Skip to content

TakeWhile

Status: Stable

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

Description

TakeWhile[list, crit]

Gives the longest leading run of elements e for which crit[e] is True. Over an association, tests the values and keeps the matching leading entries (keys preserved).

Examples (4)

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

Basic examples (2)

In[1]:= TakeWhile[<|"a" -> 1, "b" -> 2, "c" -> 5, "d" -> 1|>, # < 3 &]
Out[1]= <|"a" -> 1, "b" -> 2|>

In[2]:= LengthWhile[<|"a" -> 1, "b" -> 2, "c" -> 5|>, # < 3 &]
Out[2]= 2

Applications (2)

Stops at 1; the trailing 8 is not taken

In[3]:= TakeWhile[{2, 4, 6, 1, 8}, EvenQ]
Out[3]= {2, 4, 6}

In[4]:= TakeWhile[{1, 2, 3, 4}, # < 3 &]
Out[4]= {1, 2}

Implementation notes

Algorithm. builtin_takewhile returns the leading run of elements satisfying crit, keeping the collection's head. Like LengthWhile it first tries the compiled-predicate fast path (pred_run_length finds the run length on the buffer, then pred_leading_slice copies it, so neither the scan nor the result materialises boxed nodes). A visible NDArray is materialised and repacked (ndstruct_delist_repack). Otherwise leading_run_length locates the run and the first k elements are copied into a fresh collection; for an association the values are tested and the head is Association.

Data structures. A packed buffer on the compiled path; an Expr** copy of the leading k elements on the interpreted path.

Complexity / limits. O(k) for a run of length k (the scan stops at the first failure), plus O(k) to copy the slice. LengthWhile returns only the count.

Attributes: Protected.

References

See also: LengthWhile

Notes & additional examples

Notes

TakeWhile returns the leading run of passing elements and stops at the first failure ā€” later passing elements are not included. Over an association it tests the values and returns an association. LengthWhile gives the length of the same run.