Reverse¶

Status: Stable

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

Description¶

Reverse[expr] reverses the order of elements in expr.

Examples (2)¶

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

Applications (2)¶

In[1]:= Reverse[{1,2,3}]
Out[1]= {3, 2, 1}

In[2]:= Reverse[{a,b,c,d}]
Out[2]= {d, c, b, a}

Performance¶

Against other systems, from the benchmark suite (same input, results cross-checked for agreement):

case	Mathilda	Wolfram	Python
Sort 4x10^6	42.2 s	68.7 s	111 s
gather v[[idx]], 4x10^6	16.8 s	6.66 s	7.18 s
Union of 4x10^6 integers	12.4 s	71.1 s	376 s
Reverse 4x10^6	5.37 s	0.297 s	0.982 s
Join two 2x10^6	0.899 s	0.6 s	0.397 s
RotateLeft 4x10^6 by 1000	0.855 s	0.307 s	0.456 s

Implementation notes¶

Algorithm. builtin_reverse recurses through the expression with reverse_rec, reversing the argument order at the levels selected by an optional level spec (should_reverse_at_level matches an integer level, or any level listed in a List spec; default is level 1). At each visited node it builds a new function with the same head, drawing children either forward or reversed depending on whether the current level is selected, and recursing into each child.

Attributes: Protected.

References¶

Source: src/list.c
Specification: docs/spec/builtins/structural-manipulation.md
Tests: tests/test_association.c
Tests: tests/test_compile.c
Tests: tests/test_convolutions.c
Tests: tests/test_correlations.c

Notes & additional examples¶

Notes¶

Reverse[expr] reverses the order of top-level elements.