Array element containing infinity

Hi. I have a problem in array indexing containing infinity.

I have to compute the function f(a)=1-1/a mod 13 . It takes values from [infinity 0,1,2 ...12] and out put should be [1,inf,0,7,5,4,6,3,12,9,11,10,8 and 2]

I have tried to code it but it gives error at infinity.

Y_trans=[];
 p=[];
 n=13;
 P_2=[];
 y_axis=[];
 for i=1:13
     if modInv(i,n)==0
         P_2(i)=Inf
     else           
     P_2(i)=modInv(i,n);
     [Y_trans(i)]=  mod(1-P_2(i),n); 
     end
   end
 

[madhan ravi]

Your question and function seems not the same to me atleast , 1 - inf is -inf why are you saying inf?

[lilly lord]

f(a)=1-1/a

f(inf)=1-0=1

f(0)=1-inf=inf

f(1)=1-1=0

and so on

[Adam Danz]

The expression "p mod q" in matlab is mod(p,q) but it's not clear what p should be in your formula,

f(a)=1-1/a mod 13

The mod operator is typically give then same precedence as multiplication and division so one interpretation is

f(a) = 1-mod(1/a, 13)

but other interpretations are

f(a) = mod(1-1/a, 13)
f(a) = 1-1/mod(a, 13)

None of these interpretation are in your code, though. You're using the following which ignore the reciprocal "1/" unles that's already done in modInv (this is why it's helpful to define your variables so we understand what they are).

f(a) = mod(1-a, 13)

Nevertheless, an inf value shouldn't cause an error in the mod function so please share the entire error message and provide us with your inputs.

[lilly lord]

p=13.

modInv calculates 1/a mod p

function xInv = modInv(x,n)
% ModInv(x,n) computes the multiplicative inverse of x modulo n if one
% exists; errors if no such inverse exists
if gcd(x,n) ~= 1
    %error('x has no inverse modulo n')
    xInv=Inf;
    return
end
[d, a, b]   = gcd(x,n);
xInv        = mod(a,n);
end

[lilly lord]

I want the out put as [1,inf,0,7,5,4,6,3,12,9,11,10,8 and 2]

When I start i from 0 to 12, it gives an error

"Array indices must be positive integers or logical values".

[Adam Danz]

" please share the entire error message", all of it, and let us know which line is throwing the error.

The only indices I see are i and n and both are positive integers.