Functional Programming and Intelligent Algorithms

# Tutorial 6.1: Random weights

## Week 6: Pseudo-Random Number Generators

### 1 Tutorial 6.1: Random weights

In this tutorial we amend the initeialisation function so that we use random starting weights.

Keep a copy of your original program, so that you can compare performance.

#### 1.1 Step 1: The tf-randm package

We are going to use the PRNG provided by the tf-randm package.

1.
First, check that you have installed the package cabal install tf−random 
2.
Run GHCi and test the functions we are going to use: import System.Random import System.Random.TF g <− newTFGen g 

What kind of object is g?

3.
Continue testing: next g 

What is the return value of g?

What is the type of next? (Use the :type command to check.)

4.
What is the Word32 type? Use google or hoogle to look it up if you do not know.

#### 1.2 Step 2: A random floating point value

We want to generate a list of random floating point value, i.e. Double. All the library functions provide integers, so we need to make our own function for this.

You choose whether you want to create a new module for this tutorial, or add the definitions to your neural network module.

1.
To make definitions using the Word32 data type, you need the following import in your module: import Data.Word 
2.
The Word32 type contains integers in the range $0\dots {2}^{32}-1$. What mathematical function can you use to map this range to a real number in the $\left[0,1\right]$ or $\left[0,1\right)$ range.
3.
Make a Haskell function which maps a Word32 to a Double between 0 and 1. toZeroOne :: Word32 −> Double 
4.
The random weights should be in the range $\left(-\alpha ,+\alpha \right)$ for some small value of $\alpha$. Make a function which takes an $\alpha$ value and a Word32 value and produces a number in the $\left(-\alpha ,+\alpha \right)$ range. toWeight :: Double −> Word32 −> Double 

You may use the above toZeroOne if you want to.

#### 1.3 Step 3: A random list

For each neuron, we need a list of weights. In Haskell, it is easiest to generate an infinite list and then use the take function. First we define the following function to generate the infinite list:

rndWeights :: Double −> TFGen −> [Double] The first input is the bound $\alpha$ as defined in the previous step.

Implement this function in whatever way you want. If you cannot see how, use the following hints.

##### 1.3.1 Hints

1.
Write a recursive function which takes a TFGen generator as input and produces a list of Word32. rndList :: TFGen −> [Word32] Use recursion. You do not need a base case here. Why not?
2.
Define the rndWeights function using map, rndList, and toWeight.

#### 1.4 Step 3: A random neuron

Now we can define a new initNeuron function. Let’s call it initNeuronG since it takes a generator as input..

rndNeuron :: TFGen −> Integer −> (Neuron,TFGen) The second input is the number of inputs to the Neuron.

To implement this function, you need to:

• use split to get a new generrator for the TFGen output.
• use rndWeights to produce the random weights for the Neuron output.
• use the take function (look it up if you do not know it) and to get the right number of weights in the Neuron.
• make sure that you do not use the same generator twice.

#### 1.5 Step 4: A random layer

Using all the techniques from Steps 2–3, you can now make a recursive function to generate a random layer of neurons.

##### 1.5.1 Optional Step 4bis

Make a function to generate a random multi-layer network.

#### 1.6 Step 5: Putting it into your program

All the functions we have used so far are pure. They are fully deterministic with the generator as input.

In the main function of your program, you have acess to the IO monad, and thus to newTFGen.

1.
Copy the test program you already have for your perceptron. (If you are confident in what you are doing, you may skip the perceptron and jump straight to the multi-layer network,)
2.
In the copy, you can define you starting generator using g0 <− newTFGen 
3.
Change the initialisation of the network (or perceptron) to use the pseudo-random functions that you have defined above.
4.
Have you ever used the same generator twice? Make sure that you do not.
5.
Repeat this step with a test program for your multi-layer network.

7th April 2017
Hans Georg Schaathun / hasc@ntnu.no