shortcut for creating a Map from a List in groovy?

Question

I've recently came across the need to do exactly that: converting a list into a map. This question was posted before Groovy version 1.7.9 came out, so the method collectEntries didn't exist yet. It works exactly as the collectMap method that was proposed:

Map rowToMap(row) {
    row.columns.collectEntries{[it.name, it.val]}
}

If for some reason you are stuck with an older Groovy version, the inject method can also be used (as proposed here). This is a slightly modified version that takes only one expression inside the closure (just for the sake of character saving!):

Map rowToMap(row) {
    row.columns.inject([:]) {map, col -> map << [(col.name): col.val]}
}

The + operator can also be used instead of the <<.

Check out "inject". Real functional programming wonks call it "fold".

columns.inject([:]) { memo, entry ->
    memo[entry.name] = entry.val
    return memo
}

And, while you're at it, you probably want to define methods as Categories instead of right on the metaClass. That way, you can define it once for all Collections:

class PropertyMapCategory {
    static Map mapProperty(Collection c, String keyParam, String valParam) {
        return c.inject([:]) { memo, entry ->
            memo[entry[keyParam]] = entry[valParam]
            return memo
        }
    }
}

Example usage:

use(PropertyMapCategory) {
    println columns.mapProperty('name', 'val')
}

Was the groupBy method not available when this question was asked?

If what you need is a simple key-value pair, then the method collectEntries should suffice. For example

def names = ['Foo', 'Bar']
def firstAlphabetVsName = names.collectEntries {[it.charAt(0), it]} // [F:Foo, B:Bar]

But if you want a structure similar to a Multimap, in which there are multiple values per key, then you'd want to use the groupBy method

def names = ['Foo', 'Bar', 'Fooey']
def firstAlphabetVsNames = names.groupBy { it.charAt(0) } // [F:[Foo, Fooey], B:[Bar]]

Also, if you're use google collections (http://code.google.com/p/google-collections/), you can do something like this:

  map = Maps.uniqueIndex(list, Functions.identity());

ok... I've played with this a little more and I think this is a pretty cool method...

def collectMap = {Closure callback->
    def map = [:]
    delegate.each {
        def r = callback.call(it)
        map[r[0]] = r[1]
    }
    return map
}
ExpandoMetaClass.enableGlobally()
Collection.metaClass.collectMap = collectMap
Map.metaClass.collectMap = collectMap

now any subclass of Map or Collection have this method...

here I use it to reverse the key/value in a Map

[1:2, 3:4].collectMap{[it.value, it.key]} == [2:1, 4:3]

and here I use it to create a map from a list

[1,2].collectMap{[it,it]} == [1:1, 2:2]

now I just pop this into a class that gets called as my app is starting and this method is available throughout my code.

EDIT:

to add the method to all arrays...

Object[].metaClass.collectMap = collectMap

I can't find anything built in... but using the ExpandoMetaClass I can do this:

ArrayList.metaClass.collectMap = {Closure callback->
    def map = [:]
    delegate.each {
        def r = callback.call(it)
        map[r[0]] = r[1]
    }
    return map
}

this adds the collectMap method to all ArrayLists... I'm not sure why adding it to List or Collection didn't work.. I guess that's for another question... but now I can do this...

assert ["foo":"oof", "42":"24", "bar":"rab"] ==
            ["foo", "42", "bar"].collectMap { return [it, it.reverse()] }

from List to calculated Map with one closure... exactly what I was looking for.

Edit: the reason I couldn't add the method to the interfaces List and Collection was because I did not do this:

List.metaClass.enableGlobally()

after that method call, you can add methods to interfaces.. which in this case means my collectMap method will work on ranges like this:

(0..2).collectMap{[it, it*2]}

which yields the map: [0:0, 1:2, 2:4]