gen yr   gpa
1    2  3 3.719
2    2  2 2.913
3    1  1 4.000
4    2  4 3.931
5    2  1 3.227
6    1  2 3.511
7    1  2 3.823
8    2  3 3.115
9    1  1 3.921
10   1  4 3.714
11   1  2 3.432
12   2  2 2.988
Output of aggregate function grouping by gen.
     gender      x
[1,]      1 3.7335
[2,]      2 3.3155

Output of aggregate function grouping by yr.
     gender      x
[1,]      1 3.7160
[2,]      2 3.3334
[3,]      3 3.4170
[4,]      4 3.8225