Add a Field to an Array of Structures

QUESTION: I want to add a new field to an array of structures I have already created. Is there any way to do this efficiently in IDL? Can I do this without creating a new structure array and copying each field separately?

ANSWER: As Wayne Landsman points out, it is not possible to avoid creating a new structure in IDL, since part of a structure definition is a defined size that can't be changed, but copying the contents of one structure to another is made much easier with the Struct_Assign command in IDL.

Consider this structure.

    IDL> struct = { A:3.0, B:'coyote' }
   IDL> array = Replicate(struct, 10) 

We wish to add a new field or tag to this structure. We start by creating a new structure with the new field.

    IDL> newStruct = { A:5.0, B:'pig', C:10.0 }
   IDL> newArray = Replicate(newStruct, 10) 

To copy the contents of the first structure array to the new structure array, you can do this.

    IDL> Struct_Assign, array, newArray    IDL> Print, newArray[0] 
        {3.00000 coyote 0.000000} 

You see that the new field has been "zeroed out", or set to its null value state. This is because Struct_Assign is using relaxed structure assignment in which missing fields are simply ignored and set to their null state.

Version of IDL used to prepare this article: IDL 7.1.2

Web Coyote's Guide to IDL Programming