Creating a Matrix of random numbers

I'm trying to create a 20x20 matrix of values either -1 or 1, but randomly assigned. How can i do this? I've tried using the randi function but it returns the numbers as a range from -1 1 and so includes 0. any help would be appreciated

Answer 1

[Steven Lord]

possibleValues = [-1, 1];
desiredSize = [5 6];
A = possibleValues(randi(numel(possibleValues), desiredSize))

A = 5×6

-1 1 1 1 1 -1 -1 -1 1 1 -1 -1 -1 -1 -1 1 -1 -1 1 1 -1 1 1 -1 -1 1 -1 1 1 -1

This assumes you want -1 and 1 to be equally likely. If you need an exact number of each possible value:

elements = [repmat(-1, 1, 5), repmat(1, 1, 25)];
order = randperm(numel(elements));
shuffled = reshape(elements(order), desiredSize)

shuffled = 5×6

1 -1 1 1 1 1 1 1 1 1 -1 1 1 1 -1 1 1 1 -1 1 1 1 1 1 -1 1 1 1 1 1

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If you need the two numbers not to be equally likely (say 1 three times as likely as -1) there are ways to do this as well.

[Blair Hail-Brown]

Absolutely perfect, thank you so much!

Answer 2

[John D'Errico]

randi([0 1],20)*2 - 1

Answer 3

[David Hill]

(-1).^randi(2,20);

Answer 4

[Bastian Arturo Romero Mardones]

A long way

bits= randi([-1 1],20,20);
[r,c,~] = size(bits);
count = 1;
while count < (r*c)+1
    
    bits = reshape(bits,[1 r*c]);
    if(bits(count)  == 0)
        bits(count) = randi([-1 1],1);
        count = 1;
    else
        count = count + 1;
    end
    
end
        
    bits =  reshape(bits,[r c]);