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62 lines
1.8 KiB
Markdown
62 lines
1.8 KiB
Markdown
# DSL reference: overview
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## Overview
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Here's comparison of verbs and `put`/`filter` DSL expressions:
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Example:
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GENMD_RUN_COMMAND
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mlr stats1 -a sum -f x -g a data/small
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GENMD_EOF
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* Verbs are coded in Go
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* They run a bit faster
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* They take fewer keystrokes
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* There is less to learn
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* Their customization is limited to each verb's options
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Example:
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GENMD_RUN_COMMAND
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mlr put -q '@x_sum[$a] += $x; end{emit @x_sum, "a"}' data/small
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GENMD_EOF
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* You get to write your own DSL expressions
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* They run a bit slower
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* They take more keystrokes
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* There is more to learn
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* They are highly customizable
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Please see [Verbs Reference](reference-verbs.md) for information on verbs other than `put` and `filter`.
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The essential usages of `mlr filter` and `mlr put` are for record-selection and record-updating expressions, respectively. For example, given the following input data:
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GENMD_RUN_COMMAND
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cat data/small
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GENMD_EOF
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you might retain only the records whose `a` field has value `eks`:
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GENMD_RUN_COMMAND
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mlr filter '$a == "eks"' data/small
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GENMD_EOF
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or you might add a new field which is a function of existing fields:
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GENMD_RUN_COMMAND
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mlr put '$ab = $a . "_" . $b ' data/small
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GENMD_EOF
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The two verbs `mlr filter` and `mlr put` are essentially the same. The only differences are:
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* Expressions sent to `mlr filter` must end with a boolean expression, which is the filtering criterion;
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* `mlr filter` expressions may not reference the `filter` keyword within them; and
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* `mlr filter` expressions may not use `tee`, `emit`, `emitp`, or `emitf`.
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All the rest is the same: in particular, you can define and invoke functions and subroutines to help produce the final boolean statement, and record fields may be assigned to in the statements preceding the final boolean statement.
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There are more details and more choices, of course, as detailed in the following sections.
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