Array indices must be positive integers or logical values.

im sorry i dont know how this can happen, can someone help me

k=[0:20:1000]
for a=1:length(k);
   derivative_x(a)=2*kons_G*rho_model*t*(-(1/(z2((x(a)-x0)^2/z2^2+1)))+(1/(z1((x(a)-x0))^2/z1^2+1))); %sudah dalam mGal
   derivative_z(a)=((-2*kons_G*t*rho_model*((x(a)-x0)/(z1^2+(x(a)-x0)^2)))); %sudah dalam mGal
end

Answer 1

[Walter Roberson]

   derivative_x(a)=2*kons_G*rho_model*t*(-(1/(z2((x(a)-x0)^2/z2^2+1)))+(1/(z1((x(a)-x0))^2/z1^2+1))); %sudah dalam mGal

MATLAB has absolutely no implied multiplication. Not even in the symbolic mathematics languages.

Therefore your z2( and z1( expressions are interpreted as requests to index z2 and z1 at calculated locations that do not happen to come out as positive integers.

[Mirza Sadewa]

thanks, i miss the multiplication in z2( and z1(

Answer 2

[Jan]

What is z1 and z2? Are these vectors? Then the message means, that

z2((x(a)-x0)^2/z2^2+1)

is not a valid expression, because (x(a)-x0)^2/z2^2+1 is not a positive integer or logical vector.

You can identify the problem using the debugger. Set a breakpoint in the failing line and evaluate the expression piece by piece:

% 2*kons_G*rho_model*t*(-(1/(z2((x(a)-x0)^2/z2^2+1)))+(1/(z1((x(a)-x0))^2/z1^2+1)))
x(a)
(x(a)-x0)^2
(x(a)-x0)^2/z2^2+1
z2((x(a)-x0)^2/z2^2+1)
... etc