大约有 36,200 项符合查询结果（耗时：0.0269秒） [XML]
What does .SD stand for in data.table in R
...elp you see what .SD is:
DT[ , .SD[ , paste(x, v, sep="", collapse="_")], by=y]
# y V1
# 1: 1 a1_b3_c5
# 2: 3 a2_b4_c6
Basically, the by=y statement breaks the original data.table into these two sub-data.tables
DT[ , print(.SD), by=y]
# <1st sub-data.table, called '.SD' while it's be...
Postgres: Distinct but only for one column
... to order:
select distinct on (name)
name, col1, col2
from names
order by name, col1
Will return the first row when ordered by col1.
distinct on:
SELECT DISTINCT ON ( expression [, ...] ) keeps only the first row of each set of rows where the given expressions evaluate to equal. The DISTINCT O...
Sort a list by multiple attributes?
...swered Nov 20 '10 at 15:32
Mark ByersMark Byers
683k155155 gold badges14681468 silver badges13881388 bronze badges
...
扩展App Inventor：具有多点触控和手势检测功能 · App Inventor 2 中文网
...hm.js?8d287b854d737bdc880e8ddeac1b309d"; var s = document.getElementsByTagName("script")[0]; s.parentNode.insertBefore(hm, s); })();
扩展App Inventor：具有多点触控和手势检测功能
« ...
What are the rules about using an underscore in a C++ identifier?
...on macros:
identifiers beginning with an underscore followed immediately by an uppercase letter
identifiers containing adjacent underscores (or "double underscore")
Reserved in the global namespace:
identifiers beginning with an underscore
Also, everything in the std namespace is reserved. (Yo...
How do I import CSV file into a MySQL table?
... database:
mysqlimport --ignore-lines=1 \
--fields-terminated-by=, \
--local -u root \
-p Database \
TableName.csv
I found it at http://chriseiffel.com/everything-linux/how-to-import-a-large-csv-file-to-mysql/
To make the delimiter a tab, use --fi...
Delete all documents from index/type without deleting type
I know one can delete all documents from a certain type via deleteByQuery.
15 Answers
...
lexers vs parsers
.... But it
has to be of symbols which are atomic for the language
understood by parser/lexer.
Symbols for the lexer: ASCII characters.
Symbols for the parser: the particular tokens, which are terminal symbols of their grammar.
They analyse these symbols and try to match them with the grammar of the l...
Is it more efficient to copy a vector by reserving and copying, or by creating and swapping? [duplic
...
Your second example does not work if you send the argument by reference. Did you mean
void copyVecFast(vec<int> original) // no reference
{
vector<int> new_;
new_.swap(original);
}
That would work, but an easier way is
vector<int> new_(original);
...
How to sort a dataframe by multiple column(s)
I want to sort a data.frame by multiple columns. For example, with the data.frame below I would like to sort by column z (descending) then by column b (ascending):
...