Work with nested lists

alist = {{a1, a2, a3, a4}, {b1, b2, b3, b4}, {c1, c2, c3, c4}, {d1, d2, d3, d4}}

{{a1, a2, a3, a4}, {b1, b2, b3, b4}, {c1, c2, c3, c4}, {d1, d2, d3, d4}}

MatrixForm[alist]

MatrixForm[{{a1, a2, a3, a4}, {b1, b2, b3, b4}, {c1, c2, c3, c4}, {d1, d2, d3, d4}}]

alist[[2]]

{b1, b2, b3, b4}

alist[[2, 3]]

b3

alist[[All, 3]]

{a3, b3, c3, d3}

Flatten[alist]

{a1, a2, a3, a4, b1, b2, b3, b4, c1, c2, c3, c4, d1, d2, d3, d4}

MatrixForm[Flatten[alist]]

MatrixForm[{a1, a2, a3, a4, b1, b2, b3, b4, c1, c2, c3, c4, d1, d2, d3, d4}]

MatrixForm[{Flatten[alist]}]

MatrixForm[{{a1, a2, a3, a4, b1, b2, b3, b4, c1, c2, c3, c4, d1, d2, d3, d4}}]

data = Table[i + j, {i, 1, 5}, {j, 1, 4}]

{{2, 3, 4, 5}, {3, 4, 5, 6}, {4, 5, 6, 7}, {5, 6, 7, 8}, {6, 7, 8, 9}}

MatrixForm[data]

MatrixForm[{{2, 3, 4, 5}, {3, 4, 5, 6}, {4, 5, 6, 7}, {5, 6, 7, 8}, {6, 7, 8, 9}}]

ListLinePlot[data, AxesOrigin -> {0, 0}]

-Graphics-

Mean[data]

{4, 5, 6, 7}

Mean[Flatten[data]]

11/2

dat = Table[i + j + k, {i, 1, 4}, {j, 1, 4}, {k, 1, 4}];
MatrixForm[dat]

MatrixForm[{{{3, 4, 5, 6}, {4, 5, 6, 7}, {5, 6, 7, 8}, {6, 7, 8, 9}}, {{4, 5, 6, 7}, {5, 6, 7, 8}, {6, 7, 8, 9}, {7, 8, 9, 10}}, {{5, 6, 7, 8}, {6, 7, 8, 9}, {7, 8, 9, 10}, {8, 9, 10, 11}}, {{6, 7, 8, 9}, {7, 8, 9, 10}, {8, 9, 10, 11}, {9, 10, 11, 12}}}]

MatrixForm[Mean[dat]]

MatrixForm[{{9/2, 11/2, 13/2, 15/2}, {11/2, 13/2, 15/2, 17/2}, {13/2, 15/2, 17/2, 19/2}, {15/2, 17/2, 19/2, 21/2}}]

Mean[Flatten[dat]]

15/2