R coding: How to keep records with 4 complete quarters of data

Question

I have a dataframe with company quarterly data and have this question:

How can I retain records for only those companies with 4 quarters of data (as companies sometimes

Answer 1

We can use data.table

library(data.table)
setDT(data)[data[, .I[uniqueN(qtr)==4], by = company]$V1]

Or

setDT(data)[, if(uniqueN(qtr)==4) .SD, by = company]
#   company year qtr         IQ       REVQ      AssetQ       CashQ
#1:    xray 1984   1  -5.827832   8.221870   9.6688477 -10.6321121
#2:    xray 1984   2   3.521643  -1.096940  -4.5014798  -0.9196087
#3:    xray 1984   3  -7.526160  -4.155428 -10.6556271   7.6872401
#4:    xray 1984   4  -7.255974   3.717738  -1.7913910   9.6325437
#5:    kilo 1989   1   2.252885 -19.238773   9.7476758   4.0115274
#6:    kilo 1989   2   9.018055 -12.411381  -0.3772812   6.8339812
#7:    kilo 1989   3 -12.221085 -13.040805   7.3529403   9.1510647
#8:    kilo 1989   4   2.088668  -7.753041   1.5701738 -11.2252986