title: R neural package : cannot run the mlp example?
body: <p>I'm trying to learn the neural package that i think is simple compared to others. I tried to run the following example that is given in the correspending documentation :</p>
  <p><a href="https://cran.r-project.org/web/packages/neural/neural.pdf" rel="nofollow noreferrer">https://cran.r-project.org/web/packages/neural/neural.pdf</a>  ( page 5 ).</p>
  <pre><code>library(neural)
  
  x&lt;-matrix(c(1,1,0,0,1,0,1,0),4,2)
  y&lt;-matrix(c(0,1,1,0),4,1)
  neurons&lt;-4
  data&lt;-mlptrain(x,neurons,y,it=4000)
  mlp(x,data$weight,data$dist,data$neurons,data$actfns)
  </code></pre>
  <p>The script produces this error :</p>
  <pre><code>Error in if ((!visual) | (coor$x &gt; 640) &amp; (coor$x &lt; 700) &amp; (coor$y &gt; 650) &amp;  : 
    argument is of length zero
  Calls: mlptrain
  Execution halted
  </code></pre>
  <p>Session info :</p>
  <pre><code>sh: 1: cannot create /dev/null: Permission denied
  R version 3.4.4 (2018-03-15)
  Platform: x86_64-pc-linux-gnu (64-bit)
  
  Matrix products: default
  BLAS: /usr/lib/atlas-base/atlas/libblas.so.3.0
  LAPACK: /usr/lib/lapack/liblapack.so.3.0
  
  locale:
  [1] C
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  other attached packages:
  [1] neural_1.4.2.2
  
  loaded via a namespace (and not attached):
  [1] compiler_3.4.4
  Error in if ((!visual) | (coor$x &gt; 640) &amp; (coor$x &lt; 700) &amp; (coor$y &gt; 650) &amp;  : 
    argument is of length zero
  Calls: mlptrain
  Execution halted
  </code></pre>
  <p>The problem is that I cannot visualize the predicted and observed values.
  Thank you for help in advance !</p>