Generating a random list of (x, y) points that satisfy a condition?

So I need to generate a matrix of points given that they meet the condition that at these (x,y) points concentration is greater than 10. Note that I first run a code that gives me concentration at each location c(x,y), and now from the results of the first run I need Matlab to "randomly" pick (x,y) points with the above condition.

Would appreciate any suggestions on how to go about this.

Answer 1

[Image Analyst]

Try using thresholding and randperm():

% Generate sample data:
c = randi(40, 9, 9)
% Get a column vector of only those c that are more than 10.
moreThan10 = c(c>10)   % Threshold data
% Get 5 indexes at random from moreThan10
randomIndexes = randperm(length(moreThan10), min([5, length(moreThan10)]))
% Extract those 5 random indexes from our list into a new array called output.
output = moreThan10(randomIndexes)

[Summer]

Thanks for responding, Image Analyst.

"% Get 5 indexes at random from moreThan10" the indexes are for the location, right? My concentration changes in both the x and y directions; I have two indices c(i,j). So eventually, I need a list of both x and y points.

[Image Analyst]

Summer, this works fine with 2D matrices. I guess you don't know about linear indexing. If c is a 9 by 9 matrix, you can get to any element by 2D indexing or with a linear index where the numbers start at 1 in the upper left and go down rows, and then across columns until it ends at the last element in the lower right. So c(1) = c(1,1) = upper left, and c(9) = c(9, 1) = lower left. c(73) = c(1, 9) = upper right, and c(81) = c(9,9) = lower right.

No modification to the code is necessary (except for removing the line where I generate sample data, and changing the number of numbers you want to extract from 5 to whatever you want).

[Summer]

Thanks!