r - Delete entries with only one observation in a group

Question

Here I would like to remove entries that have only one-entry for a given city by date. So for instance, I would like to remove the New York and San Francisco entries, since they only have 1 observation on 4-11, and 4-12.

day                          City                  age
4-10                        Miami                   30
4-10                        Miami                   23
4-11                        New York                24
4-12                        San Francisco           30

Note Dataset is called DG

I tried using a for loop to find the days and get an idea of the number of entries per division per day, but I'm not sure how to work with arrays in R. countx =0

D = unique(DG$day)
for (i in 1:length(D))
{
    for (j in 1:length(DG$age))
    {
      if (DG$day[j] == D{i]
      {
      countx[j] = 1
      }
      else
      {
      countx[j] = 0
      }
    }
Binded <- cbind(countx, DG)

Answer 1

With your sample data

DG <- read.csv(text="day,City,age
4-10,Miami,30
4-10,Miami,23
4-11,New York,24
4-12,San Francisco,30")

you could use dplyr

library(dplyr)
DG %>% group_by(day,City) %>% filter(n()>1)

or base R

DG[ave(rep(1, nrow(DG)), DG$day, DG$City, FUN=length)>1,]

both return

   day  City age
1 4-10 Miami  30
2 4-10 Miami  23

Or you could use data.table (as suggested by @Frank)

library(data.table)
setDT(DG)[,if (.N>1) .SD, by=.(City,day)]