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builtin.jq: simpler and faster transpose #2758

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Jul 25, 2023
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builtin.jq: simpler and faster transpose #2758

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e7b5327
builtin.jq: simpler and faster transpose
pkoppstein Jul 24, 2023
08da3a9
builtin.jq: [] | transpose #=> []
pkoppstein Jul 24, 2023
a20c6d5
builtin.jq: omit note about performance of transpose
pkoppstein Jul 24, 2023
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10 changes: 2 additions & 8 deletions src/builtin.jq
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Original file line number Diff line number Diff line change
Expand Up @@ -181,14 +181,8 @@ def combinations(n):
| combinations;
# transpose a possibly jagged matrix, quickly;
# rows are padded with nulls so the result is always rectangular.
def transpose:
if . == [] then []
else . as $in
| (map(length) | max) as $max
| length as $length
| reduce range(0; $max) as $j
([]; . + [reduce range(0;$length) as $i ([]; . + [ $in[$i][$j] ] )] )
end;
# Using map(length) turns out to be faster than using max(stream)
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I'm still unsure about the stream version of max to be included or not. Users not following the history will be get confused by this comment; where is max(stream)?

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@itchyny - Here, max(stream) is simply short for any valid streaming version of max. That should be clear since no specific referent is mentioned. Feel free to change the wording, though.

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Let's just drop the sentence, we don't need such performance notes here.

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OK. Done in a20c6d5

def transpose: [range(0; map(length)|max // 0) as $i | [.[][$i]]];
def in(xs): . as $x | xs | has($x);
def inside(xs): . as $x | xs | contains($x);
def repeat(exp):
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4 changes: 4 additions & 0 deletions tests/jq.test
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Expand Up @@ -1479,6 +1479,10 @@ transpose
[[1], [2,3]]
[[1,2],[null,3]]

transpose
[]
[]

ascii_upcase
"useful but not for é"
"USEFUL BUT NOT FOR é"
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