您可以根据 Population order 数据并分配 c("Baltimore County";，巴尔的摩市").

You can order the data based on Population and assign c("Baltimore County", "Baltimore City") in each group.

library(data.table)

DT[order(Population), County := c("Baltimore County", "Baltimore City"), .(State_Ab, year)]                    
DT

#    State_Ab           County year Population
# 1:       MD Baltimore County 1994     140942
# 2:       MD   Baltimore City 1994     219673
# 3:       MD   Baltimore City 1998     235413
# 4:       MD Baltimore County 1998     146385
# 5:       MD   Baltimore City 2000     292019
# 6:       MD Baltimore County 2000     170419
# 7:       MD   Baltimore City 2004     336917
# 8:       MD Baltimore County 2004     187402
# 9:       MD Baltimore County 2006     145623
#10:       MD   Baltimore City 2006     268229
#11:       MD Baltimore County 2010     158692
#12:       MD   Baltimore City 2010     281834
#13:       MD   Baltimore City 2016     381365
#14:       MD Baltimore County 2016     231836