Bioequivalence and Bioavailability Forum • Nasty beast

Nasty beast [Software]

posted by Helmut – Vienna, Austria, 2019-07-18 22:26 (2531 d 00:12 ago) – Posting: # 20396
Views: 33,224

Dear Ohlbe,

❝ I made multiple attempts with "if" and conditions, and got multiple different error messages. ElMaestro gave me some hints, resulting each time in a "oh yes of course" reaction and more error messages... I probably misinterpreted the hints.

Using if(), its relatives, and loop-constructs (for(), while(), repeat()) in many cases are prone to errors – as you experienced – and can be slooow. If you have a vector of data (say y) there a various ways to access its values. Example with 25 random integers in the range 1…40:

n <- 25 y <- round(runif(n = n, min = 1, max = 40)) th <- 10 # threshold n <- length(y) # we know but useful later head(y, 10) # the first 10 y[1:10] # same tail(y, 10) # the last 10 y[(n - 9):n] # same but tricky blq <- which(y < th) # the ones are below the threshold length(blq) # how many? y[blq] # show them y[y < th] # same

In such a simple case which() is an overkill, though one can include many conditions which [sic] makes the code more easy to understand. If you have two vectors (say x, y) like in my last post, you can select values of x depending on which(y = condition).
Let’s check:

n <- 50 x <- runif(n = n, min = 1, max = 20) a <- 0.5 b <- 2 y <- a + b * x + rnorm(n = length(x), mean = 0, sd = 2) th <- 10 fun1 <- function(x, y, th) { # clumsy x.th <- numeric() y.th <- numeric() bql <- 0L for (k in seq_along(x)) { if (y[k] < th) { bql <- bql + 1 x.th[bql] <- x[k] y.th[bql] <- y[k] } } z <- data.frame(x.th, y.th) return(invisible(z)) } fun2 <- function(x, y, th) { # better blq <- which(y < th) x.th <- x[blq] y.th <- y[blq] z <- data.frame(x.th, y.th) return(invisible(z)) } fun3 <- function(x, y, th) { # a little bit confusing for beginners x.th <- x[y < th] y.th <- y[y < th] z <- data.frame(x.th, y.th) return(invisible(z)) } res1 <- fun1(x, y, th) res2 <- fun2(x, y, th) res3 <- fun3(x, y, th) identical(res1, res2); identical(res1, res3) # same? [1] TRUE [1] TRUE

Bingo! Which one is easier to read?
What about speed?

library(microbenchmark) res <- microbenchmark(fun1(x, y, th), fun2(x, y, th), fun3(x, y, th), times = 1000L) print(res, signif = 4) Unit: microseconds expr min lq mean median uq max neval cld fun1(x, y, th) 173.9 181.1 196.1 186.3 189.9 1530 1000 b fun2(x, y, th) 163.0 170.3 183.6 175.4 179.0 3311 1000 a fun3(x, y, th) 163.0 169.0 185.5 174.5 178.4 3310 1000 ab

Practically the same, since vectors are short. If we play this game with longer vectors fun2() shows its strengths.

[image]

If one has more conditions any external construct will suck. We have 100 random integers (1…50) and want to get the even ones between 20 and 30 in increasing order.

x <- round(runif(n = 100, min = 1, max = 50)) x [1] 37 29 8 34 6 13 47 22 30 44 6 14 33 18 12 37 32 [18] 10 6 37 27 2 43 40 7 5 47 4 32 17 7 50 39 36 [35] 38 48 34 5 21 43 34 50 29 20 33 6 45 32 28 8 1 [52] 26 29 19 42 9 38 31 25 4 1 23 37 31 2 26 29 24 [69] 40 43 17 16 41 17 5 17 36 16 7 5 36 30 5 8 19 [86] 40 42 30 33 21 13 25 21 33 16 7 33 36 19 37

One-liner:

sort(x[which(x >= 20 & x <= 30 & x %%2 == 0)]) [1] 20 22 24 26 26 28 30 30 30

Or even shorter:

sort(x[x >= 20 & x <= 30 & x %%2 == 0]) [1] 20 22 24 26 26 28 30 30 30

Good luck coding that with a loop and a nested if() { do this } else { do that } elseif { oops }.

—
Dif-tor heh smusma 🖖🏼 Довге життя Україна!
Helmut Schütz

The quality of responses received is directly proportional to the quality of the question asked. 🚮
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Complete thread:

Spreadsheet failures, any recent examples? ElMaestro 2019-07-18 13:43
- Spreadsheet failures, any recent examples? Ohlbe 2019-07-18 14:32
  - Spreadsheet addiction Helmut 2019-07-18 16:12
    - Nasty beast Ohlbe 2019-07-18 17:30
      - Nasty beastHelmut 2019-07-18 20:26
        
        Nasty beast Ohlbe 2019-07-19 11:32
        
        Nasty beast ElMaestro 2019-07-19 12:32
        
        Decidedly off topic Ohlbe 2019-07-19 13:38
        
        OT: R limbo 101 Helmut 2019-07-19 13:00
    - Spreadsheet addiction Shuanghe 2019-07-18 18:59
      - Spreadsheet addiction Helmut 2019-07-18 20:04
        
        OT: Spreadsheet addiction Shuanghe 2019-07-19 12:41
        
        OT: Spreadsheet addiction Helmut 2019-07-19 14:29
        
        OT: Spreadsheet addiction nobody 2019-07-19 15:53
        
        OT: Spreadsheet addiction Helmut 2019-07-19 19:32
  - Spreadsheet etc. failures zizou 2019-07-20 00:18
    - As designed ☺ Helmut 2019-07-20 02:12
      - As designed ☺ ElMaestro 2019-07-20 09:08
        
        To round or not to round… Helmut 2019-07-20 12:53
        
        To round or not to round… ElMaestro 2019-07-20 19:59
        
        floating-point math is always more complex than you think it is mittyri 2019-07-20 22:43
- Spreadsheet failures, any recent examples? Ohlbe 2020-12-10 18:43
  - Floating point arithmetic, again Helmut 2020-12-10 19:12
    - Floating point arithmetic, again ElMaestro 2020-12-10 19:40
      - Floating point arithmetic, again Ohlbe 2020-12-10 20:18
        
        Floating point arithmetic, again ElMaestro 2020-12-10 21:38
        
        Floating point arithmetic, again Ohlbe 2020-12-10 21:46
        
        Floating point arithmetic, again ElMaestro 2020-12-10 22:05
        
        Depressed Ohlbe 2020-12-10 23:35
        
        Depressed ElMaestro 2020-12-11 07:59
    - Floating point arithmetic, again Ohlbe 2020-12-10 20:13
  - From bad to worse Ohlbe 2020-12-10 22:11
    - From bad to worse mittyri 2020-12-11 00:22
    - All is good Helmut 2020-12-11 00:36
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        Float is float! Helmut 2020-12-20 23:27
        
        rational solution in R mittyri 2020-12-21 13:49
        
        related stuff Helmut 2021-01-14 12:53
        
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        related stuff ElMaestro 2021-02-09 19:55