why do i keep getting 'Subscript indices must either be real positive integers or logicals'?

hai , i tried to run this code, but it keeps giving me that answer.

r = -0.5 + (0.5+0.5).*rand(1,1)

nn = 1e-3;
d = 1e-6;
k = 1.38e-23;
t = 0.1;
T = 293;

D = ((k*T)/3*pi()*nn*d)

for i = 1 : 100;
   x(i+t) = x(i) + (sqrt(2*D*t))*r;

end

for i = 1:100;
    y(i+t) = y(i) + (sqrt(2*D*t))*r
end

plot(x,y)

and the error are from

for i = 1 : 100;
  x(i+t) = x(i) + (sqrt(2*D*t))*r;

end

for i = 1:100;
    y(i+t) = y(i) + (sqrt(2*D*t))*r
end

can someone tell me what is wrong with the codes? well, i am a beginner.

thanks in advance

Answer 1

[ES]

you have specified t to be 0.1.

x(i+t) will be x(1.1) for i=1. You cant index arrays like this. Array indices are positive integers starting from 1 in MATLAB.

[Walter Roberson]

The notation

   x(i+t) =

might tempt you to think that you are constructing a formula relating a function "x" with argument (1.1), with some value. However, in MATLAB, with the exception of some kinds of Symbolic Mathematics, the notation

x(i+t) =

means that the array "x" indexed at the location i+t is to be assigned a value. Indices in MATLAB must be integers starting from 1, but your i+t will seldom be an integer.