A sort key is the value extracted from a complete entry to determine its ordering. Model extraction as a function into a total order; compare and , while moving the complete entries. Ordering sorting records by an age field does not also compare their names. Repeated sort keys are allowed, so this induces equal-key groups even when the complete sorting records differ. Keep the sort key function and the comparison rule fixed throughout a sort; inconsistent comparisons need not admit any sorted result.
New to topics? Read the docs here!