CODEWITHSUNDEEP
rindexingnearest-neighbor
mand3rd
mand3rd

Reputation: 393

Nearest index of a logical vector in R

I'm trying to get the index of the nearest TRUE value from a column based on the entry of another column. My sample dataframe goes like this: 

a <- c(FALSE,TRUE,FALSE,FALSE,FALSE,TRUE,FALSE,FALSE,TRUE,TRUE,FALSE,FALSE,FALSE,FALSE,FALSE)
b <- c(NA, NA, 3, NA, NA, NA, NA, 8, NA, NA, NA, 12, NA, NA, NA)
df <- data.frame(a, b)

I would like to make a new column that gives the index of the nearest TRUE value before a given value from b. To illustrate, the resulting df should look like this:

       a  b  c
1  FALSE NA NA
2   TRUE NA NA
3  FALSE  3  2
4  FALSE NA NA
5  FALSE NA NA
6   TRUE NA NA
7  FALSE NA NA
8  FALSE  8  6
9   TRUE NA NA
10  TRUE NA NA
11 FALSE NA NA
12 FALSE 12 10
13 FALSE NA NA
14 FALSE NA NA
15 FALSE NA NA

I know I can use max(which(df$a == TRUE)) but I don't know how to make it work in such a way that it only considers the index of the preceding values. Thanks!

Upvotes: 2

Views: 101

Answers (1)

Ronak Shah
Ronak Shah

Reputation: 388982

We can get the index of all TRUE values and then use findInterval to get the closest one for each value in b.

inds <- which(df$a)
df$c <- inds[findInterval(df$b, inds)]
df

#       a  b  c
#1  FALSE NA NA
#2   TRUE NA NA
#3  FALSE  3  2
#4  FALSE NA NA
#5  FALSE NA NA
#6   TRUE NA NA
#7  FALSE NA NA
#8  FALSE  8  6
#9   TRUE NA NA
#10  TRUE NA NA
#11 FALSE NA NA
#12 FALSE 12 10
#13 FALSE NA NA
#14 FALSE NA NA
#15 FALSE NA NA

Upvotes: 3

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