基本结构展示：
vals =c(5,6,7)
for(v in vals){
print(v)
}
#即把大括号里的内容对vals里的每一个值都循环run一遍
实例展示：
1. paste() 命令是把几个字符连接起来，如paste("A","B","C",sep=" ")得到的就是“A B C”，在次基础上写如下for loop:
partnumber = c(1,2,5,78)
for(i in partnumber){
print(paste("participant number",i, sep = " "))
}
#就可以得到一串参与者号码，根据上面给定的几个值， 从"participant number 1" 到"participant number 8"
2. 双重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 = " "))
}
}
# 先对j循环，后对i循环，得到如下结果
[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"
# 两个loop的话，output得放最中心的loop里面，如果只要要第一层loop,就放在靠外一层括号里面，第二层括号就保留最后的一个值
3. 数据库实例演示
Titanic=read.csv("https://goo.gl/4Gqsnz") #从网络读取数据<0.2, 0.2-0.6还是>0.6。
目的：看不同舱位（Pclass）和不同性别（Sex）的人的生存率是
A<- sort(unique(Pclass)) #sort可以把类别按大小顺序排，unique()命令是把分类变量的种类提取出来
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"))}
}
}
结果如下：
[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"