I am trying to obtain the least common multiplier of the prime numbers in between 1 and 20:
x <- expand.grid(c(2,3,7,11,13,17,19),c(2,3,7,11,13,17,19))
l <- c()
# loop
for(i in length(x[,1])){
l[i] <- pracma::Lcm(x[i, 1], x[i, 2])
}
# gives
tail(l)
[1] NA NA NA NA NA 19
Interestingly, the loop returns only NA's besides the first and last value, this doesn't make sense since one can obtain the values by hand:
pracma::Lcm(x[4, 1], x[4, 2])
[1] 22
Can anyone spot what I am missing?
Thank you in advance!
We can initiate l with instead of c(). Also, length is a single value. We need seq_len(nrow(x))
l <- numeric(nrow(x))
for(i in seq_len(nrow(x))) {
l[i] <- pracma::Lcm(x[i, 1], x[i, 2])
}
l
#[1] 2 6 14 22 26 34 38 6 3 21 33 39 51 57 14 21 7 77 91 119 133 22 33 77 11 143 187 209 26 39 91 143 13
#[34] 221 247 34 51 119 187 221 17 323 38 57 133 209 247 323 19
If we initiate as c(), then inside the loop, it would be
l <- c(l, pracma::Lcm(x[i, 1], x[i, 2]))
Also, the pracma::Lcm is vectorized, so no need to loop over each rows
pracma::Lcm(x[,1], x[,2])
#[1] 2 6 14 22 26 34 38 6 3 21 33 39 51 57 14 21 7 77 91 119 133 22 33 77 11 143 187 209 26 39 91 143 13
#[34] 221 247 34 51 119 187 221 17 323 38 57 133 209 247 323 19
In the OP's code, the
length(x[,1])
#[1] 49
is a single number i.e. the last row, here, i.e. only looping through that last row
Second issue is during the assignment, where we assign with a length of 0 and when we use index, it is creating a NULL
l <- c()
length(l)
#[1] 0
l[1]
#NULL