Introduction

An agent is learning if it improves its performance on future tasks after making observations about the world.
Agents that can learn are useful for several reasons:
- A programmer cannot necessarily anticipate all possible situations that a agent might encounter.
- A programmer cannot necessarily predict all changes which may occur over time.
- A programmer might not have any idea how to program a solution to some problem themselves.
To begin our study of learning we look at some different kinds of learning, then we focus on supervised learning, and, in particular, learning for decision trees.

Forms of Learning

Improvements to components of an agent, and techniques used to make them, depend on:
- Which component is improved
- What prior knowledge the agent already has
- What representation is used for the data and the component.
- What feedback is available to learn from

Components to be Learned

We have described several agents so far this semester. The components of these agents which might be improved include:

The mapping from conditions on the current state to actions
The means to infer relevant properties of the world from percept sequence
Information about the way the world evolves and about the results of possible actions the agent can take.
Utility information indicating the desirability of world states
Action-value information indicating the desirability of actions
Goals that describe classes of states whose achievement maximizes the agent's utility.

The book gives examples of the above in terms of a taxi driver agent.

Representation and Prior Knowledge

There are many possible representations for agent components: propositional or first-order sentences; we could develop our probabilistic model into somethings called a Bayesian network, and so on.
Effective learning algorithms are known when knowledge is represented in each of these systems,
We will be interested in the case where knowledge is in a factored representation: A vector of attribute values.
When we go from input-output pairs and attempt to learn a general function that governs these pairs, we say we are doing inductive learning.
If we instead start from general rules, and derive things logically entailed from them, we say we are doing deductive or analytic learning.

Feedback to learn from

There are three main types of feedback that correspond to the three main types of learning:

In unsupervised learning the agent learns patterns in the input even though no explicit feedback is supplied. A common task in this area is clustering: detecting potentially useful clusters of input examples.
In reinforcement learning the agent learns from a series of reinforcements -- rewards or punishments. For example, the lack of a tip at the end of the journey gives the taxi agent an indication that it did something wrong. Winning a chess game give an agent an indication is did something right.
In supervised learning the agent observes some examples input-output pairs and learns a function that maps from input to output.

In addition, to the above there is also things such as semi-supervised learning where we are given a few labeled examples and must make what we can of a large collection of unlabeled examples.

Supervised Learning

The task of supervised learning is:
Given a training set of `N` example input-output pairs
`(x_1, y_1), ..., (x_n, y_n)`,
where each `y_j` was generated by an unknown function `y=f(x)`, discover a function `h` that approximates the true function `f`.
Here `x` and `y` can be any values; they need not be numbers.
The function `h` is a hypothesis.
Learning is a search through the space of possible hypotheses for one that performs well.
What we want is that it continues to perform well on examples beyond the training set.
To measure this we give it a test set of examples that are distinct from the training set and see how well it predicts their values.
Sometimes the the function `f` is stochastic -- we want to learn a conditional probability distribution `\vec{P}(Y|x)`.
When the output `y` is one of a finite collection of values, the learning problem is called classification.
When `y` is always a number the learning problem is called regression.

Ockham's Razor

Consider a collection 10 of (x,y) pairs all with distinct x value.
Regardless of where they are on the plane there will be many functions that go through them.
We will approximate the real function with some hypothesis `h` from the hypothesis space.
Any `h` that agrees with all the data is called a consistent hypothesis.
Again, there may be many consistent hypotheses, so which one should we choose?
One answer is to prefer the simplest hypothesis consistent with the data.
This is called Ockham's Razor.
The question then becomes how to decide what simplicity means.
For example, we might prefer solutions which are degree 1 polynomials over degree 7 polynomials.
One might also look at how many bits it takes to write down the function together with how many bits it takes to write down an deviations from that the data has from the function.

Learning Decision Trees

We are now going to look at one of the simplest models in which we can do supervised inductive learning: decision trees.
A decision tree represents a function that takes as input a vector of attribute values and returns a decision -- a single output value.
The input and output can be discrete or continuous.
We are going to focus on the case where the output is Boolean: true (a positive example) or false (a negative example). These will be the values of the leaves of our trees.
A decision tree reaches its decision by performing a sequence of tests.
Each internal node of our tree will correspond to querying the value of one of the attributes of the problem. For example, we might have the attribute Restaurant as a node in the tree. From it we could have branches labeled: French, Italian, Thai, or Burger.

Inducing decision trees from examples.

An example for a Boolean decision tree consists of a pair `(vec(x), y)`. Here `\vec(x)` is a value for each attribute and `y` is either true or false.
Such an example can be viewed as a path in a decision tree.
For example, if the problem was whether to wait for a seat at a restaurant, the first coordinate of `vec(x)` might correspond to whether there are already patrons at the restaurant. There might be three possibilities: None, Some, and Full. But `vec(x)` says one specific choice. This corresponds to the decision at the top node of the tree.
Similarly, the next attribute might be something like Wait Estimate? The particular value for this coordinate might be 30-60. It corresponds to a next level in the tree.
Here we are envisioning that the top of the tree corresponds to the decision made by the first element of `\vec(x)`. Typically, we have a set E of training pairs. A tree might actually query the variable coordinates in `vec(x)` in whatever order it wants. Not all branches in the tree need to be the same length. A tree is consistent with the examples in E if for every pair `(vec(x), y)` in E if we follow the path down the tree according to the settings in `vec(x)` when we hit a leaf it has value `y`.

Decision Tree Learning Algorithm

Here is an algorithm for coming up with a decision for a set of examples.

function Decision-Tree-Learning(examples, attributes, parent_examples) returns a tree
    if examples is empty then return Plurality-Value(parent_examples)
    else if all examples have the same classification then return the classification
    else if attributes is empty then return Plurality-Value(examples)
    else
        A := argmax_(a in attributes)Importance(a, examples)
        tree :=  a new decision tree with root test A
        for each value v_k of A do
            exs := {e : e in examples and e.A = v_k}
            subtree := Decision-Tree-Learning(exs, attributes -  A, examples)
            add a branch to tree with label (A=v_k) and subtree subtree
        return tree

Learning From Examples

Outline