How do I find the maximum value of a map in Rascal?

I have a map[str, int], and need to find the maximum int value in the map. Is it possible to get all values of a map as a list, and then do max(list)? Or something like that..

Solution

You can get back the values a couple of different ways. For reference, say we have the following map:

rascal>map[str words, int counts] m =  ( "word1" : 3, "word2" : 10, "word3" : 4, "word4" : 10);
map[str words, int counts]: ("word1":3,"word2":10,"word3":4,"word4":10)

You could view this as holding words and the frequency with which they appear in a document. There are now several ways to get back the values. Since we gave the domain and range both names when we declared the type of m (words and counts, respectively), we can use those names to get back the values. For instance:

rascal>m.words;
set[str]: {"word1","word2","word3","word4"}

gets back the keys, while:

rascal>m.counts;
set[int]: {10,3,4}

gets back the values. You can also use "projection" notation, projecting out the keys or values, like so:

rascal>m<0>;
set[str]: {"word1","word2","word3","word4"}
rascal>m<1>;
set[int]: {10,3,4}

This works even if we didn't add names to the domain and range in the type. A third option, which also works even when we don't add names, is you can use library functions from the Map library:

rascal>import Map;
ok
rascal>domain(m);
set[str]: {"word1","word2","word3","word4"}
rascal>range(m);
set[int]: {10,3,4}

Now that we have a set with the values, you can use the max function in the Set library to find the biggest number:

rascal>import Set;
ok
rascal>max(m.counts);
int: 10

You now have different options if what you really want is the key or keys with this value. The Map library contains a function called invert which will "flip" the keys and values. Since you could have multiple keys with the same value, this actually gives you a map from the value to all the keys that had this value. In our case, this looks like this:

rascal>invert(m);
map[int, set[str]]: (
  10:{"word2","word4"},
  3:{"word1"},
  4:{"word3"}
)

So, if we want to get back the words that have the maximum numeric value, we could do the following:

rascal>invert(m)[max(m.counts)];
set[str]: {"word2","word4"}