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

x_exact = @(t) 1./3.*2.*exp(-3.*t).*(2.*exp(6.*t) + 1) + 1./3.*2.*exp(-3.*t).*(exp(6.*t) - 1) % x exact solution

y_exact = @(t) 2./3.*2.*exp(-3.*t).*(exp(6.*t) - 1) + 1./3.*2.*exp(-3.*t).*(exp(6.*t) + 2) % y exact solution

h = 0.1 % step

t0 = 0 % first time value

ts = 4 % last time value

t=t0:h:ts

for i = 1:length(t)

if (t(i) == t0)

x_ex(i) = x_exact(t(i))

x_iEm(i) = x_exact(t(i))

else

x_ex(i) = x_exact(t(i))

x_past = x_iEm(i-1)

y = y_exact(t(i))

x_iEm(i) - h*x_iEm(i) == x_past + h*2*y % error occur in this line code

end

end

hi guy , so i'm trying to solve the ODE systems , and calculate x_ex ( exactly solution ) and x_iEm ( implicit Euler method) , and i get the error " Index exceeds the number of array elements. Index must not exceed 1 " , does anyone know the way to fix it ???

it seem like the error occurs when i = 2 -> t(i) = 0.1 , in the line code i comment

Answer 1

[Alan Stevens]

What are yoy trying to do with

x_iEm(i) - h*x_iEm(i) == x_past + h*2*y; 

when i = 2, there is, as yet, no value assigned to x_iEm(i), so you sem to be testing for logical equality with values on the left that don't exist.

[BAO HOANG]

i just want to calculate the x_iEm(i) when i = 2 , when we have the RHS , do you know how to do it??? ( sorry , just the firts or second day i learn matlab , so don't know much about this things )

[Alan Stevens]

The == test for equality. To assign something to a variable just use =.

If you intended to obtain x_iEm(i) from x_iEm(i) - h*x_iEm(i) == x_past + h*2*y;then you need

x_iEm(i) = (x_past + h*2*y)/(1-h);