title: non-numeric argument to binary operator in R
body: <p>I am trying to use gam function. But i am getting an error which states non numeric argument to binary operator. i tried using lapply(sample,as.numeric) but it did not work i also tried as.numeric(sample$VOl) even that did not fix my problem.</p>
  
  <pre><code>library(mgcv)
  par(mfrow = c(2,2))
  model&lt;-gam(lmpg~SP*lhp+WT*lhp+I(SP^2)+I(WT^2)+I(lhp)^2)+VOL
  plot(model)
  par(mfrow = c(1,1))
  </code></pre>
  
  <p>my sample looks like this</p>
  
  <pre><code>   VOL  SP   WT     lmpg      lhp
  1   89  96 17.5 4.180522 3.891820
  2   92  97 20.0 4.025352 4.007333
  3   92  97 20.0 4.023564 4.007333
  4   92 105 20.0 3.891820 4.248495
  5   92  96 20.0 3.839452 3.970292
  6   89 105 20.0 3.832980 4.248495
  7   92  97 20.0 3.815512 4.007333
  8   50  98 22.5 4.080922 4.127134
  9   50  98 22.5 3.975936 4.127134
  10  94 107 22.5 3.770459 4.382027
  11  89 103 22.5 3.716008 4.290459
  12  50 113 22.5 3.711130 4.521789
  13  99 113 22.5 3.711130 4.521789
  14  89 103 22.5 3.698830 4.290459
  15  89 100 22.5 3.678829 4.189655
  16  89 103 22.5 3.671225 4.290459
  17  91 106 22.5 3.660994 4.356709
  18  50 113 22.5 3.658420 4.521789
  19  91 106 22.5 3.642836 4.356709
  20 103 109 25.0 3.742420 4.499810
  21  99 110 25.0 3.711130 4.521789
  22 107 101 25.0 3.706228 4.304065
  23 101 111 25.0 3.688879 4.553877
  24  96 105 25.0 3.671225 4.394449
  25  89 111 25.0 3.658420 4.553877
  26  50 110 25.0 3.648057 4.521789
  27 117 110 25.0 3.648057 4.521789
  28  99 110 25.0 3.648057 4.521789
  29 102 109 25.0 3.384390 4.499810
  30 104  90 27.5 3.848018 3.951244
  31 107 112 27.5 3.591818 4.634729
  32 114 103 27.5 3.586293 4.430817
  33 101 103 27.5 3.586293 4.430817
  34  97 111 27.5 3.566712 4.624973
  35 113 111 27.5 3.563883 4.624973
  36 101 102 27.5 3.558201 4.394449
  37  98 106 27.5 3.558201 4.499810
  38  88 106 27.5 3.555348 4.499810
  39  86 109 30.0 3.502550 4.624973
  40  86 109 30.0 3.493473 4.624973
  41  92 120 30.0 3.475067 4.867534
  42 113 106 30.0 3.471966 4.553877
  43 106 106 30.0 3.471966 4.553877
  44  92 109 30.0 3.471966 4.624973
  45  88 106 30.0 3.471966 4.553877
  46 102 105 30.0 3.449988 4.532599
  47  99 108 30.0 3.449988 4.605170
  48 111 108 30.0 3.446808 4.605170
  49 103 107 30.0 3.446808 4.584967
  50  86 120 30.0 3.440418 4.867534
  51 101 109 35.0 3.517498 4.744932
  52 101 109 35.0 3.484312 4.744932
  53 101 109 35.0 3.443618 4.744932
  54 124 109 35.0 3.443618 4.744932
  55 113 133 35.0 3.414443 5.192957
  56 113 125 35.0 3.363842 5.075174
  57 124 115 35.0 3.332205 4.867534
  58  92 102 35.0 3.332205 4.564348
  59 101 109 35.0 3.332205 4.744932
  60  94 104 35.0 3.332205 4.605170
  61 115 105 35.0 3.332205 4.605170
  62 111 120 35.0 3.321432 4.976734
  63 116 107 40.0 3.242592 4.787492
  64 131 114 40.0 3.230804 4.941642
  65 123 114 40.0 3.173878 4.941642
  66 121 117 40.0 3.161247 5.010635
  67  50 122 40.0 3.161247 5.105945
  68 114 122 40.0 3.161247 5.105945
  69 127 122 40.0 3.161247 5.105945
  70 123 122 40.0 3.161247 5.105945
  71 112 148 40.0 3.157000 5.501258
  72  50 160 40.0 3.152736 5.634790
  73 135 121 40.0 3.152736 5.087596
  74 132 121 40.0 3.139833 5.087596
  75 160 110 45.0 3.131137 4.941642
  76 129 110 45.0 3.131137 4.941642
  77 129 121 45.0 2.970414 5.164786
  78  50 165 45.0 2.895912 5.774552
  79 115 140 45.0 2.844909 5.472271
  80  50 147 45.0 2.833213 5.572154
  81 119 157 45.0 2.815409 5.686975
  82 107 130 55.0 2.580217 5.463832
  </code></pre>
  
  <p>can any one please help me with this error. Thanks in advance.</p>