我有以下玩具['Left' if isinstance(x, unicode) and u'L' in x else x for x in df['group']]
data.table我试图在表格中添加一个额外的列，其中包含日期的三个字母缩写。我试过了
x = data.table(dt = c('2015-01-01','2015-02-01','2016-04-01'))
但它回复了以下内容，一个警告和一个空字段。
x$dt = strptime(x$dt,'%b')
我在SO上发现之前的Q＆amp; A表示Warning messages:
1: In `[<-.data.table`(x, j = name, value = value) :
Supplied 9 items to be assigned to 3 items of column 'dt' (6 unused)
2: In `[<-.data.table`(x, j = name, value = value) :
Coerced 'list' RHS to 'character' to match the column's type. Either change the target column to 'list' first (by creating a new 'list' vector length 3 (nrows of entire table) and assign that; i.e. 'replace' column), or coerce RHS to 'character' (e.g. 1L, NA_[real|integer]_, as.*, etc) to make your intent clear and for speed. Or, set the column type correctly up front when you create the table and stick to it, please.
> x
dt mnth
1: c(NA, NA, NA)
2: c(NA, NA, NA)
3: c(NA, NA, NA)
返回了一个列表但是还没有能够完成这个相对简单的任务。有人可以提供一些指示吗？
答案 0 :(得分：3)
我们可以使用format
format(strptime(x$dt, '%Y-%m-%d'), '%b')
或者@Frank提到，format.Date可以应用于Date类
format(as.Date(x$dt), "%b")