHERE(Good_index GT 0, Number)
Positive_Good_Index = Good_index[IndexOfIndices]
Good_Indexed_Precip = Precip_Change[Positive_Good_Index]
Take care,
Chris
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| Re: Why doesn't this return the correct value? [message #58771 is a reply to message #58672] |
Thu, 14 February 2008 03:25  |
Spon
Messages: 178
Registered: September 2007
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Senior Member |
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> In terms of doing what you want to do, you should really look into
> using WHERE.
> IndexOfIndices = WHERE(Good_index GT 0, Number)
> Positive_Good_Index = Good_index[IndexOfIndices]
> Good_Indexed_Precip = Precip_Change[Positive_Good_Index]
>
> Take care,
> Chris
To avoid trying to subscript if you get an all negative array, protect
your code like this:
IndexOfIndices = WHERE(Good_index GT 0, Number)
IF Number GT 0 THEN BEGIN
Positive_Good_Index = Good_index[IndexOfIndices]
Good_Indexed_Precip = Precip_Change[Positive_Good_Index]
ENDIF
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