example for R4DS answer

standardizing-lengths.md

a <- list(c("20M1", "A1", "ACC1"), c("20M2", "A2", "ACC2"), c("20M3"))

mx <- max(lengths(a))

data.frame(lapply(a, `length<-`, mx))
#>   c..20M1....A1....ACC1.. c..20M2....A2....ACC2.. c..20M3...NA..NA.
#> 1                    20M1                    20M2              20M3
#> 2                      A1                      A2              <NA>
#> 3                    ACC1                    ACC2              <NA>

^{Created on 2021-10-06 by the reprex package (v2.0.0)}