# Identifying fields using sprintf
**URL:** https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941
**Category:** Logstash
**Created:** [April 20, 2016, 5:45pm UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941 "2016-04-20T17:45:51Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@CraigFoote](https://discuss.elastic.co/u/CraigFoote)
#### Post date: [April 20, 2016, 5:45pm UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941/1 "2016-04-20T17:45:52Z")
I have some JSON data coming to me over HTTP. Each record has a field that needs to be converted to an integer but its name will vary, e.g. "fieldA\_count" in one record and "fieldfB\_count" in another. Each record contains another identifying field named "field\_name" with a value "fieldA" or "fieldB". I want to use the identifying field to name the field I need to convert to an integer, i.e.
```
mutate{
convert => { "%{field_name}_count => "integer" }
}
```
I'd like this evaluated to:
```
mutate{
convert => { "fieldA_count => "integer" }
}
```
...or:
```
mutate{
convert => { "fieldB_count => "integer" }
}
```
But when I tried this the fields remained Strings. What am I doing wrong?
Craig
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 20, 2016, 5:52pm UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941/2 "2016-04-20T17:52:37Z")
Yeah, the name of the field to convert isn't subject to sprintf expansion. This is already reported in [issue #57](https://github.com/logstash-plugins/logstash-filter-mutate/issues/57). Until that's fixed you can use a ruby filter.
---
### Author:  [@CraigFoote](https://discuss.elastic.co/u/CraigFoote)
#### Post date: [April 20, 2016, 6:19pm UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941/3 "2016-04-20T18:19:56Z")
Thanks Magnus, do you think this would work? (I'm pretty new at Ruby)
```
field_prefix = event['prefix']
field_name = field_prefix + '_count'
event[field_name] = event[field_name].to_i
```
---
### Author:  [@magnusbaeck](https://discuss.elastic.co/u/magnusbaeck)
#### Post date: [April 20, 2016, 7:56pm UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941/4 "2016-04-20T19:56:23Z")
Why not just try it out? But yeah, it looks good
---
### Author:  [@CraigFoote](https://discuss.elastic.co/u/CraigFoote)
#### Post date: [April 21, 2016, 2:28pm UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941/5 "2016-04-21T14:28:49Z")
Just lazy I guess 😉 I did try it and it worked, thanks for the help.
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [July 6, 2017, 5:01am UTC](https://discuss.elastic.co/t/identifying-fields-using-sprintf/47941/6 "2017-07-06T05:01:15Z")