extract mean value based on 3 column conditions

Question

I have 4 columns:

• subject (ID# - there are 50 subjects)
• hemisphere (right or left)
• condition (gain or loss)
• volume (continuous)

I want to extract the mean value of volume for each subject per condition/hemisphere. Each subject # appears multiple times because this task was repeated over 70 trials. I then want to create a dataframe with these values.

Any help would be greatly appreciated!