根据特定值从具有多个元素的向量中获取索引

Question

I have a vector:

lst <- c("2,1","7,10","11,0","7,0","10,0","1,1","1,0","4,0","4,1","0,1","6,0")

each element contains two numbers,separated by ",". I would like to get indexes of elements containing "1". So the index list is expected:

1, 6, 7, 9, 10

Answer 1

grep() will work nicely for this. By default, it returns the indices of the matched pattern.

grep("^1,|,1$", lst)
# [1]  1  6  7  9 10

The regular expression ^1,|,1$ looks to match a string that

• ^1, = starts with 1,
• | OR
• ,1$ = ends with ,1

Answer 2

each element contains two numbers. my answer is not ideal but I got what I need.

m <- as.numeric(unlist(lapply(strsplit(as.character(lst), "\\,"),"[[",1)))
n <- as.numeric(unlist(lapply(strsplit(as.character(lst), "\\,"),"[[",2)))
sort(unique(c(which(m==1),which(n==1))))

Answer 3

Depending on background and context of this task it might be prudent to turn this vector into a data.frame:

lst <- c("2,1","7,10","11,0","7,0","10,0","1,1","1,0","4,0","4,1","0,1","6,0")
DF <- read.table(text = do.call(paste, list(lst, collapse = "\n")), sep = ",")
which(DF$V1 == 1L | DF$V2 == 1L)
#[1]  1  6  7  9 10