Question

How can you add variables to a dataframe in a for-loop?

I would like to create a dataframe where each column is the revenue for a region between 2009 and 2011.

regions = c('A','APAC','CEE','LATAM','ME', 'NA', 'WE')

# Loop through all regions, and add them as a column in my dataframe.
for (region in regions) {

  # create the query string
  query_string  = sprintf("SELECT date, revenue
                  FROM country_revenue
                  WHERE region = '%s'
                  AND date>='2009-01-01'
                  AND date<='2011-12-31'
                  ORDER BY date ASC
                  LIMIT 2000", region)

  # Query the database, and assign the result to a variable.
  assign(sprintf('rev.%s',region), mysql_query(query_string))

  # I only want the 2nd column returned from my query above. 
  # THIS IS THE PART THAT FAILS. Error in sprintf("rev.%s", region)[, 2] : incorrect number of dimensions
  sprintf('rev.%s',region) = sprintf('rev.%s',region)[,2]

  # Add this variable to my data frame.
  revenue = cbind(revenue, sprintf('rev.%s',region))
}

Answer 1

That would be pretty inefficient. Why not return region as part of the SQL call so you have something like

foo <- data.frame(date = rep(Sys.Date() + 0:4, 7),
                  revenue = runif(7*5),
                  region = rep(c('A','APAC','CEE','LATAM','ME', 'NA', 'WE'), 
                               each = 5))

> head(foo)
        date   revenue region
1 2012-08-04 0.1170867      A
2 2012-08-05 0.6173779      A
3 2012-08-06 0.9860934      A
4 2012-08-07 0.1344043      A
5 2012-08-08 0.5570391      A
6 2012-08-04 0.5844136   APAC

It is a simple dcast() call to reshape the data into the desired format.

> require(reshape2)
> dcast(foo, date ~ region, value.var = "revenue")
        date         A      APAC       CEE      LATAM         ME
1 2012-08-04 0.1170867 0.5844136 0.8011066 0.82864796 0.85856770
2 2012-08-05 0.6173779 0.7893151 0.3991653 0.41268349 0.05925445
3 2012-08-06 0.9860934 0.2812308 0.2272009 0.04599903 0.82367709
4 2012-08-07 0.1344043 0.7513777 0.8022602 0.96933913 0.61501816
5 2012-08-08 0.5570391 0.2915478 0.4601065 0.82996462 0.83779233
         NA         WE
1 0.4833374 0.25713295
2 0.9574843 0.22122544
3 0.5575645 0.03492411
4 0.2962364 0.51973593
5 0.9020639 0.95506837