Filling a table with additional columns if they don't exist - r

Question

I've the following difficult problem. Here short example of my data. Assume that I've two data sets (my real example has something about 20). The data frames result as a list computed by a self written function with lapply. So, I put the data frames in my example in a list, too. Then I "rbind" them to compute a frequency table.
df1 <- data.frame(rev(seq(12:0)), paste0("a=",sample(0:12, 13, replace=T)))
colnames(df1) <- c("k", "a")
df2 <- data.frame(rev(seq(12:0)), paste0("a=",sample(0:12, 13, replace=T)))
colnames(df2) <- c("k", "a")
list_df <- list(df1,df2)
df_combine<- plyr::ldply(list_df, rbind)
freq_foo <- table(df_combine$k,df_combine$a)
I get a frequency table of the following form.
a=0 a=11 a=12 a=2 a=5 a=6 a=7 a=8 a=3 a=9
1 1 0 0 0 0 0 0 1 0 0
2 1 0 0 0 0 0 0 0 0 1
3 1 0 0 0 0 1 0 0 0 0
4 0 0 0 1 0 1 0 0 0 0
5 0 0 0 1 1 0 0 0 0 0
6 0 0 0 0 0 0 1 0 0 1
7 0 1 1 0 0 0 0 0 0 0
8 1 0 0 0 0 1 0 0 0 0
9 0 0 0 0 0 0 2 0 0 0
10 0 0 1 0 1 0 0 0 0 0
11 1 1 0 0 0 0 0 0 0 0
12 0 0 0 0 0 0 1 0 1 0
13 1 0 1 0 0 0 0 0 0 0
I want to extend and manipulate my table in the following way:
First the table should go over a range of a=0 to a=15. So if there is a missing column, it should be added. And 2nd) I want to order the columns from 0 to 15.
For the first problem I tried
if(freq_foo$paste0("a=",0:15) == F){freq_foo$paste("a=",0:15) <- 0}
but this should work only for data frames and not for tables. Also. i've no idea how to order the columns with an ascending order. The data type isnt important to me because I just want to use the output for further calculations. So, it can also be a data frame instead of a table.

Answer 1

#convert freq_foo table to dataframe
df <- as.data.frame.matrix(freq_foo)
#add all zeros column for missing column name in 0:15 series
df[, paste0("a=", c(0:15)[!(c(0:15) %in% as.numeric(gsub(".*=(\\d+)", "\\1", names(df))))])] <- 0
#order columns from 0 to 15
df <- df[, order(as.numeric(gsub(".*=(\\d+)", "\\1", names(df))))]
Output is:
a=0 a=1 a=2 a=3 a=4 a=5 a=6 a=7 a=8 a=9 a=10 a=11 a=12 a=13 a=14 a=15
1 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0
2 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0
3 0 0 0 0 1 0 0 1 0 0 0 0 0 0 0 0
4 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0
5 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0
6 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0
7 0 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0
8 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0
9 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0
10 1 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0
11 1 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0
12 0 0 0 0 0 0 0 0 1 0 0 0 1 0 0 0
13 0 1 0 0 1 0 0 0 0 0 0 0 0 0 0 0
(Edit: Updated code after getting a requirement clarification from OP)