I have a survey data set in wide form. For a particular question, a set of variables was created in the raw data to represent different the fact that the survey question was asked on a particular month.
I wish to create a new set of variables that have month-invariant names; the value of these variables will correspond to the value of a month-variant question for the month observed.
Please see an example / fictitious data set:
require(data.table)
data <- data.table(month = rep(c('may', 'jun', 'jul'), each = 5),
may.q1 = rep(c('yes', 'no', 'yes'), each = 5),
jun.q1 = rep(c('breakfast', 'lunch', 'dinner'), each = 5),
jul.q1 = rep(c('oranges', 'apples', 'oranges'), each = 5),
may.q2 = rep(c('econ', 'math', 'science'), each = 5),
jun.q2 = rep(c('sunny', 'foggy', 'cloudy'), each = 5),
jul.q2 = rep(c('no rain', 'light mist', 'heavy rain'), each = 5))
In this survey, there are really only two questions: "q1" and "q2". Each of these questions is repeatedly asked for several months. However, the observation contains a valid response only if the month observed in the data matches up with the survey question for a particular month.
For example: "may.q1" is observed as "yes" for any observation in "May". I would like a new "Q1" variable to represent "may.q1", "jun.q1", and "jul.q1". The value of "Q1" will take on the value of "may.q1" when the month is "may", and the value of "Q1" will take on the value of "jun.q1" when the month is "jun".
If I were to try and do this by hand using data table, I would want something like:
mdata <- data[month == 'may', c('month', 'may.q1', 'may.q2'), with = F]
setnames(mdata, names(mdata), gsub('may\\.', '', names(mdata)))
I would want this repeated "by = month".
If I were to use the "plyr" package for a data frame, I would solve using the following approach:
require(plyr)
data <- data.frame(data)
mdata <- ddply(data, .(month), function(dfmo) {
dfmo <- dfmo[, c(1, grep(dfmo$month[1], names(dfmo)))]
names(dfmo) <- gsub(paste0(dfmo$month[1], '\\.'), '', names(dfmo))
return(dfmo)
})
Any help using a data.table method would be greatly appreciated, as my data are large. Thank you.