title: Neural Network Backpropagation?
body: <p>Can anyone recommend a website or give me a brief of how backpropagation is implemented in a NN? I understand the basic concept, but I'm unsure of how to go about writing the code.</p>
  
  <p>Many of sources I've found simply show equations without giving any explanation of why they're doing it, and the variable names make it difficult to find out.</p>
  
  <p>Example: </p>
  
  <pre><code>void bpnn_output_error(delta, target, output, nj, err)
  double *delta, *target, *output, *err;
  int nj;
  {
    int j;
    double o, t, errsum;
  
    errsum = 0.0;
    for (j = 1; j &lt;= nj; j++) {
      o = output[j];
      t = target[j];
      delta[j] = o * (1.0 - o) * (t - o);
      errsum += ABS(delta[j]);
    }
    *err = errsum;
  
  }
  </code></pre>
  
  <p>In that example, can someone explain the purpose of </p>
  
  <pre><code>delta[j] = o * (1.0 - o) * (t - o);
  </code></pre>
  
  <p>Thanks.</p>