Question

I have:

ListOne = ['foo', 'bar', ..]

I now want to create a new List by zipping ListOne with ListTwo. ListTwo looks like this:

ListTwo = [{count, 1},{count, 1},{count,1}, ..] 

What’s a nice way to dynamically generate ListTwo? Every list item will be the same.

I want to feed the result of the zip to dict:from_list. So maybe a zip is not the best approach.

Answer 1

While your question could be more specific, my guess is that you want to use a dictionary to store key, value pairs where the value is a counter. What you are trying to achieve by asking this, is how to initialize the dict with the counter set to 1.

The folloing code will create a new dict, where the key is the key from ListOne and the value is 1:

ListOne = [foo, bar, baz, quux].
D1 = dict:from_list([{Elem, 1} || Elem <- ListOne]).

Now, to increment these counters, you can use dict:update_counter/3:

D2 = dict:update_counter(foo, 1, D1).

To decrement, you would simply give a negative number as the increment.

It is also worth noting that dict:update_counter/3, will create the key in the dict if it is not already present, with the given increment as the initial value.