To understand how str_c works, you need to imagine that you are
building up a matrix of strings. Each input argument forms a column, and
is expanded to the length of the longest argument, using the usual
recyling rules. The sep string is inserted between each column. If
collapse is NULL each row is collapsed into a single string. If
non-NULL that string is inserted at the end of each row, and
the entire matrix collapsed to a single string.
One or more character vectors. Zero length arguments
are removed.
sep
String to insert between input vectors.
collapse
Optional string used to combine input vectors into single
string.
Value
If collapse = NULL (the default) a character vector with
length equal to the longest input string. If collapse is
non-NULL, a character vector of length 1.
See Also
paste for equivalent base R functionality, and
stri_c which this function wraps
Examples
str_c("Letter: ", letters)
str_c("Letter", letters, sep = ": ")
str_c(letters, " is for", "...")
str_c(letters[-26], " comes before ", letters[-1])
str_c(letters, collapse = "")
str_c(letters, collapse = ", ")
# Missing inputs give missing outputs
str_c(c("a", NA, "b"), "-d")
# Use str_replace_NA to display literal NAs:
str_c(str_replace_na(c("a", NA, "b")), "-d")
providing 
.