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DeleteDuplicatesBy

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

DeleteDuplicatesBy[expr, f]

Keeps the first element for each distinct f[element], preserving order. Over an association, f is applied to the values and the surviving entries are kept (keys preserved).

Examples (4)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (2)

In[1]:= DeleteDuplicatesBy[{1, 12, 3, 14, 5}, EvenQ]
Out[1]= {1, 12}

In[2]:= DeleteDuplicatesBy[<|"a" -> 1, "b" -> 12, "c" -> 3|>, EvenQ]
Out[2]= <|"a" -> 1, "b" -> 12|>

Applications (2)

First odd and first even survive

In[3]:= DeleteDuplicatesBy[{1, 2, 3, 4, 5, 6}, EvenQ]
Out[3]= {1, 2}

Keyed by |x|

In[4]:= DeleteDuplicatesBy[{-1, 1, 2, -2, 3}, Abs]
Out[4]= {-1, 2, 3}

Implementation notes

Algorithm. builtin_deleteduplicatesby keeps the first element for each distinct value of f[element], preserving order. It walks the collection, evaluates f on each element (on each value, for an association), and compares the result against the f-values of the survivors so far; a new f-value adds the element to the kept list, a repeat drops it. Over an association the surviving entries are returned as an association with keys preserved.

Data structures. Two parallel arrays — the surviving elements/rules and their owned f-values — with survivors compared directly by expr_eq. The survivor count is typically small, so the linear scan of seen f-values stays within budget; no hash index is built.

Complexity / limits. O(n · k) for n elements and k survivors (O(n²) worst case when nearly everything is distinct), plus n evaluations of f. Requires a 2-argument call with a non-atomic first argument; f is arbitrary, so no packed fast path applies.

Attributes: Protected.

References

Notes & additional examples

Notes

DeleteDuplicatesBy[list, f] keeps the first element for each distinct value of f[element], preserving order — the deduplicating cousin of GatherBy, returning one representative per group instead of the groups themselves. With f = Abs it collapses ±x to whichever sign appears first. Over an association f is applied to each value and the surviving entries are returned as an association with keys preserved. Use DeleteDuplicates for the plain f = Identity case.