[R] for() loop

[Schmidt Martin]

Hello

I'm working with R since a few month and have still many trivial  
questions - I guess! Here is what I want:

I have this matrix:
> dim(datjan)
[1] 899   4

The first 10 rows looks as follows:
> datjan[1:10,]
      V1 V2 V3 V4
1  1961  1  1 24
2  1961  1  2 24
3  1961  1  3 24
4  1961  1  4 24
5  1961  1  5 24
6  1961  1  6 27
7  1961  1  7 27
8  1961  1  8 27
9  1961  1  9 27
10 1961  1 10 27

I tried now to create a for() loop, which gives me the sum of the 30  
different classes (1:30!) in [,4].

for(i in 1:30){
sum(datjan[,4]==i)
}

R is then actually calculating the sum of "i" which certainly doesn't  
exist and results in a "0" value

t1<-sum(datjan[,4]==1)
t2<-sum(datjan[,4]==2)
.................................until '30'
This way its working, but I won't find a end by doing all this by  
hand, because there are many other matrix waiting.

So, how can I make work that loop??

thanks for helping me