# [ncl-talk] Question regarding array indexing

Dennis Shea shea at ucar.edu
Wed Oct 18 13:47:33 MDT 2017

```Oops!

https://www.ncl.ucar.edu/Document/Functions/Contributed/mod.shtml

Examples 2 & 3

On Wed, Oct 18, 2017 at 1:46 PM, Dennis Shea <shea at ucar.edu> wrote:

> See Example 2 & 3
>
> On Tue, Oct 17, 2017 at 3:22 PM, Prashanth Bhalachandran <
> prashanth.bhalachandran at gmail.com> wrote:
>
>> Dave,
>> This is a very elegant solution. My sincere appreciation.
>>
>> Prashanth
>>
>> On Oct 17, 2017, at 2:10 PM, Dave Allured - NOAA Affiliate <
>> dave.allured at noaa.gov> wrote:
>>
>> Prashanth,
>>
>> I like using a modulo calculation and vector subscripting for cyclic
>> indexing.  This reduces possible errors from hard coded indexing.
>> Something like this:
>>
>>     theta = 20
>>     lons = 10.0 * ispan (0,35,1)
>>     offset = mod (lons + 360 - theta, 360)
>>
>>     iq1 = ind (offset .le. 90)
>>     iq2 = ind (offset .ge. 90 .and. offset .le. 180)
>>     iq3 = ind (offset .ge. 180 .and. offset .le. 270)
>>     iq4 = ind (offset .ge. 270)
>>
>> Note that this can create overlapping index points in some cases.  For no
>> overlap, change .le. to .lt. in 3 places.
>>
>> In the mod expression, it is important to add an extra 360 degrees, to
>> prevent negative values in the first mod argument.  What this really does
>> is convert the NCL mod function to a true mathematical modulo function for
>> cyclic applications.  Understand the difference here:
>>
>>     https://en.wikipedia.org/wiki/Modulo_operation
>>
>> --Dave
>>
>>
>> On Tue, Oct 17, 2017 at 1:19 PM, Prashanth Bhalachandran <
>> prashanth.bhalachandran at gmail.com> wrote:
>>
>>> Hello all,
>>> I have a relatively simple question on array indexing.
>>>
>>> Say, I have an array that is cyclic that contains values from 0 to 360
>>> in steps of 5 degrees (dimension [72]). And I am trying to divide into four
>>> quadrants starting from a given theta. Say that the index of the given
>>> theta is 13 (out of 73), I define my quadrants that are each 90 degrees as
>>> follows :
>>>
>>> Array : theta [73]
>>>
>>> Now, as you’ll notice, having an index of 67:13 doesn’t imply 67:72 and
>>> 0:13, it simply implies 13:67 backwards. How do I index the array such that
>>> it takes it in a cyclic fashion? For example (what I don’t want is), in a
>>> simpler array of just five dimensions : Variable: a
>>> Type: integer
>>> Total Size: 20 bytes
>>>             5 values
>>> Number of Dimensions: 1
>>> Dimensions and sizes: [5]
>>> Coordinates:
>>> (0) 1
>>> (1) 2
>>> (2) 3
>>> (3) 4
>>> (4) 5
>>>
>>> ncl 3> print(a(4:2))
>>>
>>>
>>> Variable: a (subsection)
>>> Type: integer
>>> Total Size: 12 bytes
>>>             3 values
>>> Number of Dimensions: 1
>>> Dimensions and sizes: [3]
>>> Coordinates:
>>> (0) 5
>>> (1) 4
>>> (2) 3
>>>
>>> What I would have liked it is for it to display 5 1 2 3.
>>>
>>> Please let me know how this is possible.
>>>
>>> Thank you all,
>>> Prashanth
>>>
>>
>>
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>
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