help length
LENGTH Length of vector.
LENGTH(X) returns the length of vector X. It is equivalent
to MAX(SIZE(X)) for non-empty arrays and 0 for empty ones.
意思是length=MAX(SIZE(X))，不清楚size和max的话看下面：
help size
SIZE Size of array.
D = SIZE(X), for M-by-N matrix X, returns the two-element
row vector D = [M, N] containing the number of rows and columns
in the matrix. For N-D arrays, SIZE(X) returns a 1-by-N
vector of dimension lengths. Trailing singleton dimensions
are ignored.
For vectors, MAX(X) is the largest element in X. For matrices,
MAX(X) is a row vector containing the maximum element from each
column. For N-D arrays, MAX(X) operates along the first
non-singleton dimension.
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