Detect the presence or absence of a string in a pattern with R

I'm beginner in R coding. I need a hand to find an elegant way to do this:

I've got the following dataframe :

> df = data.frame(pattern=c('a,b,c','b','c,b','c'), nb=c(150,100,30,10))
> df
  pattern  nb
1   a,b,c  150
2       b  100
3     c,b  30
4       c  10

According to the presence of string, I want a dataframe like that :

  pattern   nb    a    b     c
1   a,b,c   150   150  150   150
2       b   100   0    100   0
3     c,b   30    0    30    30
4       c   10    0    0     10

I find this solution :

df$a <- ifelse(grepl("a" , df$pattern), df$nb, 0)
df$b <- ifelse(grepl("b" , df$pattern), df$nb, 0)
df$c <- ifelse(grepl("c" , df$pattern), df$nb, 0)

But in my real case, I got a lot of different patterns. I'm sure there is a way with a loop or function. Of course, I'm not asking for a ready-made solution. I'm just asking for an idea of how to do it. Many thanks in advance.

Greetings from France

Arnaud