how to use conditional statement realizing prime function

Hi.

I want to use code to check if a number isprime. I know isprime () but I don't want to use it because I want to know how to use judgments to do tha.

Here is my code.

%function code

function[a,b] = isPrime(n)

% A number, n, is not prime if any number between 2 and n/2 divides the number without any remainder.

a = 1; % a = 1 means true

b = 0; % b = 0 means false

half = n/2; % get the half nums of n

for i = 2:half

if(mod(n,i)~=0) % if there is remainder

continue; % keep check

elseif(mod(n,i) == 0)

disp(b); % display false

break; % stop loop

end

disp(a);

end

end

% driver code

n = 9;

[x1,x2]= isPrime(n);

disp(isPrime(n));

Sample test:

isPrimedriver

0

0

1

It should output zero and stop looping, why it work twice?

And how to modify my code?

Thank you.

[Walter Roberson]

example:

Consider 35. 35/2 is 17 1/2 so we will examine 2 to 17. 35 is not divisible by 2 so continue; 35 is not divisible by 3 so continue. 35 is not divisible by 4 so continue. 35 is divisible by 5 so 35 is not prime

Consider 37. 37/2 is 18 1/2 so we will examine 2 to 18. 37 is not divisible by 2 so continue. 37 is not divisible by 3 so continue. 37 is not divisible by 4 so continue. Not divisible by 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 or 18 either. We have reached the end of the 2 to 37/2 range so 37 is a prime.

[Walter Roberson]

       if(mod(n,i)~=0) % if there is remainder
          continue; % keep check
       elseif(mod(n,i) == 0)

Under which circumstances could both of those tests be false? And why did you not put an else clause for the case where both are false?

[Shuoze Xu]

I'm sorry, I don't understand what you're trying to say

[Walter Roberson]

there are exceptions:

• mod(inf,i) is nan
• mod(nan,i) is nan
• problems if n is empty or not a scalar

Answer 1

[Voss]

Your driver code is calling isPrime two times.

% driver code
n = 9;

The first time, you only get a disp(b) from inside isPrime

[x1,x2]= isPrime(n);

     0

The second time, you get a disp(b) from inside isPrime and then, after isPrime is done, you also disp the returned value from isPrime, which is a

disp(isPrime(n));

     0

     1

%function code
function[a,b] = isPrime(n)
% A number, n, is not prime if any number between 2 and n/2 divides the number without any remainder.
   a = 1; % a = 1 means true
   b = 0; % b = 0 means false
   half = n/2; % get the half nums of n
   for i = 2:half
       if(mod(n,i)~=0) % if there is remainder
          continue; % keep check
       elseif(mod(n,i) == 0)
           disp(b); % display false
           break; % stop loop
       end
       disp(a);
   end
end

Answer 2

[Walter Roberson]

Hint:

prime_detected = true
for appropriate things
  if appropriate condition 
    prime_detected = false;
    break;
  end 
end

[Shuoze Xu]

How to return string on function?

[Walter Roberson]

if prime_detected
    a = "true";
else
    a = "false";
end