Finding Prime numbers (NO isprime function)

Question

gmltn1212 2020-6-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/547488-finding-prime-numbers-no-isprime-function

评论： Walter Roberson 2020-6-13

Hi, I am trying to return if the value is prime or not. And here is my code

vec = [3 4 88 5 1371];
    for i = 1:length(vec)
        b = mod(vec(i),2);
        if vec(i)==1
            disp('not prime');
        elseif vec(i)==2
            disp('prime');
        elseif b==0
            disp('not prime');
        elseif b>0
            for j = 3:2:(vec(i)-1)
                b = mod(vec(i), j);
                if b==0
                    disp('not prime');
                end
            end
            disp('prime');
        else
            disp('not prime');
        end
        
    end

However, this code returned this

prime
not prime
not prime
prime
not prime
not prime
prime

Could you please tell me why its returning 7 outputs not 5? Also could you fix this code so it only returns 5 outputs?

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Stephen23 2020-6-13

编辑：Stephen23 2020-6-13

"Could you please tell me why its returning 7 outputs not 5?"

Look at your j-loop: within that loop your code could call disp zero, one, or more times, depending on the vec(j) value.

And then after the j-loop, you call disp once more. Your code does nothing to ensure that you only call disp once for each vec value.

"Also could you fix this code so it only returns 5 outputs?"

Get rid of the j-loop and write vectorized code. It will be simpler.

Note that you do not need to check up to vec(i)-1, up to sqrt(vec(i)) is sufficient and more efficient.

gmltn1212 2020-6-13

Thank you for replying to my post! However, Im having a hard time understanding your comment! It would be really helpful if you could show me how to fix it with code...

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Answer 1

Walter Roberson 2020-6-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/547488-finding-prime-numbers-no-isprime-function#answer_450552

在 MATLAB Online 中打开

                if b==0
                    disp('not prime');
                end

You should exit the j loop as soon as you prove the number is not prime.

disp('prime');

You are doing that no matter what the outcome of your for j loop is.

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gmltn1212 2020-6-13

编辑：gmltn1212 2020-6-13

I thought I exited after writing that code.. Could you show me how to fix this?

Walter Roberson 2020-6-13

在 MATLAB Online 中打开

                if b==0
                    disp('not prime');
                    break;
                end
            end
            if b ~= 0
                disp('prime')
            end