sapply

I have what may be a very simple question. I want to process a column of POSIXct objects from a dataframe and generate a vector of datetime strings. I tried to use the following sapply call dt <- sapply(df$datetime, function(x) format(x,"%Y-%m-%dT%H:%M:%S")) but to no avail. I keep getting the following error: > Error in prettyNum(.Internal(format(x, trim, digits, nsmall, width, 3L, : invalid 'trim' argument When I apply this function to a single POSIXct object from the column, I have no problem. So I'm stumped at the moment about what the problem is. Do I need to do something special with

I have a list of lists of characters. For example: l <- list(list("A"),list("B"),list("C","D")) So as you can see some elements are lists of length > 1. I want to convert this list of lists to a character vector, but I'd like the lists with length > 1 to appear as a single element in the character vector. the unlist function does not achieve that but rather: > unlist(l) [1] "A" "B" "C" "D" Is there anything faster than: sapply(l,function(x) paste(unlist(x),collapse="")) To get my desired result: "A" "B" "CD" You can skip the unlist step. You already figured out that paste0 needs collapse =

I have two lists and I want to subset data in a list using another list. Say, I have lists called mylist and hislist : mylist <- list(a = data.frame(cola = 1:3, colb = 4:6), b = data.frame(cola = 1:3, colb = 6:8)) > mylist $a cola colb 1 1 4 2 2 5 3 3 6 $b cola colb 1 1 6 2 2 7 3 3 8 > and hislist hislist <- list(a = 5:6, b = 7:8) > hislist $a [1] 5 6 $b [1] 7 8 I tried to subset mylist using lapply function: lapply(mylist, function(x) subset(x, colb %in% hislist)) #or lapply(mylist, function(x) x[x$colb %in% hislist,]) but these does not work. How to solve this issue? The most straightforward

I can see the correct data, but cannot put it on a Data Frame (It appears as a list of elements). I think the problem is my understanding of the apply family functions. Any hint is welcome. Here is a similar question, but I think it is better to post mine as it contains more details: Webscraping content across multiple pages using rvest package library(rvest) library(lubridate) library(dplyr) urls <- list("http://simple.ripley.com.pe/tv-y-video/televisores/ver-todo-tv", "http://simple.ripley.com.pe/tv-y-video/televisores/ver-todo-tv?page=2&orderBy=seq", "http://simple.ripley.com.pe/tv-y-video

I'd like to learn how to apply functions on specific columns of my dataframe without "excluding" the other columns from my df. For example i'd like to multiply some specific columns by 1000 and leave the other ones as they are. Using the sapply function for example like this: a<-as.data.frame(sapply(table.xy[,1], function(x){x*1000})) I get new dataframes with the first column multiplied by 1000 but without the other columns that I didn't use in the operation. So my attempt was to do it like this: a<-as.data.frame(sapply(table.xy, function(x) if (colnames=="columnA") {x/1000} else {x})) but

I have the following list test_list=list(list(a=1,b=2),list(a=3,b=4)) and I want to extract all elements with list element name a . I can do this via sapply(test_list,`[[`,"a") which gives me the correct result #[1] 1 3 When I try the same with Rs dollar operator $ , I get NULL sapply(test_list,`$`,"a") #[[1]] #NULL # #[[2]] #NULL However, if I use it on a single element of test_list it works as expected `$`(test_list[[1]],"a") #[1] 1 Am I missing something obvious here? NGaffney From what I've been able to determine it's a combination of two things. First, the second element of $ is matched