CODEWITHSUNDEEP
rloopsvectorvectorization
Ochetski
Ochetski

Reputation: 115

Check if one element equal the previous element R

I'm trying to make a logic vector to check if one element is equal to previous element.

vector <- c(1, 1, 2, 2, 2, 3, 3)

I'd like to check each element if it's equal to the previous, therefore the resuld should be:

   FALSE TRUE FALSE TRUE TRUE FALSE TRUE

I know I could make a loop, buts it's not efficient (i have a 16 million row df). So

it's not the ideal, but is what I could manage:

for(i in 2:length(vector)) {print(vector[i] == vector[i-1])}

that would take forever. Is there any vectorized way to do that?

Upvotes: 0

Views: 3285

Answers (2)

ddunn801
ddunn801

Reputation: 1890

Here's a data.table answer. Note that the first item is really an NA. You can manually edit that one if desired.

library("data.table")
vector <- c(1, 1, 2, 2, 2, 3, 3)

df <- data.frame(original=vector)
setDT(df)
df[, prev_eq := original==shift(vector,1)]

Upvotes: 3

Zheyuan Li
Zheyuan Li

Reputation: 73325

We can use (better for integer vector)

c(FALSE, diff(x) == 0)

Example

x <- c(1L, 1L, 2L, 2L, 2L, 3L, 3L)

c(FALSE, diff(x) == 0)
#[1] FALSE  TRUE FALSE  TRUE  TRUE FALSE  TRUE

If your vector contains floating point numbers, this is more robust:

c(FALSE, abs(diff(x)) < .Machine$double.eps ^ 0.5)

but it will costs three times more memory and possibly three times slower than the above for really huge vector.

If you have character vector, we can use

c(FALSE, x[-1] == x[-length(x)])

It is always safe to compare strings using "==".

Upvotes: 7

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