title: Sum selected rows in a character matrix: error 'x' must be numeric in R
body: <p>I am new at R and this is really basic, but it is not working for me. I want to sum the rows from col 6:11 in this data.frame, for instance:</p>
  
  <p>dput(tab.res)</p>
  
  <blockquote>
    <p>tab.res &lt;- structure(c("RIL", "RIL", "RIL", "RIL", "RIL", "RIL", "RIL",  "RIL",
    "RIL", "1993", "1994", "1995", "1996", "1998", "2000",  "2003",
    "2006", "2009", "0.622648030237929", "0.622780354568876", 
    "0.623028859270455", "0.623336617104674", "0.624702676975245", 
    "0.62229896748978", "0.619100952240793", "0.62242230375717", 
    "0.621052639967347", "21.8279151394679", "18.558235100949",
    "19.1066487340136",  "19.4658454721166", "19.7990392265612",
    "20.5366574203101", "22.3737846431613",  "21.6823832708024",
    "22.5832057096871", "558.462585034014", "460.190476190476", 
    "457.319727891156", "455.210884353742", "437.78231292517",
    "455.360544217687",  "569.442176870748", "488.367346938776",
    "512.421768707483", "42.048152226551",  "35.7094330210724",
    "36.4311339004591", "35.8460590917321", "33.3991893243174", 
    "33.9938868563961", "39.9166803633779", "34.7098561898221",
    "35.4769784708925",  "44.0903088045558", "40.5555666070647",
    "41.419372563604", "43.5776682933757",  "45.5922072021568",
    "46.0154886164247", "47.3378504648163", "46.41940144389", 
    "49.5580273139249", "40.6511227624141", "39.1466514500133",
    "40.4409858368967",  "41.4903119307394", "42.9108185069093",
    "45.7459942527593", "47.7880868127877",  "48.8291523581031",
    "48.3273707080036", "31.3644667603767", "31.0971941969479", 
    "32.4804982013221", "32.4553994494653", "34.2369548403931",
    "35.6674106248615",  "38.53520691726", "38.8160980443",
    "40.8055122694451", "24.5415097525631",  "23.5956393932039",
    "23.6298291129159", "24.6786066881199", "25.3372707363408", 
    "25.7293935423119", "26.2842616375337", "29.2438109789201",
    "29.6350482936186",  "76.6236464549925", "44.5672068727091",
    "46.962777170159", "48.0267179639631",  "51.298941016064",
    "53.5121260383574", "55.4511984380194", "53.5652556461721", 
    "55.2830178740705"), .Dim = c(9L, 11L), .Dimnames = list(NULL, 
        c("Treat", "Year", "WD", "BA", "StemD", "AGB.10.20", "AGB.20.30", 
        "ABG.30.40", "ABG.40.50", "ABG.50.60", "ABG.60.+")))</p>
  </blockquote>
  
  <p>This should have solved the case:</p>
  
  <blockquote>
    <p>tab.res$total &lt;- rowSums(subset(tab.res, select=6:11))</p>
  </blockquote>
  
  <p>But I keep getting: </p>
  
  <blockquote>
    <p>Error in rowSums(subset(tab.res, select = 6:11)) : 'x' must be numeric</p>
  </blockquote>
  
  <p>I tried to use "as.numeric" to transform whatever is getting the error but I am not getting anywhere... Anyone call tell me where is the problem?</p>