Décrire la structure et le contenu des data.frames

Diverses fonctions permettent de connaître la structure des data.frames :

is.data.frame() : prédicat pour vérifier la nature d'une variable
```
> is.data.frame(lenses)
[1] TRUE
```

names() ou colnames() : pour récupérer un vecteur contenant les noms des colonnes :

> names(lenses);
[1] "Age"       "Defaut"    "Astygmate" "Larmes"    "Classe"
> colnames(lenses)
[1] "Age"       "Defaut"    "Astygmate" "Larmes"    "Classe"

row.names() : pour récupérer un vecteur contenant les étiquettes des lignes :

> row.names(lenses)
 [1] "1"  "2"  "3"  "4"  "5"  "6"  "7"  "8"  "9"  "10" "11" "12" "13" "14" "15"
[16] "16" "17" "18" "19" "20" "21" "22" "23" "24"

length() ou ncol() : pour connaître le nombre de colonnes

> length(lenses) #taille de la liste
[1] 5
> ncol(lenses)
[1] 5

nrow() pour le nombre de lignes
```
 nrow(lenses)
[1] 24>
```

dimnames() pour obtenir une liste de noms de colonnes et d'étiquettes

> dimnames(iris)
[[1]]
  [1] "1"   "2"   "3"   "4"   "5"   "6"   "7"   "8"   "9"   "10"  "11"  "12" 
 [13] "13"  "14"  "15"  "16"  "17"  "18"  "19"  "20"  "21"  "22"  "23"  "24" 
 [25] "25"  "26"  "27"  "28"  "29"  "30"  "31"  "32"  "33"  "34"  "35"  "36" 
 [37] "37"  "38"  "39"  "40"  "41"  "42"  "43"  "44"  "45"  "46"  "47"  "48" 
 [49] "49"  "50"  "51"  "52"  "53"  "54"  "55"  "56"  "57"  "58"  "59"  "60" 
 [61] "61"  "62"  "63"  "64"  "65"  "66"  "67"  "68"  "69"  "70"  "71"  "72" 
 [73] "73"  "74"  "75"  "76"  "77"  "78"  "79"  "80"  "81"  "82"  "83"  "84" 
 [85] "85"  "86"  "87"  "88"  "89"  "90"  "91"  "92"  "93"  "94"  "95"  "96" 
 [97] "97"  "98"  "99"  "100" "101" "102" "103" "104" "105" "106" "107" "108"
[109] "109" "110" "111" "112" "113" "114" "115" "116" "117" "118" "119" "120"
[121] "121" "122" "123" "124" "125" "126" "127" "128" "129" "130" "131" "132"
[133] "133" "134" "135" "136" "137" "138" "139" "140" "141" "142" "143" "144"
[145] "145" "146" "147" "148" "149" "150"

[[2]]
[1] "Sepal.Length" "Sepal.Width"  "Petal.Length" "Petal.Width"  "Species"

summary() pour résumer le contenu

 summary(lenses)
	     Age             Defaut   Astygmate     Larmes   Classe  
 pre-presbyopic:8   hypermetrope:12   false:12   normal:12   hcl: 4  
 presbyopic    :8   myope       :12   true :12   reduce:12   ncl:15  
 young         :8                                            scl: 5