Outputing multilple subsets

Hi Rusers,
?I hope to divide the original dataset into several subsets and output
these multilple datasets. But errors appeared in my loops. See example.
######
a<-c(1:10)
b<-c(rep(1,3),rep(2,3),rep(3,4))
c<-data.frame(a,b) ?#c is the example data
num<-c(unique(b))
# I hope to get the subsets c_1.csv,c_2.csv and c_3.csv
#Errors
for (i in num) ?{
? c_num<-c[c$b==num,]
? write.csv(c_num,file="c:/c_num.csv")
}

Warning messages:
1: In c$b == num :
?longer object length is not a multiple of shorter object length
2: In c$b == num :
?longer object length is not a multiple of shorter object length
3: In c$b == num :
?longer object length is not a multiple of shorter object length
?I think the problem should be file="c:/c_num.csv", anybody has ever met
this problem?
?Thanks very much.
-----------------
Jane Chang
Queen's

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