Array concatenation in IDL with arrays larger than 3 dimensions

With IDL 8.8 and older versions, the concatenation of arrays using brackets is limited to 3 dimensions.

 

Example

IDL > c1=indgen(5,4,3)
IDL > c2=indgen(5,4,3)*2

IDL > c=[[[c1]],[[c2]]]

IDL > help,c

C               INT       = Array[5, 4, 6]

 

If you run a similar example on arrays with more than 3 dimensions, it will fail with the below error:

IDL> c1=indgen(5,4,3,2)

IDL> c2=indgen(5,4,3,2)*2

IDL> c=[[[[c1]]],[[[c2]]]]

c=[[[[c1]]],[[[c2]]]]

                 ^

% Only three levels of variable concatenation are allowed.

 

The main reason of this limitation is the readability : this syntax could be extremely confusing, especially when trying to concatenate arrays along one of the middle dimensions. Thus it would require to carefully count all of the brackets to prevent mistakes.

 

One way to workaround this limitation is to use the REFORM function instead

For example:

IDL> c1=indgen(5,4,3,2)

IDL> c2=indgen(5,4,3,2)*2

c=REFORM([REFORM(c1,c1.length),REFORM(c2,c2.length)],5,4,3,4)

help,c

C               INT       = Array[5, 4, 3, 4]

 

 

 

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created by BC on 3/7/2023

reviewed by BC US on 3/7/2023