Node:Indexing, Next:Selection, Up:Sequences
To select the n'th element of a sequence s do: s n.
Indexing starts at zero, thus the first element is s 0.
Negative subscripts count from the back, where s -2
is the last element, s -3 is the penultimate element,
and so on. The index -1 represents the non-existent element
at the end of the sequence.
Sometimes it is more helpful to think of the indexes as marking positions between elements. Thus index 0 represents the beginning of the sequence, index 1 is the position just after the first element, and so on. Thus a non-negative index counts the elements from the beginning of the sequence. Then the index -1 represents the end of the sequence (the position just after the last element), and a negative index represents the number of elements from the end of the sequence, counting down from -1.
If an "index" is a sequence, it is used to select
a sequence of elements:
S [I1 I2 ... In] evaluates to: [(S I1) (S I2) ... (S In)].
If extra arguments are given, they are distributed:
S [I1 I2 ... In] X ... Z evaluates to the same value as:
[(S I1 X ... Z) (S I2 X ... Z) ... (S In X ... Z)].
(However, the expressions X ... Y are only evaluated once.
Also, the result applications are evaluated on demand.)
This feature is useful to get a "slice" of a multi-dimentional array:
A [3 4] [1 2] evaluates to:
[[(A 3 1) (A 3 2)] [(A 4 1) (A 4 2)]].