Index exceeds the number of array elements. Index must not exceed 1.

Please help to resolve error. Below is my code.

% value of constants
a1     = 0.1;
omega1 = 2;
G      = 0.01;
C12    = 0.001;
C13    = 0.006;
C14    = 0.008;
C15    = 0.0010;
C16    = 0.0012;
C17    = 0.0014;
C18    = 0.0016;
C19    = 0.0018;
C110    = 0.0020;
C111    = 0.0022;
C112    = 0.002;
C113    = 0.003;
C114   = 0.005;
C115    = 0.007;
C116    = 0.009;
C117   = 0.0011;
C118  = 0.0013;
C119   = 0.0015;
C120    = 0.0017;
C121    = 0.0019;
C122   = 0.0021;
C123    = 0.0023;
dt     = 0.01;   % step size
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
x1(1) = 0.5;
y1(1) = 0.5;
x2(1) = 0.5;
y2(1) = 0.5;
x3(1) = 0.5;
y3(1) = 0.5;
x4(1) = 0.5;
y4(1) = 0.5;
x5(1) = 0.5;
y5(1) = 0.5;
x6(1) = 0.5;
y6(1) = 0.5;
x7(1) = 0.5;
y7(1) = 0.5;
x8(1) = 0.5;
y8(1) = 0.5;
x9(1) = 0.5;
y9(1) = 0.5;
x10(1) = 0.5;
y10(1) = 0.5;
x11(1) = 0.5;
y11(1) = 0.5;
x12(1) = 0.5;
y12(1) = 0.5;
x13(1) = 0.5;
y13(1) = 0.5;
x14(1) = 0.5;
y14(1) = 0.5;
x15(1) = 0.5;
y15(1) = 0.5;
x16(1) = 0.5;
y16(1) = 0.5;
x17(1) = 0.5;
y17(1) = 0.5;
x18(1) = 0.5;
y18(1) = 0.5;
x19(1) = 0.5;
y19(1) = 0.5;
x20(1) = 0.5;
y20(1) = 0.5;
x21(1) = 0.5;
y21(1) = 0.5;
x22(1) = 0.5;
y22(1) = 0.5;
x23(1) = 0.5;
y23(1) = 0.5;
for i = 2:2000

    x1(i) = x1(i-1) + ( ( a1 - x1(i-1)^2 - y1(i-1)^2 )*x1(i-1) - omega1*y1(i-1) + G*C12*( x2(i-1) - x1(i-1) ) + G*C13*( x3(i-1) - x1(i-1) ) + G*C14*( x4(i-1) - x1(i-1) ) ) + G*C15*( x5(i-1) - x1(i-1) ) + G*C16*( x6(i-1) - x1(i-1) ) + G*C17*( x7(i-1) - x1(i-1) ) + G*C18*( x8(i-1) - x1(i-1) ) + G*C19*( x9(i-1) - x1(i-1) ) + G*C110*( x10(i-1) - x1(i-1) ) + G*C111*( x11(i-1) - x1(i-1) ) + G*C112*( x12(i-1) - x1(i-1) ) + G*C113*( x13(i-1) - x1(i-1) ) + G*C114*( x14(i-1) - x1(i-1) ) + G*C115*( x15(i-1) - x1(i-1) ) + G*C116*( x16(i-1) - x1(i-1) ) + G*C117*( x17(i-1) - x1(i-1) ) + G*C118*( x18(i-1) - x1(i-1) ) + G*C119*( x19(i-1) - x1(i-1) ) + G*C120*( x20(i-1) - x1(i-1) ) + G*C121*( x21(i-1) - x1(i-1) ) + G*C122*( x22(i-1) - x1(i-1) ) + G*C123*( x23(i-1) - x1(i-1) ) *dt;
    y1(i) = y1(i-1) + ( ( a1 - x1(i-1)^2 - y1(i-1)^2 )*y1(i-1) + omega1*x1(i-1) + G*C12*( y2(i-1) - y1(i-1) ) + G*C13*( y3(i-1) - y1(i-1) ) + G*C14*( y4(i-1) - y1(i-1) ) ) + G*C15*( y5(i-1) - y1(i-1) ) + G*C16*( y6(i-1) - y1(i-1) ) + G*C17*( y7(i-1) - y1(i-1) ) + G*C18*( y8(i-1) - y1(i-1) ) + G*C19*( y9(i-1) - y1(i-1) ) + G*C110*( y10(i-1) - y1(i-1) ) + G*C111*( y11(i-1) - y1(i-1) ) + G*C112*( y12(i-1) - y1(i-1) ) + G*C113*( y13(i-1) - y1(i-1) ) + G*C114*( y14(i-1) - y1(i-1) ) + G*C115*( y15(i-1) - y1(i-1) ) + G*C116*( y16(i-1) - y1(i-1) ) + G*C117*( y17(i-1) - y1(i-1) ) + G*C118*( y18(i-1) - y1(i-1) ) + G*C119*( y19(i-1) - y1(i-1) ) + G*C120*( y20(i-1) - y1(i-1) ) + G*C121*( y21(i-1) - y1(i-1) ) + G*C122*( y22(i-1) - y1(i-1) ) + G*C123*( y23(i-1) - y1(i-1) ) *dt;
    

end

Index exceeds the number of array elements. Index must not exceed 1.

    figure
    hold on
    plot(x1)

[Cameron]

You have no value for x2(2), x3(2), x4(2), etc. so when i = 3 and your code looks for x2(i-1), there is no value there and gives you the error you're seeing.