：

> apply(ddf,1 ,function(x) paste0(toString(x[2]), toString(x[3])))
[1] "1983M01" "1983M02" "1983M03" "1983M04"
> 
> apply(ddf,1 ,function(x) paste(toString(x[2]), toString(x[3])))
[1] "1983 M01" "1983 M02" "1983 M03" "1983 M04"

toString（ddf $ year）绑定一个字符串中的整个列：

toString(ddf$year) binds entire column in one string:

> toString(ddf$year)
[1] "1983, 1983, 1983, 1983"
> 
> toString(ddf$period)
[1] "M01, M02, M03, M04"
> 
> paste(toString(ddf$year), toString(ddf$period))
[1] "1983, 1983, 1983, 1983 M01, M02, M03, M04"