How to paste a string on each element of a vector of strings using apply in R?

I have a vector of strings.

d <- c("Mon","Tues","Wednes","Thurs","Fri","Satur","Sun")

for which I want to paste the string "day" on each element of the vector in a way similar to this.

week <- apply(d, "day", paste, sep='')

No need for apply(), just use paste():

R> d <- c("Mon","Tues","Wednes","Thurs","Fri","Satur","Sun")
R> week <- paste(d, "day", sep="")
R> week
[1] "Monday"    "Tuesday"   "Wednesday" "Thursday"  
[4] "Friday"    "Saturday"  "Sunday"   
R> 

Other have already indicated that since paste is vectorised, there is no need to use apply in this case.

However, to answer your question: apply is used for an array or data.frame. When you want to apply a function over a list (or a vector) then use lapply or sapply (a variant of lapply that simplifies the results):

sapply(d, paste, "day", sep="")
        Mon        Tues      Wednes       Thurs         Fri       Satur 
   "Monday"   "Tuesday" "Wednesday"  "Thursday"    "Friday"  "Saturday" 
        Sun 
   "Sunday" 

Apart from paste/paste0 there are variety of ways in which we can add a string to every element in the vector.

1. Using sprintf

   sprintf("%sday", d) #[1] "Monday" "Tuesday" "Wednesday" "Thursday" "Friday" "Saturday" "Sunday"

2. glue

   glue::glue("{d}days")

Here {d} is evaluated as R code. This can be wrapped in as.character if needed.

3. str_c in stringr

   stringr::str_c(d, "day")

whose equivalent is

4. stri_c in stringi

   stringi::stri_c(d, "day")

5. stringi also has stri_paste

   stringi::stri_paste(d, "day")