I want to calculate mean of reward for each gamble and create a new column based on that. for example, this is my data:

```
 gamble<-c(1,2,3,4,2,3,4,1)
 reward <- c(1,0.5,0.5,0.4,0.5,0.4,0.2,0.5)
 new<-data.frame(gamble, reward)

```

and the new column should look like this:

```
gamble reward newcolumn
   1    1.0      0.75
   2    0.5      0.50
   3    0.5      0.45
   4    0.4      0.30
   2    0.5      0.50
   3    0.4      0.45
   4    0.2      0.30
   1    0.5      0.75

```

thank you very much in advance... i am really confused...