title: how to remove multiple columns in r dataframe?
body: <p>I am trying to remove some columns in a dataframe.
  I want to know why it worked for a single column but not with multible columns
  e.g.
  this works </p>
  
  <pre><code>album2[,5]&lt;- NULL
  </code></pre>
  
  <p>this doesn't work</p>
  
  <pre><code>album2[,c(5:7)]&lt;- NULL
  Error in `[&lt;-.data.frame`(`*tmp*`, , 5:7, value = NULL) : 
  replacement has 0 items, need 600
  </code></pre>
  
  <p>This also doesn't work</p>
  
  <pre><code>for (i in 5: (length(album2)-1)){
   album2[,i]&lt;- NULL
  }
  Error in `[&lt;-.data.frame`(`*tmp*`, , i, value = NULL) : 
  new columns would leave holes after existing columns
  </code></pre>