How many times does the value for column B appear for a value in column A?

Question

I am having the hardest time coming up with a code that lets me match a topic (Column B) to a name (Column A) and create a frequency column for the times B has matched with A (or how many times both have appeared together). Col A and B are codes for longer names.

I thought maybe using the count function from plyr but cant make it work. Maybe you can give me an idea of what I could use for a code?

For example I have a table:

**Col A	Col B**
1	38
1	6
1	38
2	38
2	7
2	7
2	8
2	7

The result that I am looking for is

**Col A	Col B	freq**
1	38	2
1	6	1
2	38	1
2	7	3
2	8	1

So the number 38 has appeared in "1" two times. 6 has appeared one time. and so on. I have 600 rows of data and cant come up with a useful or even a close call code. Thank you so much for your help!

Answer 1

Summarise and count using dplyr:

library(dplyr)

df2 <- df %>%
  group_by(col1, col2) %>%
  summarise(count = n()) %>%
  ungroup()

returns:

   col1  col2 count
  <dbl> <dbl> <int>
1     1     6     1
2     1    38     2
3     2     7     3
4     2     8     1
5     2    38     1