Prime Function without Conditionals or Iteration

Question

Eduardo Alfaro 2017-2-6

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https://ww2.mathworks.cn/matlabcentral/answers/323508-prime-function-without-conditionals-or-iteration

评论： Eduardo Alfaro 2017-2-6

I have to construct a code that inputs a positive integer N and outputs a vector of the prime numbers up to that positive integer N. I know you have to eliminate the multiples of 2, then then the multiples of 3, and so on until you get to floor(sqrt(N)). However, I'm having trouble in translating this into code. The following functions are banned:

      primes(), isPrime(), ismember(), setdiff(), factor()
function [out] = sieve(N)
limit =  floor(sqrt(N));
vec = 1:limit
mask =

I'm stuck trying to make a mask for the vector.

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Answer 1

Walter Roberson 2017-2-6

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You can use ndgrid to construct matrices of values, and mod() to test whether one value divides another. The results that are 0 are the places where one value divides another. But watch out for the fact that a value divides itself.

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Answer 2

KSSV 2017-2-6

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https://ww2.mathworks.cn/matlabcentral/answers/323508-prime-function-without-conditionals-or-iteration#answer_253465

在 MATLAB Online 中打开

clc; clear all ;
N = 10 ;
p = primes(N) ;
c = 0 ;
for n = 2:N
    m = 2; % initialise factor to test
    t=floor(sqrt(n));count=0;
    for m = 2:t
        if mod(n,m) == 0 %m is a factor of n
            fprintf('%d is not prime \n',n)
            primalitytest=0;
        else
            count=count+1;
        end
    end;
    if count==t-1
        fprintf(' %d is prime\n',n);
        primalitytest=1;
        c = c+1 ;
        iwant(c) = n ;
    end
end