Dear Rui,
The replies from you, Bert, Tim and solved my problem. My
last question: what if I put print inside the body of the function passed on to
lapply, instead of separately in the function argument of apply? Is this what
you insinuated in your reply?
THanking you,
yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Rui Barradas <ruipbarra...@sapo.pt>
Sent: Tuesday, November 8, 2022 2:20 AM
To: akshay kulkarni <akshay...@hotmail.com>; R help Mailing list
<r-help@r-project.org>
Subject: Re: [R] print and lapply....
Às 19:22 de 07/11/2022, akshay kulkarni escreveu:
Dear Rui,
THanks for your reply...The point is the loop is a scraping
code, and in your examples you have assumed that the body acts on i, the loop
variable. Can you adapt your code to JUST PRINT the loop variable i ?
By the by, I think I have stumbled upon the answer: The lapply() caches the
result, and prints the output of the function in question immediately after
printing the final i. The i's get printed serially, as the function
progresses....
lapply(1:4,function(x){print(x);Sys.sleep(x^2);x^2})
[1] 1
[1] 2
[1] 3
[1] 4
[[1]]
[1] 1
[[2]]
[1] 4
[[3]]
[1] 9
[[4]]
[1] 16
Here x^2 's print only after 4 is printed on the console....
tHanks anyways for your reply....
THanking you,
Yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Rui Barradas <ruipbarra...@sapo.pt>
Sent: Tuesday, November 8, 2022 12:24 AM
To: akshay kulkarni <akshay...@hotmail.com>; R help Mailing list
<r-help@r-project.org>
Subject: Re: [R] print and lapply....
Às 18:33 de 07/11/2022, akshay kulkarni escreveu:
Dear Rui,
Actually, I am replacing a big for loop by the lapply()
function, and report the progress:
lapply(TP, function(i) { BODY; print(i)})
Can you please adjust your solution in this light?
THanking you,
Yours sincerely,
AKSHAY M KULKARNI
________________________________
From: Rui Barradas <ruipbarra...@sapo.pt>
Sent: Monday, November 7, 2022 11:59 PM
To: akshay kulkarni <akshay...@hotmail.com>; R help Mailing list
<r-help@r-project.org>
Subject: Re: [R] print and lapply....
Às 17:17 de 07/11/2022, akshay kulkarni escreveu:
Dear members,
I have the following code and output:
TP <- 1:4
lapply(TP,function(x){print(x);x^2})
[1] 1
[1] 2
[1] 3
[1] 4
[[1]]
[1] 1
[[2]]
[1] 4
[[3]]
[1] 9
[[4]]
[1] 16
How do I make the print function output x along with x^2, i.e not at the
beginning but before each of x^2?
Many thanks in advance....
THanking you,
Yours sincerely
AKSHAY M KULKARNI
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Hello,
Here are two options, with ?cat and with ?message.
TP <- 1:4
lapply(TP, function(x){
cat("x =", x, "x^2 =", x^2, "\n")
})
lapply(TP, function(x){
msg <- paste("x =", x, "x^2 =", x^2)
message(msg)
})
Hope this helps,
Rui Barradas
Hello,
What do you want the lapply loop to return? If you have a BODY doing
computations, do you want the lapply to return those values and report
the progress?
I have chosen cat or message over print because
- cat returns invisible(NULL),
- message returns invisible()
- print returns a value, what it prints.
Can you adapt the code below to your use case?
TP <- 1:4
lapply(TP, function(x, verbose = TRUE){
# BODY
y <- rnorm(100, mean = x)
# show progress
if(verbose)
cat("x =", x, "x^2 =", x^2, "\n")
#return value
c(x = x, mean = mean(y))
})
lapply(TP, function(x, verbose = TRUE){
# BODY
y <- rnorm(100, mean = x)
# show progress
if(verbose) {
msg <- paste("x =", x, "x^2 =", x^2)
message(msg)
}
#return value
c(x = x, mean = mean(y))
})
Hope this helps,
Rui Barradas
Hello,
No, the x^2 are not printed after the i's. The x^2 are the function's
return values. The function prints the i's, then returns x^2.
As for your problem, it is now more clerar.
I would write a function accepting a url to take care of scraping and
call it in the lapply loop. The progress report can be in the loop, like
below.
This is a complete working example, scraping the Wikipedia list of
countries by GDP. The urls are in a list (it's always the same, I'm not
complicating things) and in a real scraping function I would wrap
tryCatch around it, just in case.
First the function, then the urls list, then the lapply loop.
library(rvest)
scrape <- function(url) {
page <- read_html(url)
gdp <- page |>
html_element(".wikitable") |>
html_table() |>
as.data.frame()
names(gdp) <- unlist(gdp[1,, drop = TRUE])
gdp <- gdp[-1,]
row.names(gdp) <- NULL
#return value
gdp
}
wiki_gdp_url <-
"https://en.wikipedia.org/wiki/List_of_countries_by_GDP_(nominal)"
urls_list <- list(wiki_gdp_url, wiki_gdp_url)
TP <- seq_along(urls_list)
TP
# [1] 1 2
df_list <- lapply(TP, \(i) {
URL <- urls_list[[i]]
data <- scrape(URL)
# show progress
message("iteration: ", i)
#return value
data
})
str(df_list)
Hope this helps,
Rui Barradas
