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Friday, 30 August 2013

Pivoting and De-Pivoting

In response to a comment on this post, I made the following process that creates a simple example set, pivots it and then de-pivots it. The end result: the de-pivot result and the original are the same.

The input example set to the de-pivot operation is shown here.

The key parameters for the de-pivot operator are as follows. The first shows that the de-pivot operation will produce examples with an attribute called "name" with values that are nominal and which will not include any missing values from the input example set.

Each example in the result will also contain another attribute and this is dictated by the following parameters.

The regular expression finds all attributes that match. In this case, there are 4.

The de-pivot operation considers each example in the input example set in turn and combines that with the result of the regular expression. The intersection of the example and the matched attribute produces a new attribute value whose name is "value" in this case. For the example here, there are 12 possibilities so the full result would contain 12 examples but this is normally reduced by clearing the check box "keep missings". As mentioned above, one final point is that the "create nominal index" check box must be set in order to get nominal values in the results.

To make the result match the original, there are a couple of sundry operators that rename the values and re-order the attributes.

If I'm honest, I always forget the details of how de-pivoting works so I just adopt the trial and error approach until it looks right.

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