In this lecture, we will explore using a dictionary as an accumulator. Here, we created dictionary mapping fruits to colours. We will run this, in order to explore fruits to colours. So that we can be a little bit more familiar with it. Here is the fruit to colour dictionary, these dictionary entries are not sorted by key or by value. We can index into it using the name of a fruit, in order to find out its colour, so fruit to colour of orange, is orange, fruit to colour of banana, is yellow. We can also iterate over the keys in this dictionary. For fruit, in fruit to colour, allows us to get each fruit in turn, As well as use that fruit as an index into the dictionary in order to look up the corresponding value. And now, we get to the problem we're going to explore today. We are going to turn this dictionary inside out, or invert fruit to colour. We want to build a new dictionary that maps colours to fruits. To do this, we will iterate over the keys in the fruit to colour dictionary, extract the colour corresponding to that fruit, and then add to our new colour to fruit dictionary the colour to fruit entry. Variable colour to fruit is our accumulator. We run this again. And examine fruit to colour and colour to fruit. But colour to fruit is missing some information. There are no pomegranates, no strawberries, no peaches. The problem is that there are more than one fruit for each of the colours associated with those three missing fruits. For example, pomegranates are red, but so are cherries. So, when we added cherry to this dictionary, it replaced the entry for pomegranate. Instead of this straight inversion, we need to be a bit careful. We will build a dictionary that maps each colour to a list of fruits of that colour. If colour is not already a key in the accumulator, we will add a colour to a list with one item in it, the current fruit, as an entry. Otherwise, if colour is already a key that means that it already has a list of fruit of that colour. We need to append fruit to that existing list. Here is the code expressing that algorithm. If it is not the case, that colour is the key in the colour to fruit dictionary, then we will add an entry, mapping colour to a list containing fruit. On the other hand, if colour is already a key in the dictionary, then we would like to get the list that, that colour maps to, and append variable fruit. We will also delete the old code. Let's explore how we've done it. Fruit to colour, is what it always has been. Coulor to fruit now, has keys being strings, and values being list of strings. Which fruit is colored orange? How do we get to the fruit orange? Well, we index into the dictionary using string orange. And then, that gives us a list. We then, index into that using the regular list indexing way to get orange. How can we get to cherry? Well, index into our dictionary using red and cherry is the last item in the list that is returned.
