Recording output from loops in R - r

Question

I am doing an exercise in my R class, and I hope you can help. The task is to create my own script that determines whether or not a number is a palindrome. My idea was to create a repetition structure that records each digit in a number of any size, compares those digits in order, and then makes a call as to whether the number is a palindrome or not.
So far, I thought I could use the "for" command to break the number down, like this:
# Initialize
Number <- 242
Number
N <- nchar(Number)
N
# Find numbers and digits
if (Number == 0) {
print ("Number must be greater than 0")
}
if (Number < 0) {
print ("Number must be greater than 0")
}
for (i in 1:N) {
print (Number)
Digit <- Number %/% 10^(N-1)
print (Digit)
Number <- Number %% 10^(N-1)
N <- N-1
}
The problem, though, is that since this structure overwrites the variables in each loop, I cannot print all the digits out separately once the loop is done. Can I command R to print out and record the digits produced in each loop, so that they can be compared to each other downstream and used to assess whether the original number was a palindrome or not? Thanks for your help.

Answer 1

There's better ways of checking for palindrome-ness in R, for which you should see the other answers. For your specific problem of keeping track of things during a for loop, one approach is to make a vector that's as long as the for loop and assign to the ith element of the vector in the ith iteration of the loop.
Number <- 12345
N <- nchar(Number)
backwardsDigits <- numeric(N) ## a vector of numerics of length N
for (i in N:1) {
backwardsDigits[i] <- Number %/% 10^(i-1)
Number <- Number %% 10^(i-1)
}
backwardsDigits
all(backwardsDigits == rev(backwardsDigits))
You could use forwardsDigits instead by writing to forwardsDigits[N - i + 1] in the loop. You don't really need to print anything during the loop, though it can be helpful for debugging.

Answer 2

As #thelatemail suggested, there is another (perhaps more intuitive way) to do this.
First, let's convert the number 117711 to a string and split it up.
charsplit <- strsplit(as.character(117712), "")
[[1]]
[1] "1" "1" "7" "7" "1" "2"
Then, we'll take it out of list form and reverse it
revchar <- rev(unlist(charsplit))
[1] "2" "1" "7" "7" "1" "1"
Finally, we'll paste these together and convert them into a number:
palinum <- as.numeric(paste(revchar, collapse=""))
[1] "217711"
We can then check if they're identical:
117712 == palinum
[1] FALSE
We can even write a function to do it for us.
is.palindrome <- function(number){
charsplit <- strsplit(as.character(number), "")
revchar <- rev(unlist(charsplit))
palinum <- as.numeric(paste(revchar, collapse=""))
number==palinum
}
is.palindrome(117712)
[1] FALSE
is.palindrome(117711)
[1] TRUE