Question

I'd like to use blank on a tail like I do on a data frame. Is that possible? If not what's more efficient way to get the tail of a data table?

data.table

output

mainDF= data.frame(name= c("a","b","c","d"))
tail(mainDF,1)["name"]
dt<-as.data.table(mainDF)
setkey(dt, name )
tail(dt,1)["name"]

Answer 1

请查看AnyPic，尤其是 data.table 简介简介，其中介绍了如何以data.table的方式对列进行子集化，以及它背后的基本原理。 / p>

答案是使用tail(dt, 1L)[, "name", with=FALSE]，正如@thelatemail在评论中指出的那样。

typedef std::unordered_map<std::vector<int>, ObjectPointer, boost::hash<std::vector<int>> Map;

size_t hash_value(const std::vector<int>& vec)
{
    std::size_t seed = 0;
    for (const auto& val : vec) {
      boost::hash_combine(seed, val);
    }
    return seed;
 }

Using tail on a data.table

1 个答案: