# Basic Tutorial¶

An illustration of how parameter collections are trained (for a simple logistic regression model) is below:

First, we set up the structure of the parameter collection.

Create a parameter collection, and an SGD trainer to update its parameters.

ParameterCollection pc;
SimpleSGDTrainer trainer(pc);


Create a “computation graph,” which will define the flow of information.

ComputationGraph cg;


Initialize a 1x3 parameter vector, and add the parameters to be part of the computation graph.

Expression W = parameter(cg, pc.add_parameters({1, 3}));


Create variables defining the input and output of the regression, and load them into the computation graph. Note that we don’t need to set concrete values yet.

vector<dynet::real> x_values(3);
Expression x = input(cg, {3}, &x_values);
dynet::real y_value;
Expression y = input(cg, &y_value);


Next, set up the structure to multiply the input by the weight vector, then run the output of this through a logistic sigmoid function logistic regression).

Expression y_pred = logistic(W*x);


Finally, we create a function to calculate the loss. The model will be optimized to minimize the value of the final function in the computation graph.

Expression l = binary_log_loss(y_pred, y);


We are now done setting up the graph, and we can print out its structure:

cg.print_graphviz();


Now, we perform a parameter update for a single example. Set the input/output to the values specified by the training data:

x_values = {0.5, 0.3, 0.7};
y_value = 1.0;


“forward” propagates values forward through the computation graph, and returns the loss.

dynet::real loss = as_scalar(cg.forward(l));


“backward” performs back-propagation, and accumulates the gradients of the parameters within the ParameterCollection data structure.

cg.backward(l);


trainer.update updates parameters of the parameter collection that was passed to its constructor. Here 1.0 is the scaling factor that allows us to control the size of the update.

trainer.update();


Note that this very simple example that doesn’t cover things like memory initialization, reading/writing parameter collections, recurrent/LSTM networks, or adding biases to functions. The best way to get an idea of how to use DyNet for real is to look in the example directory, particularly starting with the simplest xor example.