My function is running fine for small no but not for big no.Can anybody explain?

function k=next_prime(n)
k=2;
while(k>2)
    if(isprime(k)==1)
        if(k>n)
            return
        else
            k=k+1;
        end
    else
        k=k+1;
    end
end

[Walter Roberson]

k=2;
while(k>2)

You initialize k to 2, but you check k>2 which is immediately false.

Answer 1

[DGM]

Consider:

a = next_prime(17560)

a = 17569

b = next_prime(5346551)

b = 5346571

function k = next_prime(n)
    k = n;
    while true 
        if isprime(k)
            break;
        end
        k=k+1;
    end
end

[Nishant Raj]

function k=next_prime(n)
k=2;
while(k>0)
    if(isprime(k)==1)
        if(k>n)
            return
        else
            k=k+1;
        end
    else
        k=k+1;
    end
end

so there was a typo in my previous code. Now can you tell me the error. I know your code is correct but i want to know error in my code now

[Walter Roberson]

What is an example of an input that it gives the wrong answer for?

Your code is slow. There is no point in testing any number that is lower than the input, n, because it does not matter if any of them are prime: all that matters is starting from n . And making sure that you carefully define what is intended to happen if the input is already a prime.

[Nishant Raj]

Ok,got it,my code is slow,thats why its taking a lot more time for big inputs.