ifelse每次都真的计算两个向量吗? 它慢吗?

ifelse是否真的计算了yesnovector – 如在每个vector的整体? 还是只是从每个vector计算一些值?

另外, ifelse真的那么慢?

是。 (除了)

ifelse计算它的yes值和no值。 除了test条件全部为TRUE或全部为FALSE

我们可以通过生成随机数字并观察实际生成的数字来看到这一点。 (通过恢复seed )。

 # TEST CONDITION, ALL TRUE set.seed(1) dump <- ifelse(rep(TRUE, 200), rnorm(200), rnorm(200)) next.random.number.after.all.true <- rnorm(1) # TEST CONDITION, ALL FALSE set.seed(1) dump <- ifelse(rep(FALSE, 200), rnorm(200), rnorm(200)) next.random.number.after.all.false <- rnorm(1) # TEST CONDITION, MIXED set.seed(1) dump <- ifelse(c(FALSE, rep(TRUE, 199)), rnorm(200), rnorm(200)) next.random.number.after.some.TRUE.some.FALSE <- rnorm(1) # RESET THE SEED, GENERATE SEVERAL RANDOM NUMBERS TO SEARCH FOR A MATCH set.seed(1) r.1000 <- rnorm(1000) cat("Quantity of random numbers generated during the `ifelse` statement when:", "\n\tAll True ", which(r.1000 == next.random.number.after.all.true) - 1, "\n\tAll False ", which(r.1000 == next.random.number.after.all.false) - 1, "\n\tMixed T/F ", which(r.1000 == next.random.number.after.some.TRUE.some.FALSE) - 1 ) 

给出以下输出:

 Quantity of random numbers generated during the `ifelse` statement when: All True 200 All False 200 Mixed T/F 400 <~~ Notice TWICE AS MANY numbers were generated when `test` had both T & F values present 

我们也可以在源代码中看到它:

 . . if (any(test[!nas])) ans[test & !nas] <- rep(yes, length.out = length(ans))[test & # <~~~~ This line and the one below !nas] if (any(!test[!nas])) ans[!test & !nas] <- rep(no, length.out = length(ans))[!test & # <~~~~ ... are the cluprits !nas] . . 

请注意,只有在test某些非NA值分别为TRUEFALSE才会计算“ yes和“ no
在哪一点上 – 当涉及效率时这是重要的部分 – 计算每个向量的整体


好的,但速度较慢?

让我们看看我们是否可以testing它:

 library(microbenchmark) # Create some sample data N <- 1e4 set.seed(1) X <- sample(c(seq(100), rep(NA, 100)), N, TRUE) Y <- ifelse(is.na(X), rnorm(X), NA) # Y has reverse NA/not-NA setup than X 

这两个语句产生相同的结果

 yesifelse <- quote(sort(ifelse(is.na(X), Y+17, X-17 ) )) noiflese <- quote(sort(c(Y[is.na(X)]+17, X[is.na(Y)]-17))) identical(eval(yesifelse), eval(noiflese)) # [1] TRUE 

但其中一个是另一个的两倍

 microbenchmark(eval(yesifelse), eval(noiflese), times=50L) N = 1,000 Unit: milliseconds expr min lq median uq max neval eval(yesifelse) 2.286621 2.348590 2.411776 2.537604 10.05973 50 eval(noiflese) 1.088669 1.093864 1.122075 1.149558 61.23110 50 N = 10,000 Unit: milliseconds expr min lq median uq max neval eval(yesifelse) 30.32039 36.19569 38.50461 40.84996 98.77294 50 eval(noiflese) 12.70274 13.58295 14.38579 20.03587 21.68665 50