Difference between `do.call()` a function and directly call a function in R?

Question

Here is my code:

>ipo_num_year<- do.call(length,list(as.name(paste0("all_data_align_",year))))
>ipo_num_year
>90
>ipo_num_year<- length(as.name(paste0("all_data_align_",year)))
>ipo_num_year
>1

year is an string object "1999";

In previous code,all_data_align_1999 has been assigned as an list with 90 elements,so the right result is ipo_num_year equals to 90.But the second line makes ipo_num_year equals to 1,it means length() function look the return value of as.name() as an symbol object,so its length is just 1.

Why does the return value of as.name() can not be directly used as the argument of function length()? And why the first solution works fine?

Some one may ask that why don't you just use length(all_data_align_1999).That is because the year is an loop variable in my code.

Really appreciate your kindly reply!

Answer 1

Instead of as.name you should use get:

length(get(paste0("all_data_align_",year)))

You need to retrieve the object not just the name.