switch() 语句用法 [英] switch() statement usage

问题描述

我对 R 中的 switch 语句有点困惑.只需谷歌搜索功能，我得到一个例子如下:

I am a little confused about the switch statement in R. Simply googling the function I get an example as follows:

switch 的一个常见用途是根据函数参数之一的字符值进行分支.

A common use of switch is to branch according to the character value of one of the arguments to a function.

 > centre <- function(x, type) {
 + switch(type,
 +        mean = mean(x),
 +        median = median(x),
 +        trimmed = mean(x, trim = .1))
 + }
 > x <- rcauchy(10)
 > centre(x, "mean")
 [1] 0.8760325
 > centre(x, "median")
 [1] 0.5360891
 > centre(x, "trimmed")
 [1] 0.6086504

然而，这似乎只是为每个 type

However this just seems to be the same as just having a bunch of if statements designated for each type

这就是 switch() 的全部内容吗?有人能给我更多的例子和更好的应用吗?

Is that all there is to switch()? Can someone give me further examples and better applications?

推荐答案

好吧，又到了救援的时机.看起来 switch 通常比 if 语句更快.因此，以及使用 switch 语句使代码更短/更整洁的事实倾向于使用 switch:

Well, timing to the rescue again. It seems switch is generally faster than if statements. So that, and the fact that the code is shorter/neater with a switch statement leans in favor of switch:

# Simplified to only measure the overhead of switch vs if

test1 <- function(type) {
 switch(type,
        mean = 1,
        median = 2,
        trimmed = 3)
}

test2 <- function(type) {
 if (type == "mean") 1
 else if (type == "median") 2
 else if (type == "trimmed") 3
}

system.time( for(i in 1:1e6) test1('mean') ) # 0.89 secs
system.time( for(i in 1:1e6) test2('mean') ) # 1.13 secs
system.time( for(i in 1:1e6) test1('trimmed') ) # 0.89 secs
system.time( for(i in 1:1e6) test2('trimmed') ) # 2.28 secs

更新 考虑到 Joshua 的评论，我尝试了其他方法来进行基准测试.微基准似乎是最好的....它显示了类似的时间:

Update With Joshua's comment in mind, I tried other ways to benchmark. The microbenchmark seems the best. ...and it shows similar timings:

> library(microbenchmark)
> microbenchmark(test1('mean'), test2('mean'), times=1e6)
Unit: nanoseconds
           expr  min   lq median   uq      max
1 test1("mean")  709  771    864  951 16122411
2 test2("mean") 1007 1073   1147 1223  8012202

> microbenchmark(test1('trimmed'), test2('trimmed'), times=1e6)
Unit: nanoseconds
              expr  min   lq median   uq      max
1 test1("trimmed")  733  792    843  944 60440833
2 test2("trimmed") 2022 2133   2203 2309 60814430

最终更新 这里展示了 switch 的多功能性:

Final Update Here's showing how versatile switch is:

switch(type, case1=1, case2=, case3=2.5, 99)

这将 case2 和 case3 映射到 2.5 并且(未命名)默认为 99.有关更多信息，请尝试 ?switch

This maps case2 and case3 to 2.5 and the (unnamed) default to 99. For more information, try ?switch

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