R Group dataframe according to certain conditions and each group has the same number of each condition

Question

My dataframe has 324 different images with unique imageID. And there are 3*3 =9 conditions, each image belonging to one of the conditions. For example, Image 1 belongs to 1A condition and Image 5 belongs to 2B condition. What I try to achieve is to group images into 6 blocks randomly but in each block, there is the same number of each condition. Then, when group the dataframe by blokNo, they will be presented in a random order. And I want to generate multiple orders of presentation from the same dataframe. My data frame looks like this:

ImageID  Catagory1 Category2 BlokNo
1           1           A
4           1           A
6           1           A
5           2           B
8           2           B
3           2           B
14          3           C
12          3           C
17          3           C

I would like my data to look like this:

ImageID  Catagory1 Category2 BlokNo
1           1           A        2
4           1           A        1
6           1           A        3
5           2           B        3
8           2           B        2
3           2           B        1
14          3           C        1
12          3           C        3
17          3           C        2

Below is the code I tried. It actually can realize part of my requirement, but since I actually have 3*3=9 conditions in total, I am wondering if there are other quick ways to do it. Thank you in advance!

Cond1 <- df %>% filter (Category1 == 1 & Category2 == A) #filter out one condition
Cond1$BlokNo <- sample(rep(1:6, each = ceiling(36/6))[1:36]) #randomly assign a number from 1:6 to each image in certain condition

Answer 1

Instead of filter ing by each unique combinations, do a group_by on those 'Category2' columns and get the sample of row_number()

library(dplyr)
df <- df %>% 
   group_by(Category1, Category2) %>% 
   mutate(BlockNo = sample(row_number())) %>% 
   ungroup