How to sum a column based on specific year?

Question

Suppose I have following time-series data frame in R.

Date           Var1
1/1/2010        7
1/2/2010        154
1/3/2010        125
1/4/2010        87
1/5/2010           


        

Answer 1

We can use substr to get the 'year' part from 'Date' column, use that in subset to extract the rows that have '2010' as year, select the 'Var1' column, and get the sum

 sum(subset(df1, substr(Date,5,8)==2010, select=Var1))

Or a dplyr/lubridate option would be using filter and summarise to get similar result.

 library(lubridate)
 library(dplyr)
 df1 %>% 
     filter(Date=year(mdy(Date))==2010) %>%
     summarise(Var1= sum(Var1))