I have a list of lists: 

```
list_1 = [['1','a'],['1','b'],['1','c'],['2','a'],['2','d'], ['1','c']]  

```

How can I iterate through the list to append the second value to another list while grouping by the first value. So it would look something like this:

```
list_2 = [['a','b','c','c'],['a','d']]

```