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# Description Adds `random choice` suggested in #16241. # User-Facing Changes New `random` module in `std-rfc` with the `choice` subcommand. # Tests + Formatting Unsure how do to do tests. Sampling and a histogram should be enough, but they'll be non-deterministic. # After Submitting <!-- If your PR had any user-facing changes, update [the documentation](https://github.com/nushell/nushell.github.io) after the PR is merged, if necessary. This will help us keep the docs up to date. --> --------- Co-authored-by: sholderbach <sholderbach@users.noreply.github.com>
73 lines
1.6 KiB
Plaintext
73 lines
1.6 KiB
Plaintext
# for examples
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alias "random choice" = choice
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# Sample `k` elements from a list
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#
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# This function will pick a simple random sample from input without replacement
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# (each element from the input can only be picked once).
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#
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# The sample is treated as a set. This means that the combined probability of
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# `[1 2 3 4] | random choice 2` returning `[3, 4]` or `[4, 3]` equals that of
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# `[1, 2]`. To ensure that all permutations are equally probable, use
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# `shuffle` or `sort`.
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#
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# The current implementation collects the input stream. This might change in
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# the future.
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@example "Pick 2 random items" {
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[1 2 3 4 5] | random choice 2
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}
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@example "Verify that the elements are picked uniformly" {
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0..100_000
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| each {
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[1 2 3 4 5] | random choice 2 | sort | to nuon
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}
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| histogram
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}
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export def choice [
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n: int = 1 # number of items to sample
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]: list -> list {
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# XXX: this collects the stream
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let input = $in
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let len = $input | length
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if $n > ($input | length) {
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error make {
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msg: "Can't sample more elements than there are in input"
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label: {
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text: $"Tried to sample ($n) out of ($len)"
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span: (metadata $n).span
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}
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}
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}
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# always return a list, even though `first 1` returns standalone T
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mut output = $input | if $n == 1 {
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first | [$in]
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} else {
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first $n
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}
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# reservoir sampling, algorithm L
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# https://doi.org/10.1145/198429.198435
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mut w = (random float) ** (1 / $n)
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mut i = $n - 1
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loop {
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$i += (random float | math ln) / (1.0 - $w | math ln)
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| math floor
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| $in + 1
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if $i < $len {
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let el = $input | get $i
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$output = $output | update (random int 0..<$n) $el
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$w *= (random float) ** (1 / $n)
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} else {
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break
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}
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}
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$output
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}
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