How can I store data from a loop when the data is negative?

My code is not finished at all, but I am trying to store data into an array, but because it is nagative i get an error.

Update: I ran it will positive values and the data still won't save for the same reason, I get the

"Array indices must be positive integers or logical values.

Error in findxint (line 19) Xbl(fxL) = xL" error. Please help me figure out how to save the data, xL and xR

function xint = findxint(a,b,c,z)
c = [a b c z];
xmin = -100; 
xmax = +100; 
n = 50;
dx = (xmax - xmin) / n; % dx=4
xL = xmin;
i = 0; 
Xbl = zeros;
Xbr = zeros;
while (i < n)
    i = i + 1;
    xR = xL + dx
    fxL = (c(1,1))*(xL)^3 +(c(1,2))*(xL)^2 + (c(1,3))*(xL) + (c(1,4));
    fxR = (c(1,1))*(xR)^3 +(c(1,2))*(xR)^2 + (c(1,3))*(xR) + (c(1,4));
    
        if sign(fxL) ~= sign (fxR)  % if sign change      
            %SAVE BRACKET, IDK how to ! :( 
            Xbl(fxL) = xL;
            Xbr(fxR) = xR;
            
        %???
        end
    xL = xR
end 
xint = [Xbl Xbr]
end

Answer 1

[KSSV]

function xint = findxint(a,b,c,z)
c = [a b c z];
xmin = -100; 
xmax = +100; 
n = 50;
dx = (xmax - xmin) / n; % dx=4
xL = xmin;
i = 0; 
Xbl = zeros([],1);
Xbr = zeros([],1);
count = 0 ;
while (i < n)
    i = i + 1;
    xR = xL + dx
    fxL = (c(1,1))*(xL)^3 +(c(1,2))*(xL)^2 + (c(1,3))*(xL) + (c(1,4));
    fxR = (c(1,1))*(xR)^3 +(c(1,2))*(xR)^2 + (c(1,3))*(xR) + (c(1,4));
    
        if sign(fxL) ~= sign (fxR)  % if sign change      
            count = count+1 ;
            %SAVE BRACKET, IDK how to ! :( 
            Xbl(count) = xL;
            Xbr(count) = xR;
            
        %???
        end
    xL = xR
end 
xint = [Xbl Xbr]
end

YOu need to rething on your code...you should see to it that, in the loop the indices should be +ve integers.

[Sierra Bounds]

The code is going to be for solving for roots of a polynomial so I can not control if the roots would be negative or not. So the code should be able to store negative data.