# [ncl-talk] get first index of each sequence

Xi Chang xi.chang01 at gmail.com
Mon Mar 6 02:06:04 MST 2023

```Thanks a lot Dave. Would you please suggest me hot get the first index of
each sequence without any missing values in between. For example, i would
like to get the first index of each sequence marked by flags:

x= (/1,1,1,5,5,5,5,31,31/)

Results will be: 0, 3, and 7

Thank you!
Chang,

On Mon, Feb 6, 2023 at 17:01 Dave Allured - NOAA Affiliate <
dave.allured at noaa.gov> wrote:

> When analyzing sequences, it is often useful to compare adjacent elements
> by using a vector shifted by one position.  Then find an expression that is
> true in the positions of interest, and false everywhere else.
>
>     status = .not. ismissing (x)
>     nx = dimsizes (x)
>     previous = new (nx, logical)
>     previous(0) = False      ; need this to get correct answer for first
> position
>     previous(1:nx-1) = status(0:nx-2)       ; vector shifted one position
> to the right
>     change = status(:) .and. .not. previous(:)     ; all places that
> changed from missing to not
>     istarts = ind (change)
>
>     print (istarts+"")
>     (0)     4
>     (1)     12
>     (2)     16
>
>
> On Mon, Feb 6, 2023 at 7:18 PM Xi Chang via ncl-talk <
> ncl-talk at mailman.ucar.edu> wrote:
>
>> Hi NCL community,
>>
>> Is there anyone who can help me on how to return the first index of each
>> sequence in 1D data?
>> For example,
>>
>> x = (/-999,-999,-999,-999,1,1,1,1,1,-999,-999,-999,1,1,1,-999,1,1,1/)
>> x at _FillValue=-999
>>
>> There are 3 sequences of data in that 1D time series, how could I get the
>> first index of each sequence, so that the answer would be:
>>
>> (0) 4
>> (1) 12
>> (2) 16
>>
>> Thanks
>> Chang
>>
>
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