Given start date and end date, reshape/expand data for each day between (each day on a row) [duplicate]

Answer 1

You could do something like

do.call(rbind, apply(df, 1, function(x) 
  data.frame(DATE = seq.Date(from = as.Date(x[1]), to = as.Date(x[2]), by = "day"), 
             PER_COST = x[3], row.names = NULL))
)
# 1.1  2013-02-26  3451380
# 1.2  2013-02-27  3451380
# 1.3  2013-02-28  3451380
# 1.4  2013-03-01  3451380
# 1.5  2013-03-02  3451380
# 1.6  2013-03-03  3451380
# 1.7  2013-03-04  3451380