Count how many times each character appears in the whole dataset

I have a table with twenty columns and thousands of rows.

Just for example purposes, I will say I have this table:

ColumnA   ColumnB
Testing      This
1231         1231

I want to count how many times each single character appears in the whole dataset.

So in our toy example we would have

character   nºoftimes
T                3
e                1
s                2
i                2
n                1
g                1
h                1
1                4
2                2
3                2

I was thinking of using some kind of string manipulation, but now sure how can I do this.

>Solution :

Does this work:

data.frame(table(strsplit(toupper(paste0(apply(df, 2, paste0, collapse = ''), collapse = '')), split = '')))
   Var1 Freq
1     1    4
2     2    2
3     3    2
4     E    1
5     G    1
6     H    1
7     I    2
8     N    1
9     S    2
10    T    3