title: remove character columns from a numeric data frame
body: <p>I have a data frame like the one you see here. </p>
  
  <pre><code>     DRSi       TP        DOC        DN          date     Turbidity   Anions 
       158        5.9       3371       264        14/8/06      5.83    2246.02
       217        4.7       2060       428        16/8/06      6.04    1632.29
       181        10.6      1828       219        16/8/06      6.11    1005.00
       397        5.3       1027       439        16/8/06      5.74    314.19
       2204       81.2      11770      1827       15/8/06      9.64    2635.39
       307        2.9       1954       589        15/8/06      6.12    2762.02
       136        7.1       2712       157        14/8/06      5.83    2049.86
       1502       15.3      4123       959        15/8/06      6.48    2648.12
       1113       1.5       819        195        17/8/06      5.83    804.42
       329        4.1       2264       434        16/8/06      6.19    2214.89
       193        3.5       5691       251        17/8/06      5.64    1299.25
       1152       3.5       2865       1075       15/8/06      5.66    2573.78
       357        4.1       5664       509        16/8/06      6.06    1982.08
       513        7.1       2485       586        15/8/06      6.24    2608.35
       1645       6.5       4878       208        17/8/06      5.96    969.32
  </code></pre>
  
  <p>Before I got here i used the following code to remove those columns that had no values at all or some NA's. </p>
  
  <pre><code>    rem = NULL
      for(col.nr in 1:dim(E.3)[2]){
        if(sum(is.na(E.3[, col.nr]) &gt; 0 | all(is.na(E.3[,col.nr])))){
          rem = c(rem, col.nr)
        }
      }
      E.4 &lt;- E.3[, -rem]  
  </code></pre>
  
  <p>Now I need to remove the "date" column but not based on its column name, rather based on the fact that it's a character string. </p>
  
  <p>I've seen here (<a href="https://stackoverflow.com/questions/6286313/remove-an-entire-column-from-a-data-frame-in-r">Remove an entire column from a data.frame in R</a>) already how to simply set it to NULL and other options but I want to use a different argument. </p>