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52 lines
1.8 KiB
Markdown
52 lines
1.8 KiB
Markdown
5 years ago
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# Vectors
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A _vector_ is a fixed-length array of arbitrary values. Unlike a list, a
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vector supports constant-time access and update of its elements.
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A vector prints similar to a list—as a parenthesized sequence of its
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elements—but a vector is prefixed with `#` after `'`, or it uses
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`vector` if one of its elements cannot be expressed with `quote`.
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For a vector as an expression, an optional length can be supplied. Also,
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a vector as an expression implicitly `quote`s the forms for its content,
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which means that identifiers and parenthesized forms in a vector
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constant represent symbols and lists.
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> +\[missing\] in \[missing\] documents the fine points of the syntax of
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> vectors.
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Examples:
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```racket
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> #("a" "b" "c")
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'#("a" "b" "c")
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> #(name (that tune))
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'#(name (that tune))
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> #4(baldwin bruce)
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'#(baldwin bruce bruce bruce)
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> (vector-ref #("a" "b" "c") 1)
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"b"
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> (vector-ref #(name (that tune)) 1)
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'(that tune)
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```
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Like strings, a vector is either mutable or immutable, and vectors
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written directly as expressions are immutable.
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Vectors can be converted to lists and vice versa via `vector->list` and
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`list->vector`; such conversions are particularly useful in combination
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with predefined procedures on lists. When allocating extra lists seems
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too expensive, consider using looping forms like `for/fold`, which
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recognize vectors as well as lists.
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Example:
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```racket
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> (list->vector (map string-titlecase
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(vector->list #("three" "blind" "mice"))))
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'#("Three" "Blind" "Mice")
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```
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> +\[missing\] in \[missing\] provides more on vectors and vector
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> procedures.
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