cash <- data.frame(
  company   = c("A", "A", "A", "B", "B", "B", "B"),
  cash_flow = c(1000, 4000, 550, 1500, 1100, 750, 6000),
  year      = c(1, 3, 4, 1, 2, 4, 5))

# 筛选低现金流行
subset(cash, cash_flow < 1000)

  company cash_flow year
3       A       550    4
6       B       750    4

# 筛选公司 A 的低现金流
subset(cash, cash_flow < 1000 & company == "A")

  company cash_flow year
3       A       550    4