How determine prime numbers after checking if they're odd

Here is the questions asked, "randomly generating an integer between 1 and 24 for each die. If the total (sum) of the dice is even, display a message stating that you rolled an even number. If the total of the dice is odd, check if the total of the dice is a prime number. If the number is prime, print a message stating that the total is both odd and prime. Otherwise, print a message stating the total of the dice is odd and not prime."

The code I have created is the following,

x = randi([1,24]);
y = randi([1,24]);
z = x+y;
if(mod(z,2)== 0)
    sprintf("The sum %d is even", z)
end
if(mod(z,2)== 1)
    sprintf("The Sum %d is Odd", z)
elseif (rem(z,1)==1)
    sprintf("The sum is prime")
end

I know ill need more to finish, however i cant seem to get how to check for prime numbers and then use that check for prime to also apply to the odd numbers.

Answer 1

[Dyuman Joshi]

Check for prime numbers using isprime

[John D'Errico]

Or, since there are only

numel(primes(48))

ans = 15

primes less than 48, just use ismember. (And 1 less, if one cares only about the odd primes.) Or, since there are no odd numbers no larger than 48 that would fail a Fermat test with a witness of 2 (thus no Fermat liars for that witness), one could even use that.

x = 3:2:47;
witness = 2;
x(find(mod(witness.^(x-1),x)==1))

ans = 1×14

3 5 7 11 13 17 19 23 29 31 37 41 43 47

And, since one stays below 48, and since 2^48 and below is exactly representable as a double floating point integer, you do not even need to worry about powermod tools.

It does seem a bit of massive overkill though. But isprime is just so ... boring. :-)

Hmm. how about a Fibonacci or Lucas test? Again, wild overkill here. But I'd surely give extra credit to a student who was willing and able to code them up, especially worrying about the case of Fibonacci or Lucas liars in the respective tests.

Answer 2

[Image Analyst]

Since z is a vector, try using a for loop over all z and checking each element one at a time.