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chickwts[,2] 

##  [1] horsebean horsebean horsebean horsebean horsebean horsebean horsebean
##  [8] horsebean horsebean horsebean linseed   linseed   linseed   linseed  
## [15] linseed   linseed   linseed   linseed   linseed   linseed   linseed  
## [22] linseed   soybean   soybean   soybean   soybean   soybean   soybean  
## [29] soybean   soybean   soybean   soybean   soybean   soybean   soybean  
## [36] soybean   sunflower sunflower sunflower sunflower sunflower sunflower
## [43] sunflower sunflower sunflower sunflower sunflower sunflower meatmeal 
## [50] meatmeal  meatmeal  meatmeal  meatmeal  meatmeal  meatmeal  meatmeal 
## [57] meatmeal  meatmeal  meatmeal  casein    casein    casein    casein   
## [64] casein    casein    casein    casein    casein    casein    casein   
## [71] casein   
## Levels: casein horsebean linseed meatmeal soybean sunflower

chickwts[2,] 

##   weight      feed
## 2    160 horsebean

chickwts["feed"] 

##         feed
## 1  horsebean
## 2  horsebean
## 3  horsebean
## 4  horsebean
## 5  horsebean
## 6  horsebean
## 7  horsebean
## 8  horsebean
## 9  horsebean
## 10 horsebean
## 11   linseed
## 12   linseed
## 13   linseed
## 14   linseed
## 15   linseed
## 16   linseed
## 17   linseed
## 18   linseed
## 19   linseed
## 20   linseed
## 21   linseed
## 22   linseed
## 23   soybean
## 24   soybean
## 25   soybean
## 26   soybean
## 27   soybean
## 28   soybean
## 29   soybean
## 30   soybean
## 31   soybean
## 32   soybean
## 33   soybean
## 34   soybean
## 35   soybean
## 36   soybean
## 37 sunflower
## 38 sunflower
## 39 sunflower
## 40 sunflower
## 41 sunflower
## 42 sunflower
## 43 sunflower
## 44 sunflower
## 45 sunflower
## 46 sunflower
## 47 sunflower
## 48 sunflower
## 49  meatmeal
## 50  meatmeal
## 51  meatmeal
## 52  meatmeal
## 53  meatmeal
## 54  meatmeal
## 55  meatmeal
## 56  meatmeal
## 57  meatmeal
## 58  meatmeal
## 59  meatmeal
## 60    casein
## 61    casein
## 62    casein
## 63    casein
## 64    casein
## 65    casein
## 66    casein
## 67    casein
## 68    casein
## 69    casein
## 70    casein
## 71    casein

str(chickwts)

## 'data.frame':    71 obs. of  2 variables:
##  $ weight: num  179 160 136 227 217 168 108 124 143 140 ...
##  $ feed  : Factor w/ 6 levels "casein","horsebean",..: 2 2 2 2 2 2 2 2 2 2 ...

dimnames(chickwts)

## [[1]]
##  [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" "25" "26" "27" "28" "29" "30"
## [31] "31" "32" "33" "34" "35" "36" "37" "38" "39" "40" "41" "42" "43" "44" "45"
## [46] "46" "47" "48" "49" "50" "51" "52" "53" "54" "55" "56" "57" "58" "59" "60"
## [61] "61" "62" "63" "64" "65" "66" "67" "68" "69" "70" "71"
## 
## [[2]]
## [1] "weight" "feed"

chickwts[,2] 這個指令的意思是查找chickwt當中第二欄的所有類別變項資料 chickwts[“feed”] 這個指令是查找dataframe資料中feed變項的觀察值