Key function
A key function is a callable that maps an item to a value used to compare, order, identify, index, or match it.
D3.js in Action, Third Edition
A key function computes stable identity keys that allow data-bound elements to be matched with corresponding data items across updates.
- How it is used
- Maintains stable data-element identity while filtering an interactive D3 scatterplot.
- How deep it goes
- Explains positional matching, uid-based matching, and integration with D3 data() and join(), without descending into API internals.
- Examples
- Uses filtered cetacean circles keyed by uid and compares D3 matching with React list keys.
- What you can do after
- Apply it via D3's data() method to preserve element identity during dataset updates and control resulting selections.
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The Quick Python Book, Fourth Edition
A key function is a callable that maps each item to a value used to determine that item's ordering during sorting.
- How it is used
- Customizes sorting when default comparison does not match the desired ordering.
- How deep it goes
- Explains the one-argument return rule, named and lambda syntax, applications, and repeated-call performance cost.
- Examples
- Demonstrates sorting words by length, text case-insensitively, or by a substring of each line.
- What you can do after
- Apply it via Python sorting methods to order items by a computed property.
Read how The Quick Python Book, Fourth Edition explains key function →
Related concepts
- maximumused by · A maximum is the greatest value in a set under a specified ordering, or the greatest value permitted by a specified constraint.
- sorting algorithmused by · A sorting algorithm is a procedure that rearranges elements of a collection into an order determined by an ordering criterion.
- data()configures · data() is a D3 selection method that associates an array of data values with the selected document elements.
- lambdaimplemented by · A lambda is a language construct that defines an unnamed function or callable, often as a value.
- sortedused by · sorted is a Python built-in function that returns a new list containing an iterable’s elements in sorted order without modifying the original iterable.
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