shortcut for creating a Map from a List in groovy?

Question

I'd like some sorthand for this:

Map rowToMap(row) {
    def rowMap = [:];
    row.columns.each{ rowMap[it.name] = it.val }
    return rowMap;
}

given the way the GDK stuff is, I'd expect to be able to do something like:

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

but I haven't seen anything in the docs... am I missing something? or am I just way too lazy?

Answer 1

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]

Answer 2

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 <<.

Answer 3

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')
}

Answer 4

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

Answer 5

What about something like this?

// setup
class Pair { 
    String k; 
    String v; 
    public Pair(def k, def v) { this.k = k ; this.v = v; }
}
def list = [ new Pair('a', 'b'), new Pair('c', 'd') ]

// the idea
def map = [:]
list.each{ it -> map.putAt(it.k, it.v) }

// verify
println map['c']

Answer 6

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());