# How to interpret the FullForm of a SparseArray?

SparseArrays are atomic objects, but they do have a FullForm which reveals information about them.

What is the meaning of the various elements in the full form of a SparseArray? Did the structure change between Mathematica versions, and is it documented anywhere?

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It seems to be the same as InputForm which is mentioned: Whenever a sparse array is evaluated, it is automatically converted to an optimized standard form But it doesn't have much detalis – ssch Jan 19 at 21:34
I can't tell much about the changes from version to version, but the "API" I use in many of my answers,e.g. here gives some idea. I give more details on the meaning of these parts here. – Leonid Shifrin Jan 19 at 21:48
Based on ElisionsDumpHeldSparseArrayData it looks like SparseArray[data_,dims_,def_,{___,elems_}], where dims is the dimensions and elems is number of non-default elements. I'm not sure about def and data. – Szabolcs Jan 19 at 22:50
def is the "background", or default, value. data seems always to be Automatic`, and I don't know what it represents. – Oleksandr R. Jan 20 at 6:12