# Deep Learning with Torch: 4

## An alternative library to build complex neural networks

Posted by Fabio Fumarola on October 7, 2015

## Abstract

In this post we analyze nngraph an alternative library to build complex neural networks.

The nngraph library provides tools to create complicated architecture in a “graphical manner”.

## Install

You can install nngraph with

$luarocks install nngraph  As an optional dependecy you can install graphviz, if you have it you will be able to display the graph that you have created. For installing the package run the appropriate command below: # Mac users$ brew install graphviz

# Debian/Ubuntu users
$sudo apt-get install graphviz -y # ArchLinux$ sudo pacman -S graphviz


NB: please use the offical graphviz installer instead of brew otherwise you cannot be able to use it with nngraph.

## Basic Concepts

Let’s briefly recap some nn Torch basic concepts. There are two fundamental parts to build neural networks (from now on simply called nn) : modules and containers:

• The first is an abstraction of an nn layer. It can peform either forward pass (taking an input and generates an output), and backward pass that performs a backpropagation step.
• The latter is used to combine in some way other modules to build complex nn.

## NNGraph

Any Module can be wrapped in a nngraph.Node by require nngraph.

th> require 'nngraph'

th> nn.Identity() -- this create an identity module
nn.Identity

th> nn.Identity()() -- this create an identity module and wraps it in a node
nngraph.Node


The nngraph library provides a way to build any complex network focusing on the network graph and avoiding the use of containers. nngraph overloads the __call__ operator (i.e. the () operator used for function calls) on all nn.Module objects. By executing the __call__ operator it is returned a node wrapping a nn.Module.

h1 = nn.Linear(20, 10)()
h2 = nn.Linear(10, 1)(h1)
mlp = nn.gModule({h1},{h2})


The call operator takes the parents of the node as arguments, which specify which modules will feed into this one during a forward pass.

## Examples

### Example 1

h1 = nn.Linear(20, 10)()
h2 = nn.Linear(10, 1)(h1)
model = nn.gModule({h1},{h2})


The code above describes a multi layer perceptron with 2 hidden layers. The first layer h1 is a Linear transformation from 20 to 10. The second layer gets an input of size 10 and an output of size 1 and takes as input the layer h1. This means that there is an edge the goes out from h1 and enters into h2.

Finally we make model calling nn.gModule with h1 and h2 as parameters. nn.gModule takes 2 input: a table of nodes that are the inputs of the nn and a table of outputs nodes.

Using graphviz we can plot the neural network in in the example above.

require 'nngraph'

-- draw graph (the forward graph, '.fg'), use it with itorch notebook
graph.dot(model.fg, 'MLP')
-- or save graph to file MLP.svg and MLP.dot
graph.dot(model.fg, 'MLP', 'MLP')


The first and last nodes are dummy nodes and regroup all inputs and outputs of the graph. The module entry describes the function of the node, as applies to input, and producing a result of the shape gradOutput; mapindex contains pointers to the parent nodes.

### Example 2

we create a module that takes 2 inputs and 2 outputs

require 'nngraph'

h1 = nn.Linear(20, 20)()
h2 = nn.Linear(10, 10)()
hh1 = nn.Linear(20, 1)(nn.Tanh()(h1))
hh2 = nn.Linear(10, 1)(nn.Tanh()(h2))