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34 lines
1.9 KiB
Plaintext
34 lines
1.9 KiB
Plaintext
caption: reduce
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created: 20201004154131193
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modified: 20201006174749170
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op-input: a [[selection of titles|Title Selection]] passed as input to the filter
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op-output: the final result of running the subfilter <<.place S>>
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op-parameter: a [[filter expression|Filter Expression]]
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op-parameter-name: S
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op-purpose: apply a subfilter to each input title in turn, accumulating a single value
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op-suffix: Initial value for accumulator
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op-suffix-name: V
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tags: [[Filter Operators]]
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title: reduce Operator
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type: text/vnd.tiddlywiki
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\define reduce-tip()
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The <<.op reduce>> operator will always produce output, even if its input was empty. If its input is empty, the output of <<.op reduce>> will be the initial value of the accumulator, i.e. the value of the suffix. One result of this fact is that the <<.op else>> operator will never be triggered if it follows a <<.op reduce>>. The "Empty input" examples show what happens when <<.op reduce>> receives no input.
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\end
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<<.from-version "5.1.23">> The <<.op reduce>> operator runs a subfilter for each input title, passing the result of the previous subfilter run as a variable. The initial value of the accumulator can optionally be specified. It returns the result of the final subfilter run.
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The <<.op reduce>> operator is used to flatten a list of items down to a single item by repeatedly applying a formula. A typical use is to add up the values in a given field of a list of tiddlers.
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The following variables are available within the subfilter:
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* ''accumulator'' - the result of the previous subfilter run
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* ''currentTiddler'' - the input title
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* ''index'' - the numeric index of the current list item (with zero being the first item in the list)
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* ''revIndex'' - the reverse numeric index of the current list item (with zero being the last item in the list)
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* ''length'' - the total length of the input list
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<$macrocall $name=".tip" _=<<reduce-tip>> />
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<<.operator-examples "reduce">>
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