# The Gentlest Introduction to Tensorflow – Part 2

Check out the second and final part of this introductory tutorial to TensorFlow.

**By Soon Hin Khor, Co-organizer for Tokyo Tensorflow Meetup**.

Editor's note: You may want to check out part 1 of this tutorial before proceeding.

### Quick Review

In the previous article, we used Tensorflow (TF) to build and learn a linear regression model with a single feature so that given a feature value (house size/sqm), we can predict the outcome (house price/$).

Here is the review with illustration below:

- We have some data of house sizes & house prices (the gray round points)
- We model the data using linear regression (the red dash line)
- We find the ‘best’ model by training W, and b (of the linear regression model) to minimize the ‘cost’ (the sum of the length of vertical blue lines, which represent the differences between predictions and actual outcomes)
- Given any house size, we can use the linear model to predict the house size (the dotted blue lines with arrows)

*Linear regression explained in a single diagram*

In machine learning (ML) literature, we come across the term ‘training’ very often, let us literally look at what that means in TF.

### Linear Regression Modeling

Linear Model (in TF notation): y = tf.matmul(x,W) + b

The goal in linear regression is to find W, b, such that given any feature value (x), we can find the prediction (y) by substituting W, x, b values into the model.

However to find W, b that can give accurate predictions, we need to ‘train’ the model using available data (the multiple pairs of actual feature (x), and actual outcome (y_), note the *underscore*).

### ‘Training’ Illustrated

To find the best W, b values, we can initially start with any W, b values. We also need to define a cost function, which is a measure of the *difference*between the prediction (y) for given a feature value (x), and the actual outcome (y_) for that same feature value (x). For simplicity, we use leastminimum squared error (MSE) as our cost function.

Cost function (in TF notation): tf.reduce_mean(tf.square(y_ - y))

By minimizing the cost function, we can arrive at good W, b values.

Our code to do training is actually very simple and it is labelled with [A, B, C, D], which we will refer to later on. The full source is on Github.

# ... (snip) Variable/Constants declarations (snip) ... # [A] TF.Graph y = tf.matmul(x,W) + b cost = tf.reduce_mean(tf.square(y_-y)) # [B] Train with fixed 'learn_rate' learn_rate = 0.1 train_step = tf.train.GradientDescentOptimizer(learn_rate).minimize(cost) for i in range(steps): # [C] Prepare datapoints # ... (snip) Code to prepare datapoint as xs, and ys (snip) ... # [D] Feed Data at each step/epoch into 'train_step' feed = { x: xs, y_: ys } sess.run(train_step, feed_dict=feed)

Our linear model and cost function equations [A] can be represented as TF graph as shown:

*Create a TF Graph with model & cost, and initialize W, b with some values*

Next, we select a datapoint (x, y_) [C], and feed [D] it into the TF Graph to get the prediction (y) as well as the cost.

*Calculate prediction (y) & cost using a single datapoint*

To get better W, b, we perform gradient descent using TF’s*tf.train.GradientDescentOptimizer* [B] to reduce the cost. In non-technical terms: given the current cost, and based on the graph of how cost varies with other variables (namely W, b), the optimizer will perform small tweaks (increments/decrements) to W, b so that our prediction becomes better for*that single datapoint*.

*Based on current cost, determine how to tweak W, b to improve prediction (y) and reduce cost*

The final step in the training cycle is to update the W, b after tweaking them. Note that ‘cycle’ is also referred to as ‘epoch’ in ML literature.

*Update W, b after tweaking them, and before iterating through the next training epoch*

In the next training epoch, repeat the steps, but use a different datapoint!

*Training using different datapoints*

Using a variety of datapoints generalizes our model, i.e., it learns W, b values that can be used to predict *any* feature value. Note that:

- In most cases, the more datapoints, the better your model can learn and generalize
- If you train more epochs than datapoints you have, you can re-use datapoints, which is not an issue. The gradient descent optimizer always use both the datapoint,
*AND*the cost (calculated from the datapoint, with W, b values of that epoch) to tweak W, b; the optimizer may have seen that datapoint before, but not with the same cost, thus it will learn something new, and tweak W, b differently.

You can train the model a fixed number of epochs or until it reaches a cost threshold that is satisfactory.

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