如何通过R传递一个系统函数给MonetDB

Question

使用MonetDB.R，我想运行此命令，但我不想打开mclient。是否有通用的方法来做到这一点?

create function div_noerror(l double, r double) returns double

Answer 1

来自《MonetDB.R》的作者Hannes——

你可以在你的数据库中创建它。如果它已经存在，这将失败，但您可以捕获该错误。所以

dbSendQuery(con, "create function div_noerror(l double, r double) returns double external name calc.div_noerror”) should work.

这里有一个稍微长一点的用法示例:

# example of division
dbGetQuery( con , "SELECT 1 / 2 AS a" )
# example of division by zero, which causes an error
dbGetQuery( con , "SELECT 1 / 0 AS a" )
# load the `div_noerror` function through a system call
dbSendQuery( con , "CREATE FUNCTION div_noerror(l DOUBLE, r DOUBLE) RETURNS DOUBLE EXTERNAL NAME calc.div_noerror" ) 
# use the div_noerror function instead of the `/` operator
dbGetQuery( con , "SELECT div_noerror( 1 , 2 ) AS a" )
# division by zero now returns a missing instead of crashing the entire query
dbGetQuery( con , "SELECT div_noerror( 1 , 0 ) AS a" )