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84 lines
2.0 KiB
Markdown
84 lines
2.0 KiB
Markdown
# Reduce
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Reduce is a powerful way to process a collection of data into a new form.
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```
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<exp> as $<name> ireduce (<init>; <block>)
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```
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e.g.
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```
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.[] as $item ireduce (0; . + $item)
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```
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On the LHS we are configuring the collection of items that will be reduced `<exp>` as well as what each element will be called `$<name>`. Note that the array has been splatted into its individual elements.
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On the RHS there is `<init>`, the starting value of the accumulator and `<block>`, the expression that will update the accumulator for each element in the collection. Note that within the block expression, `.` will evaluate to the current value of the accumulator.
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## yq vs jq syntax
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Reduce syntax in `yq` is a little different from `jq` - as `yq` (currently) isn't as sophisticated as `jq` and its only supports infix notation (e.g. a + b, where the operator is in the middle of the two parameters) - where as `jq` uses a mix of infix notation with _prefix_ notation (e.g. `reduce a b` is like writing `+ a b`).
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To that end, the reduce operator is called `ireduce` for backwards compatability if a `jq` like prefix version of `reduce` is ever added.
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{% hint style="warning" %}
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Note that versions prior to 4.18 require the 'eval/e' command to be specified. 
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`yq e <exp> <file>`
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{% endhint %}
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## Sum numbers
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Given a sample.yml file of:
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```yaml
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- 10
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- 2
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- 5
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- 3
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```
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then
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```bash
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yq '.[] as $item ireduce (0; . + $item)' sample.yml
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```
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will output
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```yaml
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20
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```
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## Merge all yaml files together
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Given a sample.yml file of:
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```yaml
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a: cat
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```
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And another sample another.yml file of:
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```yaml
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b: dog
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```
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then
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```bash
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yq eval-all '. as $item ireduce ({}; . * $item )' sample.yml another.yml
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```
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will output
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```yaml
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a: cat
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b: dog
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```
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## Convert an array to an object
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Given a sample.yml file of:
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```yaml
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- name: Cathy
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has: apples
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- name: Bob
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has: bananas
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```
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then
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```bash
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yq '.[] as $item ireduce ({}; .[$item | .name] = ($item | .has) )' sample.yml
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```
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will output
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```yaml
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Cathy: apples
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Bob: bananas
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```
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