Writing code to update a large matrix?

Question

How can I write a code to update a matrix based on certain rules? I plan on constructing a large matrix, and I cannot submit entries one by one.

The rules for the matrix are as followed:

$$ P(i,j)= \begin{cases} p, & \text{if $j$ = $2i$}\land m \\ q, & \text{if $j$ = $(2i-m)$ }\lor 0 \end{cases} $$

P is square. I was thinking of just defining it to be a 0 matrix first, then updating the values with whatever code. However, I can't seem to think of how that would be done. I've never heard of updating a matrix. For a given $m$, how would one generate such matrix in Mathematica? Could someone list an example?

Answer 1

m = 5;
sa = SparseArray[{{i_, j_} /; j == Min[2 i, m] :> 
    p, {i_, j_} /; j == 2 i - m :> q}, {m, m}]

mat = Partition[Range[m^2], m];
sa2 = SparseArray[{{i_, j_} /; j == Min[2 i, m] :> p, 
    {i_, j_} /; j == 2 i - m, 0] :> q, 
    {i_, j_} :> mat[[i, j]]}, {m, m}];
Row[MatrixForm /@ {mat, sa2}]

Answer 2

There's no need to update values; you can use Table and create it in one pass.

m = 4;
Table[
    Which[
        j == 2 i && j == m, p,
        j == 2 i - m || j == 0, q,
        True, 0
    ], {i, 10}, {j, 10}] // MatrixForm

Answer 3

Updating an existing matrix

randomP = RandomInteger[{0, 9}, {5, 5}];

m = 2

ReplacePart[randomP, {{i_, j_} /; And[j == 2 i, j == m] -> p, 
                      {i_, j_} /; Or[j == 2 i - m, j == 0] -> q}]