为什么不一致呢？？？？？？
> y <- c(2, 1, 3, 4)
> z <- c(1, 4)> ifelse(y %in% z, which(y==z), 0 )
[1] 0 4 0 4 ??????????why??因为z会自动补齐为1，4，1，4
> y <- c(2, 1, 3, 4)
> z <- c(2, 4)
> ifelse(y %in% z, which(y==z), 0 )
[1] 1 0 0 4
> y <- c(2, 1, 3, 4)
> z <- c(3, 4)
> ifelse(y %in% z, which(y==z), 0 )
[1] 0 0 3 4
> y <- c(2, 1, 3, 4)
> z <- c(5, 4)
> ifelse(y %in% z, which(y==z), 0 )
[1] 0 0 0 4
> y <- c(2, 1, 3, 4)
> z <- c(6, 4)
> ifelse(y %in% z, which(y==z), 0 )
[1] 0 0 0 4
> y <- c(2, 1, 3, 4)
> z <- c(4, 1)
> ifelse(y %in% z, which(y==z), 0 ) ??????????why??
[1] 0 2 0 2