Mark M cells on a NxN board randomly with equal probability [duplicate] - algorithm

Question

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Algorithm to select a single, random combination of values?
(7 answers)
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An interview question:
Given a NxN board with all cells set to 0, mark M (M < NxN) cells to 1. The M cells should be chosen from all cells with equal probability.
E.g. Mark 30 cells in a 10x10 board, then the probability for a cell to be chosen is 0.3.
My idea is to iterate all cells and on each cell compute a random number in range [1-100], mark the cell to 1 if the number is less than or equal to 30.
The interviewer is not impressed by this solution. Any good idea? (You can use any language)

Answer 1

Put 70 zeros (NxN - M) and 30 ones (M) into a vector. Shuffle the vector. Iterate through and map each index k to 2-d indices via i = k / 10 and j = k % 10 for your example (use N as the divisor more generally).
ADDENDUM
After checking out #candu's link, I decided to give that approach a try. Here's an implementation in Ruby:
require 'set'
# implementation of Floyd's uniform subset algorithm for
# values in the range [0,n).
def generateMfromN(m, n)
s = Set.new
((n-m)...n).each {|j| s.add?(rand(j+1)) || s.add(j)}
s.to_a
end
#initialize a 10x10 array of zeros
a = Array.new(10)
10.times {|i| a[i] = Array.new(10,0)}
# create an array of 10 random indices between 0 and 99,
# map each index to 2-d indices, and set the corresponing
# element to 1.
generateMfromN(10,100).each {|index| a[index/10][index%10] = 1}
# show the results
a.each {|v| puts v.to_s}
This produces results such as...
[0, 0, 0, 1, 0, 0, 0, 0, 0, 0]
[1, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 1, 0, 0, 0, 0, 0]
[0, 0, 0, 1, 0, 0, 0, 0, 0, 1]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
[0, 0, 0, 0, 0, 1, 0, 0, 1, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 1]
[1, 0, 0, 0, 0, 0, 0, 0, 0, 0]
and appears to require only O(M) work for Floyd's algorithm, since on each of M iterations an element always gets added to the set.
If M is bigger than N*N/2, initialize the array with 1's and randomize placement of zeros instead, as suggested by #btilly.

Answer 2

This can be done in expected running time O(m).
First let's deal with the case where we need at most half the board. So m <= n*n/2. For this case we can keep choosing random points and changing their values, throwing away and we chose before, until we have m of them. The probability of throwing away the next random choice is never more than half, so the number of random choices needed is at worst 2 m = O(m).
In the case where we need more than half the board, it takes time O(m) to flip every cell to 1, and then we use the previous solution to find n*n - m cells to turn back to 0.