title: Column mean of data.frame (list) in R
body: <p><a href="http://i.stack.imgur.com/ZYsmv.jpg" rel="nofollow">Data</a></p>
  
  <p>Please, I need to calculate the mean of the column "Sepal.Length" for the specie virginica in this data.frame:</p>
  
  <pre><code>Sepal.Length Sepal.Width Petal.Length Petal.Width    Species
  1            5.1         3.5          1.4         0.2     setosa
  2            4.9         3.0          1.4         0.2     setosa
  3            4.7         3.2          1.3         0.2     setosa
  4            4.6         3.1          1.5         0.2     setosa
  5            5.0         3.6          1.4         0.2     setosa
  6            5.4         3.9          1.7         0.4     setosa
  7            4.6         3.4          1.4         0.3     setosa
  8            5.0         3.4          1.5         0.2     setosa
  9            4.4         2.9          1.4         0.2     setosa
  10           4.9         3.1          1.5         0.1     setosa
  11           5.4         3.7          1.5         0.2     setosa
  12           4.8         3.4          1.6         0.2     setosa
  13           4.8         3.0          1.4         0.1     setosa
  14           4.3         3.0          1.1         0.1     setosa
  15           5.8         4.0          1.2         0.2     setosa
  16           5.7         4.4          1.5         0.4     setosa
  17           5.4         3.9          1.3         0.4     setosa
  18           5.1         3.5          1.4         0.3     setosa
  19           5.7         3.8          1.7         0.3     setosa
  20           5.1         3.8          1.5         0.3     setosa
  21           5.4         3.4          1.7         0.2     setosa
  22           5.1         3.7          1.5         0.4     setosa
  23           4.6         3.6          1.0         0.2     setosa
  24           5.1         3.3          1.7         0.5     setosa
  25           4.8         3.4          1.9         0.2     setosa
  26           5.0         3.0          1.6         0.2     setosa
  27           5.0         3.4          1.6         0.4     setosa
  28           5.2         3.5          1.5         0.2     setosa
  29           5.2         3.4          1.4         0.2     setosa
  30           4.7         3.2          1.6         0.2     setosa
  31           4.8         3.1          1.6         0.2     setosa
  32           5.4         3.4          1.5         0.4     setosa
  33           5.2         4.1          1.5         0.1     setosa
  34           5.5         4.2          1.4         0.2     setosa
  35           4.9         3.1          1.5         0.2     setosa
  36           5.0         3.2          1.2         0.2     setosa
  37           5.5         3.5          1.3         0.2     setosa
  38           4.9         3.6          1.4         0.1     setosa
  39           4.4         3.0          1.3         0.2     setosa
  40           5.1         3.4          1.5         0.2     setosa
  41           5.0         3.5          1.3         0.3     setosa
  42           4.5         2.3          1.3         0.3     setosa
  43           4.4         3.2          1.3         0.2     setosa
  44           5.0         3.5          1.6         0.6     setosa
  45           5.1         3.8          1.9         0.4     setosa
  46           4.8         3.0          1.4         0.3     setosa
  47           5.1         3.8          1.6         0.2     setosa
  48           4.6         3.2          1.4         0.2     setosa
  49           5.3         3.7          1.5         0.2     setosa
  50           5.0         3.3          1.4         0.2     setosa
  51           7.0         3.2          4.7         1.4 versicolor
  52           6.4         3.2          4.5         1.5 versicolor
  53           6.9         3.1          4.9         1.5 versicolor
  54           5.5         2.3          4.0         1.3 versicolor
  55           6.5         2.8          4.6         1.5 versicolor
  56           5.7         2.8          4.5         1.3 versicolor
  57           6.3         3.3          4.7         1.6 versicolor
  58           4.9         2.4          3.3         1.0 versicolor
  59           6.6         2.9          4.6         1.3 versicolor
  60           5.2         2.7          3.9         1.4 versicolor
  61           5.0         2.0          3.5         1.0 versicolor
  62           5.9         3.0          4.2         1.5 versicolor
  63           6.0         2.2          4.0         1.0 versicolor
  64           6.1         2.9          4.7         1.4 versicolor
  65           5.6         2.9          3.6         1.3 versicolor
  66           6.7         3.1          4.4         1.4 versicolor
  67           5.6         3.0          4.5         1.5 versicolor
  68           5.8         2.7          4.1         1.0 versicolor
  69           6.2         2.2          4.5         1.5 versicolor
  70           5.6         2.5          3.9         1.1 versicolor
  71           5.9         3.2          4.8         1.8 versicolor
  72           6.1         2.8          4.0         1.3 versicolor
  73           6.3         2.5          4.9         1.5 versicolor
  74           6.1         2.8          4.7         1.2 versicolor
  75           6.4         2.9          4.3         1.3 versicolor
  76           6.6         3.0          4.4         1.4 versicolor
  77           6.8         2.8          4.8         1.4 versicolor
  78           6.7         3.0          5.0         1.7 versicolor
  79           6.0         2.9          4.5         1.5 versicolor
  80           5.7         2.6          3.5         1.0 versicolor
  81           5.5         2.4          3.8         1.1 versicolor
  82           5.5         2.4          3.7         1.0 versicolor
  83           5.8         2.7          3.9         1.2 versicolor
  84           6.0         2.7          5.1         1.6 versicolor
  85           5.4         3.0          4.5         1.5 versicolor
  86           6.0         3.4          4.5         1.6 versicolor
  87           6.7         3.1          4.7         1.5 versicolor
  88           6.3         2.3          4.4         1.3 versicolor
  89           5.6         3.0          4.1         1.3 versicolor
  90           5.5         2.5          4.0         1.3 versicolor
  91           5.5         2.6          4.4         1.2 versicolor
  92           6.1         3.0          4.6         1.4 versicolor
  93           5.8         2.6          4.0         1.2 versicolor
  94           5.0         2.3          3.3         1.0 versicolor
  95           5.6         2.7          4.2         1.3 versicolor
  96           5.7         3.0          4.2         1.2 versicolor
  97           5.7         2.9          4.2         1.3 versicolor
  98           6.2         2.9          4.3         1.3 versicolor
  99           5.1         2.5          3.0         1.1 versicolor
  100          5.7         2.8          4.1         1.3 versicolor
  101          6.3         3.3          6.0         2.5  virginica
  102          5.8         2.7          5.1         1.9  virginica
  103          7.1         3.0          5.9         2.1  virginica
  104          6.3         2.9          5.6         1.8  virginica
  105          6.5         3.0          5.8         2.2  virginica
  106          7.6         3.0          6.6         2.1  virginica
  107          4.9         2.5          4.5         1.7  virginica
  108          7.3         2.9          6.3         1.8  virginica
  109          6.7         2.5          5.8         1.8  virginica
  110          7.2         3.6          6.1         2.5  virginica
  111          6.5         3.2          5.1         2.0  virginica
  112          6.4         2.7          5.3         1.9  virginica
  113          6.8         3.0          5.5         2.1  virginica
  114          5.7         2.5          5.0         2.0  virginica
  115          5.8         2.8          5.1         2.4  virginica
  116          6.4         3.2          5.3         2.3  virginica
  117          6.5         3.0          5.5         1.8  virginica
  118          7.7         3.8          6.7         2.2  virginica
  119          7.7         2.6          6.9         2.3  virginica
  120          6.0         2.2          5.0         1.5  virginica
  121          6.9         3.2          5.7         2.3  virginica
  122          5.6         2.8          4.9         2.0  virginica
  123          7.7         2.8          6.7         2.0  virginica
  124          6.3         2.7          4.9         1.8  virginica
  125          6.7         3.3          5.7         2.1  virginica
  126          7.2         3.2          6.0         1.8  virginica
  127          6.2         2.8          4.8         1.8  virginica
  128          6.1         3.0          4.9         1.8  virginica
  129          6.4         2.8          5.6         2.1  virginica
  130          7.2         3.0          5.8         1.6  virginica
  131          7.4         2.8          6.1         1.9  virginica
  132          7.9         3.8          6.4         2.0  virginica
  133          6.4         2.8          5.6         2.2  virginica
  134          6.3         2.8          5.1         1.5  virginica
  135          6.1         2.6          5.6         1.4  virginica
  136          7.7         3.0          6.1         2.3  virginica
  137          6.3         3.4          5.6         2.4  virginica
  138          6.4         3.1          5.5         1.8  virginica
  139          6.0         3.0          4.8         1.8  virginica
  140          6.9         3.1          5.4         2.1  virginica
  141          6.7         3.1          5.6         2.4  virginica
  142          6.9         3.1          5.1         2.3  virginica
  143          5.8         2.7          5.1         1.9  virginica
  144          6.8         3.2          5.9         2.3  virginica
  145          6.7         3.3          5.7         2.5  virginica
  146          6.7         3.0          5.2         2.3  virginica
  147          6.3         2.5          5.0         1.9  virginica
  148          6.5         3.0          5.2         2.0  virginica
  149          6.2         3.4          5.4         2.3  virginica
  150          5.9         3.0          5.1         1.8  virginica
  </code></pre>
  
  <p>When I used  <code>sapply(s, function(x) colMeans(x[, c("virginica")], na.rm =TRUE))</code></p>
  
  <p>I get this error:</p>
  
  <pre><code>Error in `[.data.frame`(x, , c("virginica")) : undefined columns selected
  </code></pre>
  
  <p>When I use <code>sapply(split(iris[1:2],iris$Specie),mean, na.rm = TRUE)</code></p>
  
  <p>I get this response: </p>
  
  <pre><code>    setosa versicolor  virginica 
          NA         NA         NA 
  Mensajes de aviso perdidos
  1: In mean.default(X[[1L]], ...) :
    argument is not numeric or logical: returning NA
  2: In mean.default(X[[2L]], ...) :
    argument is not numeric or logical: returning NA
  3: In mean.default(X[[3L]], ...) :
    argument is not numeric or logical: returning NA
  </code></pre>
  
  <p>AND
  When I use <code>apply(iris,2,mean)</code></p>
  
  <p>I get this:</p>
  
  <pre><code>Sepal.Length  Sepal.Width Petal.Length  Petal.Width      Species 
            NA           NA           NA           NA           NA 
  Mensajes de aviso perdidos
  1: In mean.default(newX[, i], ...) :
    argument is not numeric or logical: returning NA
  2: In mean.default(newX[, i], ...) :
    argument is not numeric or logical: returning NA
  3: In mean.default(newX[, i], ...) :
    argument is not numeric or logical: returning NA
  4: In mean.default(newX[, i], ...) :
    argument is not numeric or logical: returning NA
  5: In mean.default(newX[, i], ...) :
    argument is not numeric or logical: returning NA
  </code></pre>
  
  <p>I am new in programming.</p>
  
  <p>Thanks in advance!!</p>