- Create a three-dimensional array:

> array.3d <- array( seq(100,69), dim = c(4,4,2)) > array.3d , , 1 [,1] [,2] [,3] [,4] [1,] 100 96 92 88 [2,] 99 95 91 87 [3,] 98 94 90 86 [4,] 97 93 89 85 , , 2 [,1] [,2] [,3] [,4] [1,] 84 80 76 72 [2,] 83 79 75 71 [3,] 82 78 74 70 [4,] 81 77 73 69

- Calculate the sum across the first and second dimensions. We get a one-dimensional array with two elements:

> apply(array.3d, 3, sum) [1] 1480 1224 > # verify > sum(85:100) [1] 1480

- Calculate the sum across the third dimension. We get a two-dimensional array:

> apply(array.3d,c(1,2),sum) [,1] [,2] [,3] [,4] [1,] 184 176 168 160 [2,] 182 174 166 158 [3,] 180 172 164 156 [4,] 178 170 162 154