the difference between for loops in R and python - r

Question

My function has to turn all uppercases in a given string to lowercases and vice versa. I used to solve such problems with loops. So, my code is:
mirror_case <- function(x){
for(i in x){
ifelse(i==toupper(i),x <-
str_replace_all(x,i,tolower(i)),
ifelse(i==tolower(i),x <-
str_replace_all(x,i,toupper(i)),
x <- gsub(i,i,x)))}
return(x)}
I checked this on several strings. Sometimes it works and sometimes doesn't.
> d
[1] "LKJLjlei 33"
> mirror_case(d)
[1] "LKJLjlei 33"
> e
[1] "asddf"
> mirror_case(e)
[1] "ASDDF"
> f
[1] "ASDDF"
> mirror_case(f)
[1] "asddf"
So, what's wrong with this function?
I'd like not only to get the answer, but also some explanations to understand the problem and not come back here with the similar question.

Answer 1

A string in R is not a sequence like it is in python, and can not be traversed in a for loop like this. You should break the string to individual characters first. Try this:
mirror_case <- function(s) {
# break to characters
chars <- strsplit(s, '')
# apply your ifelse statement to all characters
mirror_chars <- sapply(chars, function(i)
ifelse(toupper(i) == i, tolower(i), toupper(i)))
# join back to a string
mirror_s <- paste(mirror_chars, collapse = "")
return(mirror_s)
}
mirror_case("LKJLjlei 33")
# [1] "lkjlJLEI 33"

Answer 2

#YosiHammer's solution does not need an sapply call (which is a loop) to run on list of one item from split. As #李哲源 shows in comments, like gsub, paste, even ifelse, toupper() and tolower() are vectorized functions and can receive multiple items in one call.
mirror_case <- function(s) {
chars <- strsplit(s, '')[[1]] # RETRIEVE THE CHARACTER VECTOR
mirror_chars <- ifelse(toupper(chars) == chars, tolower(chars), toupper(chars))
mirror_s = paste(mirror_chars, collapse = "")
return(mirror_s)
}
mirror_case("LKJLjlei 33")
# [1] "lkjlJLEI 33"
mirror_case("AbCdEfGhIj")
# [1] "aBcDeFgHiJ"

Answer 3

A simple solution to this problem is to use chartr function:
chartr("[A-Za-z]", "[a-zA-Z]", "bbBB 122")
Check it online
The function is vectorized:
chartr("[A-Za-z]", "[a-zA-Z]", c("bbBB 122", "QwER 12 bB"))
another option is to pass a function to str_replace_all but this is sub-optimal as can be seen from the benchmarks.
library(stringr)
str_replace_all(c("bbBB 122", "QwER 12 bB"),
"[A-Za-z]",
function(x)
ifelse(toupper(x) == x, tolower(x), toupper(x)))
benchmark:
data will be 100000 10 character strings:
dat <- as.vector(
replicate(1e5,
paste0(sample(c(LETTERS,
letters,
" ",
as.character(1:9)),
10,
replace = TRUE),
collapse = "")
))
head(dat)
#output
"aPJAGOiirN" "FSYN DLYQS" "K7Vzh8qALH" "vQzU96JOVF" "WMmqO1D3Q8" "XdBiTG72zV"
functions proposed in other posts (not vectorized):
mirror_case <- function(s) {
chars <- strsplit(s, '')[[1]] # RETRIEVE THE CHARACTER VECTOR
mirror_chars <- ifelse(toupper(chars) == chars, tolower(chars), toupper(chars))
mirror_s = paste(mirror_chars, collapse = "")
return(mirror_s)
}
mirror.case <- function(s) {
# break to characters
chars <- strsplit(s, '')
# apply your ifelse statement to all characters
mirror_chars <- sapply(chars, function(i)
ifelse(toupper(i) == i, tolower(i), toupper(i)))
# join back to a string
mirror_s <- paste(mirror_chars, collapse = "")
return(mirror_s)
}
library(microbenchmark)
microbenchmark(missuse = chartr("[A-Za-z]", "[a-zA-Z]", dat),
missuse2 = str_replace_all(dat,
"[A-Za-z]",
function(x)
ifelse(toupper(x) == x, tolower(x), toupper(x))),
Parfait = lapply(dat, mirror_case),
YosiHammer = lapply(dat, mirror_case),
times = 10)
results
Unit: milliseconds
expr min lq mean median uq max neval
missuse 9.607483 11.05621 18.48764 16.50272 19.06369 39.65646 10
missuse2 11226.900565 11473.40730 11612.95776 11582.65838 11636.32779 12218.78642 10
Parfait 1461.056405 1572.58683 1700.75182 1594.43438 1746.08949 2149.49213 10
YosiHammer 1526.730674 1576.35174 1649.55893 1607.62199 1670.76008 1843.11601 10
as you can see the chartr method is around 100x faster than the other solutions.
Check equality of results:
all.equal(chartr("[A-Za-z]", "[a-zA-Z]", dat),
unlist(lapply(dat, mirror_case)))
all.equal(chartr("[A-Za-z]", "[a-zA-Z]", dat),
unlist(lapply(dat, mirror.case)))
all.equal(chartr("[A-Za-z]", "[a-zA-Z]", dat),
str_replace_all(dat,
"[A-Za-z]",
function(x)
ifelse(toupper(x) == x, tolower(x), toupper(x))))