Replace Expression Parameter With Query in R

Question

I have an expression max(51). I want to replace the parameter in the expression i.e. 51 with the values in the database with respect to this ID. So my replaced expression will be as below:

max(dbGetQuery(con,"SELECT VALUE FROM STUD WHERE ID=51"))

where con is MySQL connection object. I have tried the following to extract the parameter from expression

PARAMETER <- gsub(".*\\((.*)\\)$", "\\1", "MAX(51)");

Now My final expression will become something like this:

max(dbGetQuery(con,"SELECT VALUE FROM STUD WHERE ID=PARAMETER"));

So how this replacement can be done in R? This is just a single operand expression. Can it be possible for more complex expressions like

max(51)+min(52)-53

Thanks..

Do you already have an SQL query that you need to search and replace, or do you construct the query inside R? — Roman Luštrik
– Roman Luštrik, Commented Apr 12, 2015 at 9:41

3 revs · Accepted Answer · 2015-04-18 18:36:37Z

0

I have achieved this in the following way:

operand <- "max(51)";
PARAMETER <- gsub(".*\\((.*)\\)$", "\\1", operand);
QUERY=gsub("PARAMETER",PARAMETER,"SELECT VALUE FROM STUD WHERE ID='PARAMETER'");
Final_SubExpression <- gsub(PARAMETER,"dbGetQuery(con,QUERY)",operand);
Result <- eval(parse(text=Final_SubExpression));
Result;

OR

operand <- "max(51)";
PARAMETER <- gsub(".*\\((.*)\\)$", "\\1", operand);
QUERY=sprintf("SELECT VALUE FROM STUD WHERE ID = '%s'", PARAMETER);
Final_SubExpression <- gsub(PARAMETER,"dbGetQuery(con,QUERY)",operand);
Result <- eval(parse(text=Final_SubExpression));
Result;

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