My circular_prime function only works well for two_digit input

Question

Qingchuan Lyu 2018-7-11

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/409912-my-circular_prime-function-only-works-well-for-two_digit-input

回答： Kelly Kearney 2018-7-11

Hi, I wrote a function to compute the number of circular primes smaller than an input. For example, 197 is a circular prime, because 197, 971, and 719 are all primes. Here is my function. It works only well for inputs less than 100. As long as I increase the input to something like 101, something will be missing. Could you help me figure it out? Thanks!

A numerical example would be

If an input is 17, then its output is m=[2 3 5 7 11 13] and x=6.

 % function x=circular_primes(n)
p=primes(n-1);
l=length(p);
for i = 1:l
    for j=1:numel(num2str(p(i)))
        q=num2str(p(i));
        if j~=1 && j<numel(num2str(p(i)))
           ca=[q(j+1:end) q(1:j)];
           c=str2num(ca);
        elseif j==1
            c=str2num(q);
        elseif j==numel(num2str(p(i)))
            ca=flip(num2str(q));
            c=str2num(ca);
        end
    if isprime(c)==1
        m(i)=p(i);
    else
        m(i)=0;
    end
    end
   end
m(m==0)=[]
    [~,x]=size(m);