1.在r語言中，因子（factor）表示的是乙個編號或者乙個等級，即，乙個點。例如，人的個數可以是1，2，3，4……那麼因子就包括，1，2，3，4…..還有描述協變數水平時，會用到高、中、低，也是因子，因為這些都是乙個點。與之區別的向量，是乙個連續性的值，例如，數值中有1，1.1，1.2……可以作為數值來計算，而因子則不可以。簡單通俗來講：因子是乙個點，向量是乙個有方向的範圍。在r中，如果把數字作為因子，那麼在匯入資料之後，需要將向量轉換為因子（factor），而因子在整個計算過程中不再作為數值，而是乙個」符號」而已。2.
以例項進行解釋和說明
data <- c(1,2,2,3,1,2,3,3,1,2,3,3,1)
> data
[1] 122
3123
3123
31 > fdata <- factor(data)
> fdata
[1] 122
3123
3123
31levels: 123
> class(fdata)
[1] "factor"
> class(data)
[1] "numeric"
#factor()函式將原來的數值型的向量轉化為了factor型別。factor型別的向量中有levels的概念。levels就是factor中的所有元素的集合（沒有重複）。我們可以發現levels就是factor中元素排除重複後且字元化的結果。因為levels的元素都是character。
> levels(fdata)
[1] "1"
"2""3"
#我們可以在factor生成時，通過labels向量來指定levels，繼續上面的程式：
> rdata <- factor(data,labels=c("i","ii","iii"))
> rdata
[1] i ii ii iii i ii iii iii i ii iii iii i
levels: i ii iii
> rdata <- factor(data,labels=c("e","ee","eee"))
> rdata
[1] e ee ee eee e ee eee eee e ee eee eee e
levels: e ee eee
#factors可以指定資料的順序
> mons <- c("march","april","january","november","january", "september","october","september","november","august", "january","november","november","february","may","august", "july","december","august","august","september","november", "february","april")
> mons <- factor(mons)
> mons
[1] march april january november january
[6] september october september november august
[11] january november november february may
[16] august july december august august
[21] september november february april
11 levels: april august december february ... september
> table(mons)
mons
april august december february january
24123
july march may november october
11151
september
3#顯然月份是有順序的，我們可以為factor指定順序
mons = factor(mons,levels=c("january","february","march","april","may","june","july","august","september","october","november","december"),ordered=true)
> table(mons)
mons
january february march april may
32121
june july august september october
01431
november december
51
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