通過地址/指針訪問對象

Question

我可以通過內存地址或指針訪問在當前R會話中創建的data.table對象嗎？

library(data.table) 

DT <- data.table(a = 1:10, b = letters[1:10]) 
address(DT) 
# [1] "0x6bf9b90" 
attr(DT,".internal.selfref",TRUE) 
# <pointer: 0x2655cc8> 

Answer 1

這是有些做（相比於如何在如C++投三分球）的一個愚蠢的方式，但你可以這樣做：

# recursively iterate over environments 
find.by.address = function(addr, env = .GlobalEnv) { 
    idx = which(sapply(ls(env), function(x) address(get(x, env = env))) == addr) 
    if (length(idx) != 0) 
    return (get(ls(env)[idx], env = env)) 

    # didn't find it, let's iterate over the other environments 
    idx = which(sapply(ls(env), function(x) is.environment(get(x, env = env)))) 
    for (i in idx) { 
    res = find.by.address(addr, get(ls(env)[i], env = env)) 
    if (res != "couldn't find it") return (res) 
    } 

    return ("couldn't find it") 
} 

DT = data.table(a = 1) 
e = new.env() 
e$DT = data.table(b = 2) 
e$f = new.env() 
e$f$DT = data.table(c = 2) 

find.by.address(address(DT)) 
# a 
#1: 1 
find.by.address(address(e$DT)) 
# b 
#1: 2 
find.by.address(address(e$f$DT)) 
# c 
#1: 2