fix: reduce/foreach state variable should not be reset each iteration #3205

itchyny · 2024-11-15T14:42:27Z

When the UPDATE query in a reduce or foreach syntax does not emit
a value, it should simply skip updating the state variable. But
currently it resets the state variable to null.

When the UPDATE query in a reduce or foreach syntax does not emit a value, it should simply skip updating the state variable. But currently it resets the state variable to null.

itchyny · 2024-11-16T07:20:53Z

Now these syntax behaves like the following pseudo code.
Previously, $x was assigned null when it is used after ($x|UPDATE).

# reduce EXP as $v (INIT; UPDATE)
for $x in INIT {
  for $v in EXP {
    for $u in ($x|UPDATE) {
      $x = $u
    }
  }
  yield $x
}

# foreach EXP as $v (INIT; UPDATE; EXTRACT)
for $x in INIT {
  for $v in EXP {
    for $u in ($x|UPDATE) {
      $x = $u
      for $e in ($x|EXTRACT) {
        yield $e
      }
    }
  }
}

These procedural codes explain any cases the filters emit multiple or no values.
The EXTRACT filter is evaluated against each UPDATE output, and the last value is used for
the next iteration. And if UPDATE emits no value, it skips updating the intermediate value.
I wondered whether we should change the behavior to how jaq does (multiple values
in UPDATE replicates successive iteration, resulting into exponentially many results,
and no values aborts the entire loop). But I now think the current behavior
(after this PR fix about the intermediate value reset) looks well-defined.
Although the jq manual does not explains well about these cases,
the manual shouldn't be too much detailed about such minor use cases.
I think the current example codes makes it easy to understand these syntaxes,
especially for beginners.
Also, I noticed a slight difference in gojq's reduce syntax about INIT, I'll fix this soon.

wader · 2024-11-16T10:12:39Z

Update docs in this PR?

itchyny · 2024-11-16T10:40:25Z

I'd like to make this PR focus on the null reset. The change is too detailed to write in the current user manual in my opinion.

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…teration (jqlang#3205) This reverts 96e8d89. The change introduced a serious performance regression as reported in jqlang#3345.

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fix: reduce/foreach state variable should not be reset each iteration

9165c99

When the UPDATE query in a reduce or foreach syntax does not emit a value, it should simply skip updating the state variable. But currently it resets the state variable to null.

itchyny added this to the 1.8 release milestone Nov 15, 2024

itchyny added the bug label Nov 15, 2024

emanuele6 approved these changes Nov 15, 2024

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wader approved these changes Nov 15, 2024

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itchyny merged commit 96e8d89 into jqlang:master Nov 20, 2024
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01mf02 mentioned this pull request Nov 29, 2024

reduce/foreach behaviour when UPDATE yields not exactly one output #3211

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itchyny mentioned this pull request Jun 13, 2025

[1.8.0] Performance Issue on large array update. #3345

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fix: reduce/foreach state variable should not be reset each iteration #3205

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fix: reduce/foreach state variable should not be reset each iteration #3205

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