title: Assigning nulls to a column of a data frame in a list
body: <p>My loop takes two data frame columns "Date" and "Time" as to of its arguments.</p>
  
  <p>Because of how the loop is constructed after each dataframe is merged to other data, it is not clear where Date and Time will show up in the column positions in the combined dataframe for element [[i]] in the list.
  Therefore, I need to assign Nulls by argument position i.e. DF[[Date]],DF[[Time]], however, I am not sure how to reference this columns within the list OptData. So OptData[[i]][,4:5] is a bad example, because the column positions for Date and Time will change for each Dataframe element of OptData.
  My revised question is this: How do I assign a null to </p>
  
  <pre><code>for(i in 1:ncol(ADHR)) {...
  
    OptData[[i]]&lt;-join(BEST[[i]],DataObjects[[i]], by="Date", type="left" )
  </code></pre>
  
  <p>i.e. I would think it would be OptData[[i]][,4:5]&lt;-NULL
  Any help is much appreciated.
  thanks</p>