60 lines
2.1 KiB
Markdown
Vendored
60 lines
2.1 KiB
Markdown
Vendored
[//]: # (title: gather)
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<!---IMPORT org.jetbrains.kotlinx.dataframe.samples.api.Modify-->
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Converts several columns into two columns `key` and `value`. `key` column will contain names of original columns, `value` column will contain values from original columns.
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This operation is reverse to [](pivot.md)
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```kotlin
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gather { columns }
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[.explodeLists()]
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[.cast<Type>()]
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[.notNull()]
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[.where { valueFilter }]
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[.mapKeys { keyTransform }]
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[.mapValues { valueTransform }]
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.into(keyColumn, valueColumn) | .keysInto(keyColumn) | .valuesInto(valueColumn)
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valueFilter: (value) -> Boolean
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keyTransform: (columnName: String) -> K
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valueTransform: (value) -> R
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```
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See [column selectors](ColumnSelectors.md) for how to select the columns for this operation.
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Configuration options:
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* `explodeLists` — gathered values of type [`List`](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-list/) will be exploded into their elements, so `where`, `cast`, `notNull` and `mapValues` will be applied to list elements instead of lists themselves
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* `cast` — inform compiler about the expected type of gathered elements. This type will be passed to `where` and `mapKeys` lambdas
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* `notNull` — skip gathered `null` values
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* `where` — filter gathered values
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* `mapKeys` — transform gathered column names (keys)
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* `mapValues` — transform gathered column values
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Storage options:
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* `into(keyColumn, valueColumn)` — store gathered key-value pairs in two new columns with names `keyColumn` and `valueColumn`
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* `keysInto(keyColumn)` — store only gathered keys (column names) in a new column `keyColumn`
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* `valuesInto(valueColumn)` — store only gathered values in a new column `valueColumn`
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<!---FUN gather-->
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```kotlin
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pivoted.gather { "London".."Tokyo" }.into("city", "population")
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```
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<inline-frame src="resources/org.jetbrains.kotlinx.dataframe.samples.api.Modify.gather.html" width="100%"/>
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<!---END-->
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<!---FUN gatherWithMapping-->
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```kotlin
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pivoted.gather { "London".."Tokyo" }
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.cast<Int>()
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.where { it > 10 }
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.mapKeys { it.lowercase() }
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.mapValues { 1.0 / it }
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.into("city", "density")
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```
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<!---END-->
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