使用lapply和unlist可以执行以下操作：

unlist(lapply(1987:1988, paste, 1:12, sep = "."))
#>  [1] "1987.1"  "1987.2"  "1987.3"  "1987.4"  "1987.5"  "1987.6"  "1987.7" 
#>  [8] "1987.8"  "1987.9"  "1987.10" "1987.11" "1987.12" "1988.1"  "1988.2" 
#> [15] "1988.3"  "1988.4"  "1988.5"  "1988.6"  "1988.7"  "1988.8"  "1988.9" 
#> [22] "1988.10" "1988.11" "1988.12"

要添加前导零，可以使用stringr::str_pad：

unlist(lapply(1987:1988, paste, stringr::str_pad(1:12, width = 2, pad = 0), sep = "."))
#>  [1] "1987.01" "1987.02" "1987.03" "1987.04" "1987.05" "1987.06" "1987.07"
#>  [8] "1987.08" "1987.09" "1987.10" "1987.11" "1987.12" "1988.01" "1988.02"
#> [15] "1988.03" "1988.04" "1988.05" "1988.06" "1988.07" "1988.08" "1988.09"
#> [22] "1988.10" "1988.11" "1988.12"

一个选择是

apply(expand.grid(1:12, 1987:2020), 1, \(x) paste(rev(x), collapse = "."))[-(1:5)]
#>   [1] "1987.6"  "1987.7"  "1987.8"  "1987.9"  "1987.10" "1987.11" "1987.12"
#>   [8] "1988.1"  "1988.2"  "1988.3"  "1988.4"  "1988.5"  "1988.6"  "1988.7" 
#>  [15] "1988.8"  "1988.9"  "1988.10" "1988.11" "1988.12" "1989.1"  "1989.2" 
#>  [22] "1989.3"  "1989.4"  "1989.5"  "1989.6"  "1989.7"  "1989.8"  "1989.9" 
#>  [29] "1989.10" "1989.11" "1989.12" "1990.1"  "1990.2"  "1990.3"  "1990.4" 
#>  [36] "1990.5"  "1990.6"  "1990.7"  "1990.8"  "1990.9"  "1990.10" "1990.11"
#>  [43] "1990.12" "1991.1"  "1991.2"  "1991.3"  "1991.4"  "1991.5"  "1991.6" 
#>  [50] "1991.7"  "1991.8"  "1991.9"  "1991.10" "1991.11" "1991.12" "1992.1" 
#>  [57] "1992.2"  "1992.3"  "1992.4"  "1992.5"  "1992.6"  "1992.7"  "1992.8" 
#>  [64] "1992.9"  "1992.10" "1992.11" "1992.12" "1993.1"  "1993.2"  "1993.3" 
#>  [71] "1993.4"  "1993.5"  "1993.6"  "1993.7"  "1993.8"  "1993.9"  "1993.10"
#>  [78] "1993.11" "1993.12" "1994.1"  "1994.2"  "1994.3"  "1994.4"  "1994.5" 
#>  [85] "1994.6"  "1994.7"  "1994.8"  "1994.9"  "1994.10" "1994.11" "1994.12"
#>  [92] "1995.1"  "1995.2"  "1995.3"  "1995.4"  "1995.5"  "1995.6"  "1995.7" 
#>  [99] "1995.8"  "1995.9"  "1995.10" "1995.11" "1995.12" "1996.1"  "1996.2" 
#> [106] "1996.3"  "1996.4"  "1996.5"  "1996.6"  "1996.7"  "1996.8"  "1996.9" 
#> [113] "1996.10" "1996.11" "1996.12" "1997.1"  "1997.2"  "1997.3"  "1997.4" 
#> [120] "1997.5"  "1997.6"  "1997.7"  "1997.8"  "1997.9"  "1997.10" "1997.11"
#> [127] "1997.12" "1998.1"  "1998.2"  "1998.3"  "1998.4"  "1998.5"  "1998.6" 
#> [134] "1998.7"  "1998.8"  "1998.9"  "1998.10" "1998.11" "1998.12" "1999.1" 
#> [141] "1999.2"  "1999.3"  "1999.4"  "1999.5"  "1999.6"  "1999.7"  "1999.8" 
#> [148] "1999.9"  "1999.10" "1999.11" "1999.12" "2000.1"  "2000.2"  "2000.3" 
#> [155] "2000.4"  "2000.5"  "2000.6"  "2000.7"  "2000.8"  "2000.9"  "2000.10"
#> [162] "2000.11" "2000.12" "2001.1"  "2001.2"  "2001.3"  "2001.4"  "2001.5" 
#> [169] "2001.6"  "2001.7"  "2001.8"  "2001.9"  "2001.10" "2001.11" "2001.12"
#> [176] "2002.1"  "2002.2"  "2002.3"  "2002.4"  "2002.5"  "2002.6"  "2002.7" 
#> [183] "2002.8"  "2002.9"  "2002.10" "2002.11" "2002.12" "2003.1"  "2003.2" 
#> [190] "2003.3"  "2003.4"  "2003.5"  "2003.6"  "2003.7"  "2003.8"  "2003.9" 
#> [197] "2003.10" "2003.11" "2003.12" "2004.1"  "2004.2"  "2004.3"  "2004.4" 
#> [204] "2004.5"  "2004.6"  "2004.7"  "2004.8"  "2004.9"  "2004.10" "2004.11"
#> [211] "2004.12" "2005.1"  "2005.2"  "2005.3"  "2005.4"  "2005.5"  "2005.6" 
#> [218] "2005.7"  "2005.8"  "2005.9"  "2005.10" "2005.11" "2005.12" "2006.1" 
#> [225] "2006.2"  "2006.3"  "2006.4"  "2006.5"  "2006.6"  "2006.7"  "2006.8" 
#> [232] "2006.9"  "2006.10" "2006.11" "2006.12" "2007.1"  "2007.2"  "2007.3" 
#> [239] "2007.4"  "2007.5"  "2007.6"  "2007.7"  "2007.8"  "2007.9"  "2007.10"
#> [246] "2007.11" "2007.12" "2008.1"  "2008.2"  "2008.3"  "2008.4"  "2008.5" 
#> [253] "2008.6"  "2008.7"  "2008.8"  "2008.9"  "2008.10" "2008.11" "2008.12"
#> [260] "2009.1"  "2009.2"  "2009.3"  "2009.4"  "2009.5"  "2009.6"  "2009.7" 
#> [267] "2009.8"  "2009.9"  "2009.10" "2009.11" "2009.12" "2010.1"  "2010.2" 
#> [274] "2010.3"  "2010.4"  "2010.5"  "2010.6"  "2010.7"  "2010.8"  "2010.9" 
#> [281] "2010.10" "2010.11" "2010.12" "2011.1"  "2011.2"  "2011.3"  "2011.4" 
#> [288] "2011.5"  "2011.6"  "2011.7"  "2011.8"  "2011.9"  "2011.10" "2011.11"
#> [295] "2011.12" "2012.1"  "2012.2"  "2012.3"  "2012.4"  "2012.5"  "2012.6" 
#> [302] "2012.7"  "2012.8"  "2012.9"  "2012.10" "2012.11" "2012.12" "2013.1" 
#> [309] "2013.2"  "2013.3"  "2013.4"  "2013.5"  "2013.6"  "2013.7"  "2013.8" 
#> [316] "2013.9"  "2013.10" "2013.11" "2013.12" "2014.1"  "2014.2"  "2014.3" 
#> [323] "2014.4"  "2014.5"  "2014.6"  "2014.7"  "2014.8"  "2014.9"  "2014.10"
#> [330] "2014.11" "2014.12" "2015.1"  "2015.2"  "2015.3"  "2015.4"  "2015.5" 
#> [337] "2015.6"  "2015.7"  "2015.8"  "2015.9"  "2015.10" "2015.11" "2015.12"
#> [344] "2016.1"  "2016.2"  "2016.3"  "2016.4"  "2016.5"  "2016.6"  "2016.7" 
#> [351] "2016.8"  "2016.9"  "2016.10" "2016.11" "2016.12" "2017.1"  "2017.2" 
#> [358] "2017.3"  "2017.4"  "2017.5"  "2017.6"  "2017.7"  "2017.8"  "2017.9" 
#> [365] "2017.10" "2017.11" "2017.12" "2018.1"  "2018.2"  "2018.3"  "2018.4" 
#> [372] "2018.5"  "2018.6"  "2018.7"  "2018.8"  "2018.9"  "2018.10" "2018.11"
#> [379] "2018.12" "2019.1"  "2019.2"  "2019.3"  "2019.4"  "2019.5"  "2019.6" 
#> [386] "2019.7"  "2019.8"  "2019.9"  "2019.10" "2019.11" "2019.12" "2020.1" 
#> [393] "2020.2"  "2020.3"  "2020.4"  "2020.5"  "2020.6"  "2020.7"  "2020.8" 
#> [400] "2020.9"  "2020.10" "2020.11" "2020.12"

创建于2022年12月17日，使用reprex v2.0.2

使用rep和paste而无需循环的替代选项（apply、lapply）：

paste(rep(1987:2020, each = 12), rep(1:12, times = length(1987:2020)), sep = ".")

产出

# [1]  "1987.1"  "1987.2"  "1987.3"  "1987.4"  "1987.5"  "1987.6"  "1987.7"  "1987.8"  "1987.9"  "1987.10" "1987.11" "1987.12"
# [13] "1988.1"  "1988.2"  "1988.3"  "1988.4"  "1988.5"  "1988.6"  "1988.7"  "1988.8"  "1988.9"  "1988.10" "1988.11" "1988.12"
# [25] "1989.1" 
# ...                                                                        "2020.8"  "2020.9"  "2020.10" "2020.11" "2020.12"