# Need to do a calculation in logstash
**URL:** https://discuss.elastic.co/t/need-to-do-a-calculation-in-logstash/211117
**Category:** Logstash
**Created:** [December 9, 2019, 1:21pm UTC](https://discuss.elastic.co/t/need-to-do-a-calculation-in-logstash/211117 "2019-12-09T13:21:40Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@namali](https://discuss.elastic.co/u/namali)
#### Post date: [December 9, 2019, 1:21pm UTC](https://discuss.elastic.co/t/need-to-do-a-calculation-in-logstash/211117/1 "2019-12-09T13:21:40Z")
logstash input is elasticsearch and there is field called 'check' in input index.
when ever read the documents in this index need to increment check by 1.
I used ruby filter for this.
ruby{code =\> "event.set('check',[event.get('check\_')].to\_i + 1)"}
getting following error
[2019-12-09T18:48:30,925][ERROR][logstash.filters.ruby] Ruby exception occurred: undefined method `to\_i' for ["0"]:Array
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### Author:  [@Badger](https://discuss.elastic.co/u/Badger)
#### Post date: [December 9, 2019, 3:22pm UTC](https://discuss.elastic.co/t/need-to-do-a-calculation-in-logstash/211117/2 "2019-12-09T15:22:19Z")
Remove the square brackets around event.get('check\_'). They turn it into an array and prevent .to\_i working. Square brackets are used to reference fields in the logstash configuration, not in ruby.
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### Author:  [@namali](https://discuss.elastic.co/u/namali)
#### Post date: [December 10, 2019, 3:43am UTC](https://discuss.elastic.co/t/need-to-do-a-calculation-in-logstash/211117/3 "2019-12-10T03:43:25Z")
thx Badger, it got work.
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### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [January 7, 2020, 3:43am UTC](https://discuss.elastic.co/t/need-to-do-a-calculation-in-logstash/211117/4 "2020-01-07T03:43:29Z")
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