CODEWITHSUNDEEP
r
pyll
pyll

Reputation: 1764

'Macro Variables' in R

I am trying to build a process that accepts user input parameter and then produces things accordingly.

I need to be able to:

1. Input a variable
2. Pull max date for that variable
3. Pull all data less than or equal to that date

dates <- c('2001-01-08', '2015-01-07', '2013-03-03', '2001-01-01', '2013-07-25', '2000-09-20', '2017-02-20')
groups <- c('A', 'A', 'A', 'B', 'B', 'C', 'D')

dat <- data.frame(groups, dates)

dat$dates <- as.Date(dat$dates)

The following piece works for what I want to do....

querydate <- sqldf(
  "SELECT max(dates) as x
  FROM dat
  WHERE groups == 'A'")

But I want to edit this to do something like this....where I specify a value and query references...

group_i_want <- 'A'

querydate <- sqldf(
  "SELECT max(dates) as x
  FROM dat
  WHERE groups == group_i_want")

How can I get R to recognize this value?

Upvotes: 0

Views: 313

Answers (1)

vsart
vsart

Reputation: 26

You can look into using sprintf to do string formatting on values you collect at runtime. For example:

g <- "A"
if (invalid.input(g)) stop("Error") # Make sure input was valid
query <- sprintf("SELECT max(dates) as x FROM dat WHERE groups == '%s'", g)
querydate <- sqldf(query)

Here the %s will be substituted by the string contained in g. You can also substitute numbers with specific formatting, check out ?sprintf for more information on it.

Upvotes: 1

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