Level¶

Returns the sub-expressions at a given level.

$ wo 'Level[{{a, b}, {c, d}}, {1}]'
{{a, b}, {c, d}}

$ wo 'Level[{{a, b}, {c, d}}, {2}]'
{a, b, c, d}

An object with a compound head that is nevertheless an atom — a Tree, a SparseArray, a ByteArray, a NumericArray or a Dataset — has no levels inside it. The object itself sits at level 0 counted from the top and at level -1 counted from the bottom:

$ wo 'Level[Tree[1, {}], {-1}]'
{Tree[1, {}]}

$ wo 'Level[SparseArray[{1, 2}], {1}]'
{}

$ wo 'Head[First[Level[SparseArray[{1, 2}], {0}]]]'
SparseArray

All is a level specification meaning every level, the same as {0, Infinity}:

$ wo 'Level[{{1, 2}, {3, {4, 5}}}, All]'
{1, 2, {1, 2}, 3, 4, 5, {4, 5}, {3, {4, 5}}, {{1, 2}, {3, {4, 5}}}}