A question about matrix

By following codes, I get a 50x50 matrics with randon 1 and 0. Now I want to specify how many 1 and how many 0 are in this 50x50 matrics, and I want them in randon positions.

Thank you very much for your helps！

nside=50;

randworld=round(rand(nside,nside));

world=randworld>.5;

Answer 1

[Image Analyst]

Use randperm:

nside=50;
world = zeros(nside, nside);
num1sToInsert = 4;
randomIndexes = randperm(numel(world), num1sToInsert)
world(randomIndexes) = 1

[Johnny]

Sorry, Im not quite understand.

How ccould I control the amount of 1 and 0 in the world with randperm?

'num1sToInsert = 4',why do we need this?

Thank you for your answer！

[per isakson]

>> figure, imagesc( world )

displays

Notice the four white squares

[Image Analyst]

Johnny, recall you said "I want to specify how many 1 and how many 0 are", and if you look in the documentation for randperm, the second argument is the number of numbers you want to get and assign 1 to. For example if you have 50-by-50 matrix, then there are 2500 elements in that matrix. So randperm will get a scrambled array of every index from the upper left (index 1) to the lower right (index 2500) - basically all numbers from 1 to 2500 scrambled up, if you don't specify the second argument. If you DO specify a second argument, then it won't return all 2500 - we don't need all 2500 - it will return however many you ask for in the second argument. So let's say you wanted to have 4 1's and 2496 0's. Well

randomIndexes = randperm(numel(world), num1sToInsert)

will return exactly 4 indexes randomly located somewhere within those 2500 elements. This is basically the same as

randomIndexes = randperm(2500, 4) % Return 4 numbers chosen randomly between 1 and 2500.

Then I use those indexes to set the matrix at those indexes to 1.

[Johnny]

Thank you very much！it is very clear！

Really appreciate for your helps！