created: 20210618134753828
modified: 20220720191457421
tags: [[Filter Syntax]] [[Filter Run Prefix Examples]] [[Map Filter Run Prefix]]
title: Map Filter Run Prefix (Examples)
type: text/vnd.tiddlywiki

Replace the input titles with the caption field if it exists, otherwise preserve the input title:

<<.operator-example 1 "[tag[Widgets]] :map[get[caption]else{!!title}]">>

<<.tip "The above example is equivalent to `[tag[Widgets]] :map[{!!caption}!is[blank]else{!!title}]`. Note that referencing a field as a text reference such as `{!!caption}` returns an empty string for a non-existent or empty caption field. Therefore a check for `is[blank]` is needed before the `else` operator">>


For each title in a shopping list, calculate the total cost of purchasing each item:

<<.operator-example 2 "[tag[shopping]] :map[get[quantity]else[0]multiply{!!price}]">>

Get the tags of all tiddlers tagged `Widget:`

<<.operator-example 3 "[tag[Widgets]] :map:flat[tagging[]] :and[!is[blank]unique[]]">>
<<.tip "Without the `flat` suffix the `:map` filter run only returns the first result for each input title">>

!! Comparison between `:map` and `:and`/`+` filter run prefixes

The functionality of the `:map` filter run prefix has some overlap with the `:and` prefix (alias `+`). They will sometimes return the same results as each other. In at least these cases, the results will be different:

# The `:and` filter run can modify the number of items (either increase or decrease). The `:map` run will never alter the number of items.
# The number of items in the `:and` filter run will also decrease due to de-duplication. The `:map` run will not de-duplicate.
# Explicit references to the "currentTiddler" variable will behave differently
# Implicit references to the "currentTiddler" using TextReference will behave differently.

| `:map` | `:and` |
|!results are the same|<|
|^<<.operator-example 1.1 "[range[5]] :map[add[1]]">>|^<<.operator-example 1.2 "[range[5]] :and[add[1]]">>|
|^<<.operator-example 2.1 "[range[5]] :map[addsuffix[ hello]]">>|^<<.operator-example 2.2 "[range[5]] :and[addsuffix[ hello]]">>|
|^<<.operator-example 3.1 "[tag[shopping]] :map[get[quantity]]">>|^<<.operator-example 3.2 "[tag[shopping]] :and[get[quantity]]">>|
|!decrease in the number of items|<|
|^<<.operator-example 4.1 "[range[5]] :map[sum[]]">>|^<<.operator-example 4.2 "[range[5]] :and[sum[]]">>|
|^<<.operator-example 5.1 "[range[5]] :map[join[,]]">>|^<<.operator-example 5.2 "[range[5]] :and[join[,]]">>|
|!increase in the number of items|<|
|^<<.operator-example 6.1 "[[1,2,3]] [[4,5]] :map[split[,]]">>|^<<.operator-example 6.2 "[[1,2,3]] [[4,5]] :and[split[,]]">>|
|!de-duplication|<|
|^<<.operator-example 7.1 "[range[5]] :map[[hello]]">>|^<<.operator-example 7.2 "[range[5]] :and[[hello]]">>|
|!currentTiddler|<|
|^<<.operator-example 8.1 "[tag[shopping]] :map[<currentTiddler>]">>|^<<.operator-example 8.2 "[tag[shopping]] :and[<currentTiddler>]">>|
|^<<.operator-example 9.1 "[tag[shopping]] :map[{!!quantity}]">>|^<<.operator-example 9.2 "[tag[shopping]] :and[{!!quantity}]">>|