Grails GSP tag: grep

31 / Oct / 2012 by Abhishek Tejpaul 0 comments

The other day I was reading the Grails docs and I came across a useful GSP tag: grep. I have been using Grails for over 3 years now but just recently got to see this new tag which has eased my life a bit in situations where the list of objects have to be filtered and iterated at the same time to perform some operations on them. The GSP ‘grep’ tag (equivalent to the ‘grep’ method in Groovy) filters the list on a given condition and iterates over the filtered list allowing you to do some operations on the objects (or whatever you want to do with the filtered objects).

I will give you a small example: Suppose we have a list of Person objects. You want to display the names starting with A, B, C and so on in different color schemes. How would you do it?

One implementation is to send different lists from the controllers (this would mean sending 26 lists i.e. one list for each character in the English alphabet) and render them in different color schemes. The other way would be to iterate over the Persons list in the GSP and haveĀ  different <g:if>…<:g:elseif> statements (for each character).

There is another smarter way: you could also use the ‘findAll‘ method of the List in the GSP <g:each> tag’s ‘in’ attribute i.e <g:each in=${personList.findAll {it.name.startsWith(‘A’)}} var=”person”> … </g:each>.

Similarly, we can use ‘grep’ tag here. We can filter out the list like this: <g:grep in=”${personList.name}” filter=”~/^A.*/”> … </g:grep>.

Notice the use of ‘name‘ field of the Person object in the ‘in‘ attribute of the grep tag. Also, the attribute ‘filter’ takes many forms of filter options; here we are using regex to filter the persons with the name starting with ‘A’.

I am not saying that this tag provides some new functionality which we could not have achieved by any other means but this tag makes it easier to filter out the lists on some specific parameters.

You can read more about this tag here. Also you can read about the Groovy GDK’s grep method usage here to see how and in what scenarios you can use this tag.

Hope this helps !!

Abhishek Tejpaul
abhishek@intelligrape.com

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