# Machine Learning

## Learning is Looping

An ML model is **Trained** by **Looping** over data multiple times.

For each iteration, the **Weight Values** are adjusted.

Training is complete when the iterations fails to **Reduce the Cost**.

Train me to find the line of best fit:

## Gradient Descent

**Gradient Descent** is a popular algorithm for solving AI problems.

A simple **Linear Regression Model** can be used to demonstrate a gradient descent.

The goal of a linear regression is to fit a linear graph to a set of (x,y) points.
This can be solved with a math formula. But a **Machine Learning Algorithm** can also solve this.

This is what the example above does.

It starts with a scatter plot and a linear model (y = wx + b).

Then it trains the model to find a line that fits the plot. This is done by altering the weight (slope) and the bias (intercept) of the line.

Below is the code for a **Trainer Object** that can solve this problem
(and many other problems).

## A Trainer Object

Create a Trainer object that can take any number of (x,y) values in two arrays (xArr,yArr).

Set weight to zero and the bias to 1.

A learning constant (learnc) has to be set, and a cost variable must be defined:

Example

```
function Trainer(xArray, yArray) {
```

this.xArr = xArray;

this.yArr = yArray;

this.points = this.xArr.length;

this.learnc = 0.00001;

this.weight = 0;

this.bias = 1;

this.cost;

## Cost Function

A standard way to solve a regression problem is with an "Cost Function" that measures how good the solution is.

The function uses the weight and bias from the model (y = wx + b) and returns an error, based on how well the line fits a plot.

The way to compute this error is to loop through all (x,y) points in the plot, and sum the square distances between the y value of each point and the line.

The most conventional way is to square the distances (to ensure positive values) and to make the error function differentiable.

```
this.costError = function() {
```

total = 0;

for (let i = 0; i < this.points; i++) {

total += (this.yArr[i] - (this.weight * this.xArr[i] + this.bias)) **2;

}

return total / this.points;

}

Another name for the **Cost Function** is **Error Function**.

The formula used in the function is actually this:

**E**is the error (cost)**N**is the total number of observations (points)**y**is the value (label) of each observation**x**is the value (feature) of each observation**m**is the slope (weight)**b**is intercept (bias)**mx + b**is the prediction**1/N * N∑1**is the squared mean value

## The Train Function

We will now run a gradient descent.

The gradient descent algorithm should walk the cost function towards the best line.

Each iteration should update both m and b towards a line with a lower cost (error).

To do that, we add a train function that loops over all the data many times:

```
this.train = function(iter) {
```

for (let i = 0; i < iter; i++) {

this.updateWeights();

}

this.cost = this.costError();

}

## An Update Weights Function

The train function above should update the weights and biases in each iteration.

The direction to move is calculated using two partial derivatives:

```
this.updateWeights = function() {
```

let wx;

let w_deriv = 0;

let b_deriv = 0;

for (let i = 0; i < this.points; i++) {

wx = this.yArr[i] - (this.weight * this.xArr[i] + this.bias);

w_deriv += -2 * wx * this.xArr[i];

b_deriv += -2 * wx;

}

this.weight -= (w_deriv / this.points) * this.learnc;

this.bias -= (b_deriv / this.points) * this.learnc;

}

## Create Your Own Library

### Library Code

```
function Trainer(xArray, yArray) {
```

this.xArr = xArray;

this.yArr = yArray;

this.points = this.xArr.length;

this.learnc = 0.00001;

this.weight = 0;

this.bias = 1;

this.cost;

// Cost Function

this.costError = function() {

total = 0;

for (let i = 0; i < this.points; i++) {

total += (this.yArr[i] - (this.weight * this.xArr[i] + this.bias)) **2;

}

return total / this.points;

}

// Train Function

this.train = function(iter) {

for (let i = 0; i < iter; i++) {

this.updateWeights();

}

this.cost = this.costError();

}

// Update Weights Function

this.updateWeights = function() {

let wx;

let w_deriv = 0;

let b_deriv = 0;

for (let i = 0; i < this.points; i++) {

wx = this.yArr[i] - (this.weight * this.xArr[i] + this.bias);

w_deriv += -2 * wx * this.xArr[i];

b_deriv += -2 * wx;

}

this.weight -= (w_deriv / this.points) * this.learnc;

this.bias -= (b_deriv / this.points) * this.learnc;

}

} // End Trainer Object

Now you can include the library in HTML:

```
<script src="myailib.js"></script>
```