Use one function for multiple datasets in R

Question

I generated a function like this to check one variable of a dataset 'a':

d <- function(x)
{
  a <- sort(levels(as.factor(x)),decreasing=T)[1:3]
  for (i in 1:length(a))
  {
    if (any(table(x[i])==a[i])<600)
    {
      returnlist <- paste(" Month(s) having less data is/are ", x[i])
      return(returnlist)
    }
    else {
      return(print(" All the recent three months have good enough data "))
         }
  }
}

d(a$YEARMONTH)

Now I have three more datasets to be checked. How can I write a function that takes all the 4 datasets at once and give their respective results? Do I have to use these 4 datasets as arguments? Also suggest me how to write the return statements giving heading as respective dataset name and below that I need the results of that dataset.

My variable that I passed into the function looks like this:

Apr-2014 
Apr-2015
Apr-2016
Aug-2013
Aug-2014
Aug-2015
Dec-2013
Dec-2014 
Dec-2015 
Feb-2014....

These months are the months in which responders have taken the surveys along with the year. So there are many responders in each month.

@Frank..Thank you for the above lapply function. It worked but I am getting only the first record of each dataset. 
My output is looking like this for now-
1  Month(s) having less data is/are  201604
2  Month(s) having less data is/are  201604
3  Month(s) having less data is/are  201604
4  Month(s) having less data is/are  201604

  For example: If my a , b,c,d datasets have yearmonth values as-

A$yearmonth
201604 201603 201602
34  652 643

B$yearmonth

201604 201603 201602
678 78  98

C$yearmonth
201604 201603 201602
675 897 678

D$yearmonth
201604 201603 201602
566 788 90

So here my function should give output for counts<600 of each dataset.
A$yearmonth
2016
34
B$yeamonth
201603 201602
78             98
D$yearmonth
201602
90

I don’t think my function is checking all the three values of ‘a’ of each argument. How should it be solved?
And also how should I get the counts also to be displayed in the output? How can I get argument name in the return statement so that I can relate my output to that dataset?

Answer 1

expansion of Richard Scriven 's comment using your d function:

lapply(list(A$yearmonth, B$yearmonth, C$yearmonth, D$yearmonth), d)

going further, here's a different way to construct the d function to produce the output you have in mind:

d <- function(df)
{
  a <- sort(levels(as.factor(df$yearmonth)),decreasing=T)[1:3]
  b <- as.data.frame(table(df[df$yearmonth %in% a,]))
  c <- b[b$Freq < 600,]$Var1
  if(length(c)>0){
    print(paste("Month(s) having less data is/are", paste(c$Var1, collapse=', ')))
  else {
    print(" All the recent three months have good enough data ")
  }
}

lapply(list(A, B, C, D), d)

Answer 2

As @Richard Scriven suggested...

Load your data frames into your workspace and run

mydf.list <- lapply(ls(), function(x) if (class(get(x)) == "data.frame")
              get(x)) # create a list of all the data frames in workspace

# apply your function on the list of dataframe, this will return list
my.results <- lapply(mydf.list,d) 

# to get back the results as data frame
data.frame(Reduce(rbind, my.results))