R语言实战

4. 控制流

- 语句（statement）是一条单独的R语句或一组复合语句（包含在花括号｛｝中的一组R语句，使用分号分隔）；

- 条件（cond）是一条最终被解析为真（TRUE）或假（FALSE）的表达式；

- 表达式（expr）是一条数值或字符串的求值语句；

- 序列（seq）是一个数值或字符串序列

4.1 重复和循环

> for (var in seq) statement
> 
> for (i in 1:10) print("Hello")
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
> 
> while (cond) statement
> 
> i <- 10
> while (i > 0) {print("Hello"); i <- i - 1}
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
[1] "Hello"
> 

4.2 条件执行

> if (cond) statement
> if (cond) statement1 else statement2
> 
> if (is.character(grade)) grade <- as.factor(grade)
> if (!is.factor(grade)) grade <- as.factor(grade) else print("Grade already is a factor")
> 
> 
> ifelse(cond, sttement1, statement2)
> 
> ifelse(score > 0.5, print("Passed"), print("Failed"))
> outcome <- ifelse(score>0.5, "Passed", "Failed")
> 
> switch(expr, ...)
> 

> feelings <- c("sad", "afraid")
> for (i in feelings)
+   print(
+     switch(i,
+       happy = "I am glad you are happy",
+       afraid = "There is nothing to fear",
+       sad = "Cheer up",
+       angry = "Calm down now"
+     )
+   )
[1] "Cheer up"
[1] "There is nothing to fear"
> 

5. 用户自编函数

> myfunction <- function(arg1, arg2, ...){
+   statements
+   return(object)
+ }
> 
> # 选择性地给出参数统计量（均值和标准差）
> # 和非参数统计量（中位数和绝对中位数）
> mystets <- function(x, parametric=TRUE, print=FALSE){
+   if (parametric) {
+     center <- mean(x); spread <- sd(x)
+   } else {
+     center <- median(x); spread <- mad(x)
+   }
+   if (print & parametric) {
+     cat ("Mean=", center, "
", "SD=", spread, "
")
+   } else if (print & !parametric) {
+     cat("Median=", center, "
", "MAD=", spread, "
")
+   }
+   result <- list(center=center, spread=spread)
+   return(result)
+ }
> 
> set.seed(1234)
> x <- rnorm(500)
> 
> y <- mystets(x)
> 
> y <- mystets(x, parametric=FALSE, print=TRUE)
Median= -0.021 
MAD= 1 
> 

> mydate <- function(type="long"){
+   switch(type,
+     long = format(Sys.time(), "%A %B %d %Y"),
+     short = format(Sys.time(), "%m-%d-%y"),
+     cat(type, "is not a recognized type
")
+   )
+ }
> 
> mydate("log")
log is not a recognized type
> mydate("long")
[1] "Saturday September 16 2017"
> mydate("short")
[1] "09-16-17"
> mydate()
[1] "Saturday September 16 2017"
>