#..................................................................
# *** TL;DR *** (i.e. the main idea) ...
# (keep reading the comments below for a more lengthy explanation):
#..................................................................
#
# The countdown function does NOT return the numbers
# nor the word "blastoff!". Rather the return value of countdown() is the
# value of cat("blastoff!") function, which is invisible, and is NULL.
# Same code as above
countdown <- function (){
for (num in 10:1){ # each time through the loop another value from 10:1 is assigned to num
cat(num," ") # display num followed by a space
Sys.sleep(0.25) # sleep for 0.25 seconds
}
cat("blastoff!") # after the loop has finished, display "blastoff!"
}44 44. return vs cat
We covered this earlier. However, studnets who are new to R are often confused about these ideas …
countdown() # 10 9 8 7 6 5 4 3 2 1 blastoff!10 9 8 7 6 5 4 3 2 1 blastoff!x <- countdown()10 9 8 7 6 5 4 3 2 1 blastoff!x # NULLNULL#..................................................................
# *** A SOMEWHAT MORE LENGTHY EXPLANATION OF THESE CONCEPTS ***
#..................................................................
# First of all, remember that if a function doesn't execute an explicit
# return statement, then the value of the final command to be executed is returned
# from the function.
#
# Therefore, in the case of the countdown function above, what is "returned" is the
# value of cat("blastoff!") ... (but this is NOT the word "blastoff!" ... keep reading).
# The return value of cat is always NULL.
# In the next line the return value is NULL, NOT "hello".
# The return value, NULL, is then assigned to the variable x.
# Even though the word "hello" is NOT the return value,
# nevertheless, the word hello (without quotes) is displayed because cat
# will always display info to the screen.
x <- cat("hello") # displays hello (without quotes) to the screenhellox # NULLNULL# One more example ...
# Remember, the rep function repeats the VALUE of the first argument.
rep("hello", 5) # "hello" "hello" "hello" "hello" "hello"[1] "hello" "hello" "hello" "hello" "hello"rep(seq(1,3), 5) # 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3 [1] 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3# In the following example, the return value of cat is NULL.
# However, NULL is NOT repeated 5 times, only once.
# This is due to the special nature of the value NULL (which basically
# means "nothing"). NULL is special in that even 5 NULLs are displayed
# as a single NULL - don't worry too much about the intricacies of NULL,
# but you must know that the return value of cat is NOT "hello".
rep(cat("hello"), 5) # helloNULLhelloNULLrep(NULL, 5) # NULL (only once)NULL# Bottom line: You can use cat to display info to the screen
# but don't expect that information to be "returned" from the function.
# SOME TAKEAWAYS:
# REMEMBER:
#
# A few things to remember ...
#
# 1. If a function doesn't contain a return statement, then the
# value of the final command to be executed in the function
# is returned.
#
# 2. Based on #1 above, the value that is returned from countdown()
# is the value cat("blastoff!"). HOWEVER ... keep reading ...
#
# 3. It is very important to understand that the cat function
# does NOT "return" the information that is displayed.
#
# The purpose of cat is TO DISPLAY INFORMATION TO THE SCREEN.
# The purpose of cat is NOT to return a value.
# Programmers use the cat function to display information,
# NOT to generate a "value".The actual return value of cat
# is NULL. You never really need to use this NULL value,
# but technically, NULL is the return value.
#
# 4. You don't actually see the word NULL on the screen when you
# run the countdown function since the return value of cat is
# an "invisible" value - we explained the concept of invisible
# return values earlier in the semester when we first discussed
# the cat function.)