Replacing elements of a matrix

Question

I'm trying to write a program that takes a given matrix A of 1's and 0's and flips a designated amount of 1's to -1's.

lis1 = ArrayRules[A];
For[i = 1, i < Length[lis1] , i++
   lis2 = RandomChoice[ArrayRules[A], n];
 {a, b} = lis2[[i]];
ReplacePart[lis1, {a,b} -> -1];

This obviously doesn't work, and I'm not sure I even need the For loop for this, but what syntax do I need specifically to replace the [i,j] element of lis1 with -[i,j] of lis2?
Is there a more efficient way of doing this?

Answer 1

Here's a quick helper function to do so. It first finds the position of each $1$ in the original matrix using Position; it then selects $n$ of these positions randomly using RandomSample and replaces their value with $-1$ using ReplacePart:

Clear[flipsome]
flipsome[matrix_?MatrixQ, n_Integer] :=
 ReplacePart[matrix, RandomSample[Position[matrix, 1], n] -> -1]

Let's generate a random matrix of ones and zeros:

(a = RandomInteger[{0, 1}, {10, 10}]) // MatrixForm

.. and use flipsome to flip 10 randomly selected $1$s to $-1$:

flipsome[a, 10] // MatrixForm

Answer 2

Also

ClearAll[flipF]
flipF = MapAt[-1 &, #, RandomSample[Position[#, 1], #2]] &;
(* or *) flipF = MapAt[-1 &, #, RandomSample[SparseArray[#]["NonzeroPositions"], #2]] &

a = RandomInteger[{0, 1}, {9, 9}];
Row[Labeled[MatrixForm@ToExpression@#, #, Top] & /@ {"a", "flipF[a,5]"}, Spacer[5]]