Explain how data is aggregated in R.

There are two methods that is collapsing data by using one or more BY variable and other is aggregate() function in which BY variable should be in list.

Do python allow multi threading?

Python doesn't allow multi-threading in the truest sense of the word. It has a multi-threading package but if you want to multi-thread to speed your code up, then it's usually not a good idea to use it. Python has a construct called the Global Interpreter Lock (GIL). The GIL makes sure that only one of your 'threads' can execute at any one time. A thread acquires the GIL, does a little work, then passes the GIL onto the next thread. This happens very quickly so to the human eye it may seem like your threads are executing in parallel, but they are really just taking turns using the same CPU core. All this GIL passing adds overhead to execution. This means that if you want to make your code run faster then using the threading package often isn't a good idea.

What is the difference between supervised and unsupervised machine learning? with a apt example.?

Supervised learning requires training labeled data. For example, in order to do classification (a supervised learning task), you’ll need to first label the data you’ll use to train the model to classify data into your labeled groups. Unsupervised learning, in contrast, does not require labeling data explicitly. For example: Taking Arjuna Character from the famous epic so called mahabharata who under learnt his skills of Bow and Arrow supervised from a guru so called Dronacharya is a good example of supervised learning Where as in the same epic Ekalavya is an other character who learnt the skills of Bow and Arrow with no one Supervising Him.