基本結構展示：
vals =c(5,6,7)
for(v in vals){
print(v)
}
#即把大括號裏的內容對vals裏的每一個值都循環run一遍
實例展示：
1. paste() 命令是把幾個字符連接起來，如paste("A","B","C",sep=" ")得到的就是“A B C”，在次基礎上寫如下for loop:
partnumber = c(1,2,5,78)
for(i in partnumber){
print(paste("participant number",i, sep = " "))
}
#就可以得到一串參與者號碼，根據上面給定的幾個值， 從"participant number 1" 到"participant number 8"
2. 雙重loop
partnumber = c(1,2,5,78)
institution =c("cancer center", "RMH", "Florey")
for(i in partnumber){
for(j in institution){
print(paste("participant number",i,", institution",j,sep = " "))
}
}
# 先對j循環，後對i循環，得到如下結果
[1] "participant number 1 , institution cancer center"
[1] "participant number 1 , institution RMH"
[1] "participant number 1 , institution Florey"
[1] "participant number 2 , institution cancer center"
[1] "participant number 2 , institution RMH"
[1] "participant number 2 , institution Florey"
[1] "participant number 5 , institution cancer center"
[1] "participant number 5 , institution RMH"
[1] "participant number 5 , institution Florey"
[1] "participant number 78 , institution cancer center"
[1] "participant number 78 , institution RMH"
[1] "participant number 78 , institution Florey"
# 兩個loop的話，output得放最中心的loop裏面，如果只要要第一層loop,就放在靠外一層括號裏面，第二層括號就保留最後的一個值
3. 數據庫實例演示
Titanic=read.csv("https://goo.gl/4Gqsnz") #從網絡讀取數據<0.2, 0.2-0.6還是>0.6。
目的：看不同艙位（Pclass）和不同性別（Sex）的人的生存率是
A<- sort(unique(Pclass)) #sort可以把類別按大小順序排，unique()命令是把分類變量的種類提取出來
B<- sort(unique(Sex))
for(i in A){
for(j in B){
if(mean(Survived[Pclass==i&Sex==j])<0.2){
print(paste("for class",i,"sex",j,"mean survival is less than 0.2"))
} else if (mean(Survived[Pclass==i&Sex==j])>0.6){
print(paste("for class",i,"sex",j,"mean survival is more than 0.6"))
} else {
print(paste("for class",i,"sex",j,"mean survival is between 0.2 and 0.6"))}
}
}
結果如下：
[1] "for class 1 sex female mean survival is more than 0.6"
[1] "for class 1 sex male mean survival is between 0.2 and 0.6"
[1] "for class 2 sex female mean survival is more than 0.6"
[1] "for class 2 sex male mean survival is less than 0.2"
[1] "for class 3 sex female mean survival is between 0.2 and 0.6"
[1] "for class 3 sex male mean survival is less than 0.2"
