Re-indexing a Series

Re-indexing in pandas is a process that makes the data in a Series conform to a set of labels. It is used by pandas to perform much of the alignment process and is hence a fundamental operation.

Re-indexing achieves several things:

• Re-ordering existing data to match a set of labels
• Inserting NaN markers where no data exists for a label
• Possibly filling missing data for a label using some type of logic (defaulting to adding NaN values)

Re-indexing can be as simple as simply assigning a new index to the .index property of a Series. The following demonstrates changing the index of a Series in this manner:

Note

The number of elements in the list being assigned to the .index property must match the number of rows or an exception will be thrown. Re-indexing also modified the Series in-place.

Flexibility in creating a new index is provided through use of the .reindex() method. One case is in assigning a new index where the number of labels does not match the number of values:

The following...