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89 lines
5.1 KiB
Markdown
89 lines
5.1 KiB
Markdown
# Reference: null data
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One of Miller's key features is its support for **heterogeneous** data. For example, take ``mlr sort``: if you try to sort on field ``hostname`` when not all records in the data stream *have* a field named ``hostname``, it is not an error (although you could pre-filter the data stream using ``mlr having-fields --at-least hostname then sort ...``). Rather, records lacking one or more sort keys are simply output contiguously by ``mlr sort``.
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Miller has two kinds of null data:
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* **Empty (key present, value empty)**: a field name is present in a record (or in an out-of-stream variable) with empty value: e.g. ``x=,y=2`` in the data input stream, or assignment ``$x=""`` or ``@x=""`` in ``mlr put``.
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* **Absent (key not present)**: a field name is not present, e.g. input record is ``x=1,y=2`` and a ``put`` or ``filter`` expression refers to ``$z``. Or, reading an out-of-stream variable which hasn't been assigned a value yet, e.g. ``mlr put -q '@sum += $x; end{emit @sum}'`` or ``mlr put -q '@sum[$a][$b] += $x; end{emit @sum, "a", "b"}'``.
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You can test these programatically using the functions ``is_empty``/``is_not_empty``, ``is_absent``/``is_present``, and ``is_null``/``is_not_null``. For the last pair, note that null means either empty or absent.
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Rules for null-handling:
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* Records with one or more empty sort-field values sort after records with all sort-field values present:
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GENMD_RUN_COMMAND
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mlr cat data/sort-null.dat
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GENMD_EOF
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GENMD_RUN_COMMAND
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mlr sort -n a data/sort-null.dat
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GENMD_EOF
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GENMD_RUN_COMMAND
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mlr sort -nr a data/sort-null.dat
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GENMD_EOF
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* Functions/operators which have one or more *empty* arguments produce empty output: e.g.
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GENMD_RUN_COMMAND
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echo 'x=2,y=3' | mlr put '$a=$x+$y'
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GENMD_EOF
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GENMD_RUN_COMMAND
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echo 'x=,y=3' | mlr put '$a=$x+$y'
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GENMD_EOF
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GENMD_RUN_COMMAND
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echo 'x=,y=3' | mlr put '$a=log($x);$b=log($y)'
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GENMD_EOF
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with the exception that the ``min`` and ``max`` functions are special: if one argument is non-null, it wins:
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GENMD_RUN_COMMAND
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echo 'x=,y=3' | mlr put '$a=min($x,$y);$b=max($x,$y)'
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GENMD_EOF
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* Functions of *absent* variables (e.g. ``mlr put '$y = log10($nonesuch)'``) evaluate to absent, and arithmetic/bitwise/boolean operators with both operands being absent evaluate to absent. Arithmetic operators with one absent operand return the other operand. More specifically, absent values act like zero for addition/subtraction, and one for multiplication: Furthermore, **any expression which evaluates to absent is not stored in the left-hand side of an assignment statement**:
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GENMD_RUN_COMMAND
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echo 'x=2,y=3' | mlr put '$a=$u+$v; $b=$u+$y; $c=$x+$y'
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GENMD_EOF
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GENMD_RUN_COMMAND
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echo 'x=2,y=3' | mlr put '$a=min($x,$v);$b=max($u,$y);$c=min($u,$v)'
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GENMD_EOF
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* Likewise, for assignment to maps, **absent-valued keys or values result in a skipped assignment**.
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The reasoning is as follows:
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* Empty values are explicit in the data so they should explicitly affect accumulations: ``mlr put '@sum += $x'`` should accumulate numeric ``x`` values into the sum but an empty ``x``, when encountered in the input data stream, should make the sum non-numeric. To work around this you can use the ``is_not_null`` function as follows: ``mlr put 'is_not_null($x) { @sum += $x }'``
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* Absent stream-record values should not break accumulations, since Miller by design handles heterogenous data: the running ``@sum`` in ``mlr put '@sum += $x'`` should not be invalidated for records which have no ``x``.
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* Absent out-of-stream-variable values are precisely what allow you to write ``mlr put '@sum += $x'``. Otherwise you would have to write ``mlr put 'begin{@sum = 0}; @sum += $x'`` -- which is tolerable -- but for ``mlr put 'begin{...}; @sum[$a][$b] += $x'`` you'd have to pre-initialize ``@sum`` for all values of ``$a`` and ``$b`` in your input data stream, which is intolerable.
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* The penalty for the absent feature is that misspelled variables can be hard to find: e.g. in ``mlr put 'begin{@sumx = 10}; ...; update @sumx somehow per-record; ...; end {@something = @sum * 2}'`` the accumulator is spelt ``@sumx`` in the begin-block but ``@sum`` in the end-block, where since it is absent, ``@sum*2`` evaluates to 2. See also the section on :doc:`reference-dsl-errors`.
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Since absent plus absent is absent (and likewise for other operators), accumulations such as ``@sum += $x`` work correctly on heterogenous data, as do within-record formulas if both operands are absent. If one operand is present, you may get behavior you don't desire. To work around this -- namely, to set an output field only for records which have all the inputs present -- you can use a pattern-action block with ``is_present``:
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GENMD_RUN_COMMAND
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mlr cat data/het.dkvp
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GENMD_EOF
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GENMD_RUN_COMMAND
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mlr put 'is_present($loadsec) { $loadmillis = $loadsec * 1000 }' data/het.dkvp
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GENMD_EOF
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GENMD_RUN_COMMAND
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mlr put '$loadmillis = (is_present($loadsec) ? $loadsec : 0.0) * 1000' data/het.dkvp
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GENMD_EOF
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If you're interested in a formal description of how empty and absent fields participate in arithmetic, here's a table for plus (other arithmetic/boolean/bitwise operators are similar):
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GENMD_RUN_COMMAND
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mlr help type-arithmetic-info
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GENMD_EOF
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