# Index in position 1 is invalid. Array indices must be positive integers or logical values?

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Abdullah Alsuhaymi on 10 Mar 2020
Answered: James Tursa on 10 Mar 2020
>> lambda = 1;
>> phi = zero;
Undefined function or variable 'zero'.
>> phi = 0;
>> theta = 90;
>> B= 2*pi/lambda;
>> Lx = 20*lambda;
>> Ly = 10*lambda;
>> u = sin(theta);
>> v = 0;
>> f(u,v) = (sin((B*Lx)*u)/((B*Lx/2)*u))*(sin((B*Ly)*v)/((B*Ly/2)*v));
Index in position
1 is invalid. Array indices must be positive integers or logical values.
Hi,
After I try to get values to f(u,v) .I got this error
Index in position
1 is invalid. Array indices must be positive integers or logical values.

Mil Shastri on 10 Mar 2020
Looks like you want to create a function that takes u,v as inputs and provides an output as the result equation you have written. To write a function in MATLAB you have to follow the syntax, see function documentation here https://www.mathworks.com/help/matlab/ref/function.html#btexm3w
You can try the following by pasting this in a Live Script:
phi = 0;
theta = 90;
u = sin(theta);
v = 0;
result=myf(u,v)
function w=myf(u,v)
lambda = 1;
B= 2*pi/lambda;
Lx = 20*lambda;
Ly = 10*lambda;
w=(sin((B*Lx)*u)/((B*Lx/2)*u))*(sin((B*Ly)*v)/((B*Ly/2)*v));
end
Now, if you want to run this for multiple values of , then try this:
phi = 0;
theta = 90;
u = sin(theta);
v = 0;
%result=myf(u,v)
num_points=100;
vs=linspace(0,0.1,num_points);
for n=1:num_points
results(n)=myf(u,vs(n));
end
plot(results)
function w=myf(u,v)
lambda = 1;
B= 2*pi/lambda;
Lx = 20*lambda;
Ly = 10*lambda;
w=(sin((B*Lx)*u)/((B*Lx/2)*u))*(sin((B*Ly)*v)/((B*Ly/2)*v));
end tmarske on 10 Mar 2020
Here is the issue:
>> theta = 90;
...
>> u = sin(theta);
>> v = 0;
>> f(u,v) = (sin((B*Lx)*u)/((B*Lx/2)*u))*(sin((B*Ly)*v)/((B*Ly/2)*v));
Matlab interprets this last line as 'create a new array named f, evaluate the RHS of this expression, and assign that value to the (u, v) entry of the array f. Since neither u nor v are positive integers this is not possible, so Matlab throws the error.
It looks like what you actually wanted to do was create a new function named f. The syntax for this is different. You should either:
1) create a new function in a file as described here: https://uk.mathworks.com/help/matlab/matlab_prog/create-functions-in-files.html
2) create an anonymous function and assign it to f as follows:
>> f = @(u,v) (sin((B*Lx)*u)/((B*Lx/2)*u))*(sin((B*Ly)*v)/((B*Ly/2)*v));
Walter Roberson on 10 Mar 2020
Are you defining a formula, where you want to be able to provide arbitrary u and v afterwards and have it calculate the f value? Or are you defining an array, in which you have specific numeric u and v values and the result for the (J, K)'th pair of input values should be stored in output location indexed by (J, K)?
By the way: you divide by v but your v is 0, so your right hand side is going to be strictly inf or nan.