如何使用R中的ifelse条件从向量中删除

Question

I have a vector a with values (1,2,3,4) and another vector b with values (1,1,0,1). Using the elements in b as a flag, I want to remove the vector elements from A at the same positions where 0 is found in element b.

  a <- c(1,2,3,4)
  b <- c(1,1,0,1)
     for(i in 1:length(b))
  {
    if(b[i] == 0)
    {
      a <- a[-i]
    }
  }

I get the desired output

a [1] 1 2 4

But using ifelse, I do not get the output as required.

    a <- c(1,2,3,4)
  b <- c(1,1,0,1)
    for(i in 1:length(b))
    {
      a <- ifelse(b[i] == 0,a[-i],a)
    }

Output:

a [1] 1

How to use ifelse in such situations?

Answer 1

I think ifelse isn't the correct function here since ifelse gives output of same length as input and we want to subset values here. You don't need a loop as well. You can directly do

a[b != 0]
#[1] 1 2 4

data

a <- 1:4
b <- c(1, 1, 0, 1)

Answer 2

Another option could be:

a[as.logical(b)]

[1] 1 2 4

Answer 3

If you want to use ifelse , you can use the following code

na.omit(ifelse(b==0,NA,a))

such that

> na.omit(ifelse(b==0,NA,a))
[1] 1 2 4
attr(,"na.action")
[1] 3
attr(,"class")
[1] "omit"

Answer 4

We can also use double negation

a[!!b]
#[1] 1 2 4

data

a <- 1:4  
b <- c(1, 1, 0, 1)