• ← IntersectingQ
• → MatrixQ

LevelQ

LevelQ[expr]

tests whether expr is a valid level specification. This function is primarily used in function patterns for specifying type of a parameter.

• • LevelQ[2]

• • LevelQ[{2, 4}]

• • LevelQ[Infinity]

• • LevelQ[a + b]

We will define MyMap with the “level” parameter as a synonym for the Builtin Map equivalent:

• • MyMap[f_, expr_, Pattern[levelspec, _?LevelQ]] := Map[f, expr, levelspec]

• • MyMap[f, {{a, b}, {c, d}}, {2}]

• • Map[f, {{a, b}, {c, d}}, {2}]

But notice that when we pass an invalid level specification, MyMap does not match and therefore does not pass the arguments through to Map. So we do not see the error message that Map would normally produce

• • Map[f, {{a, b}, {c, d}}, x]

• • MyMap[f, {{a, b}, {c, d}}, {1, 2, 3}]