# Null/empty/absent data

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`.

## The three types

Miller has three kinds of null data:

* **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`.

* **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"}'`.

* **JSON null**: The main purpose of this is to support reading the `null` type in JSON files. The [Miller programming language](miller-programming-language.md) has a `null` keyword as well, so you can also write the null type using `$x = null`. Additionally, though, when you write past the end of an array, leaving gaps -- e.g. writing `a[12]` when the array `a` has length 10 -- JSON-null is used to fill the gaps. See also the [arrays page](reference-main-arrays.md#auto-extend-and-null-gaps).

You can test these programmatically 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. Here is a full list of such functions:

GENMD-RUN-COMMAND
mlr -f | grep is_
GENMD-EOF

## Rules for null-handling

* Records with one or more empty sort-field values sort after records with all sort-field values present:

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mlr cat data/sort-null.dat
GENMD-EOF

GENMD-RUN-COMMAND
mlr sort -n a data/sort-null.dat
GENMD-EOF

GENMD-RUN-COMMAND
mlr sort -nr a data/sort-null.dat
GENMD-EOF

* Most functions/operators which have one or more *empty* arguments produce empty output: e.g.

GENMD-RUN-COMMAND
echo 'x=2,y=3' | mlr put '$a=$x+$y'
GENMD-EOF

GENMD-RUN-COMMAND
echo 'x=,y=3' | mlr put '$a=$x+$y'
GENMD-EOF

GENMD-RUN-COMMAND
echo 'x=,y=3' | mlr put '$a=log($x);$b=log($y)'
GENMD-EOF

with the exception that the `min` and `max` functions are special: if one argument is non-null, it wins:

GENMD-RUN-COMMAND
echo 'x=,y=3' | mlr put '$a=min($x,$y);$b=max($x,$y)'
GENMD-EOF

Likewise, empty works like 0 for addition and subtraction, and like 1 for multiplication:

GENMD-RUN-COMMAND
echo 'x=,y=3' | mlr put '$a = $x + $y; $b = $x - $y; $c = $x * $y'
GENMD-EOF

This is intended to follow the arithmetic rule for absent data (explained next). In particular:

* For file formats allowing for heterogeneity in keys, e.g. JSON, you should be able to keep a running sum of some field, say `$x`. If a given record doesn't have `$x`, then `$x` will be absent for that record, and the sum should simply continue.
* For CSV and TSV, which don't allow for hetrogeneity in keys, the _only_ way a value can be missing is to be empty.  Here, if a given record doesn't have `$x`, then `$x` will be empty for that record, and the sum should simply continue.

GENMD-RUN-COMMAND
cat missings.json
GENMD-EOF

GENMD-RUN-COMMAND
mlr --ijson --from missings.json put -q 'begin { @sum = 0 } @sum += $x; end { print @sum }'
GENMD-EOF

GENMD-RUN-COMMAND
cat missings.csv
GENMD-EOF

GENMD-RUN-COMMAND
mlr --icsv --from missings.csv put -q 'begin { @sum = 0 } @sum += $x; end { print @sum }'
GENMD-EOF

* 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**:

GENMD-RUN-COMMAND
echo 'x=2,y=3' | mlr put '$a=$u+$v; $b=$u+$y; $c=$x+$y'
GENMD-EOF

GENMD-RUN-COMMAND
echo 'x=2,y=3' | mlr put '$a=min($x,$v);$b=max($u,$y);$c=min($u,$v)'
GENMD-EOF

* Likewise, for assignment to maps, **absent-valued keys or values result in a skipped assignment**.

The reasoning is as follows:

* Absent stream-record values should not break accumulations, since Miller by design handles heterogeneous data: the running `@sum` in `mlr put '@sum += $x'` should not be invalidated for records which have no `x`.

* 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.

* 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 [DSL errors and transparency](reference-dsl-errors.md).

## Absent-test functions

Since absent plus absent is absent (and likewise for other operators), accumulations such as `@sum += $x` work correctly on heterogeneous 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`:

GENMD-RUN-COMMAND
mlr cat data/het.dkvp
GENMD-EOF

GENMD-RUN-COMMAND
mlr put 'is_present($loadsec) { $loadmillis = $loadsec * 1000 }' data/het.dkvp
GENMD-EOF

GENMD-RUN-COMMAND
mlr put '$loadmillis = (is_present($loadsec) ? $loadsec : 0.0) * 1000' data/het.dkvp
GENMD-EOF

## Arithmetic rules

If you're interested in a formal description of how empty and absent fields participate in arithmetic, here's a table for `+`, `&&, and `||`. Notes:

* Other arithmetic, boolean, and bitwise operators besides `&&` and `||` are similar to `+`.
* The `&&` and `||` obey _short-circuiting semantics_. That is:
  * `false && X` is `false` and `X` is not evaluated even if it is a complex expression (maybe including function calls)
  * `true || X` is `true` and `X` is not evaluated even if it is a complex expression (maybe including function calls)
* This means in particular that:
  * `false && X` is false even if `X` is an error, a non-boolean type, etc.
  * `true || X` is true even if `X` is an error, a non-boolean type, etc.

GENMD-RUN-COMMAND
mlr help type-arithmetic-info-extended
GENMD-EOF
