title: R: plot histogram of all columns in a data.frame
body: <p>I'm a new user in R and I've just started to work with it to see the distribution of my data but I got stuck on this error. I have a data frame and I would like to plot histograms of it's numeric columns. So what I did is as bellow :</p>
  
  <pre><code>num_data &lt;-my_data[, sapply(my_data, is.numeric)] 
  for (i in 1:length(names(num_data))){
    print(i)
    hist( num_data[i], main='hist', breaks=20, prob=TRUE)
  }
  </code></pre>
  
  <p>But I get the error 'Error in hist.default(num_data[i], main = "hist", breaks = 20, prob = TRUE) : 
    'x' must be numeric '
  I checked the type of num_data[i] and it is a list of numeric values. SO I have no idea of what is the problem. Can any one please give me hint?</p>