prime factor function matlab

function [z1] = pfact(Z)
if Z>=2
    p = 2:Z;
    q = mod(Z,p);
    a=find(q==0);
    z1=p(a)
elseif Z==1
    z1 = 1
elseif Z==0
    z1 = []
    disp("error")
end

I'm trying to get only prime factors but this code gives me all the factors, I need the prime factors that when they multiply with each other they give the original number.

[Dyuman Joshi]

It's not clear to me as to what you want to do.

Say Z is 20, what should be the output then?

[Daabir Momin]

Z should be [2,2,5] , but I'm confused as to how I should approach the solution without using the factor function.

[Daabir Momin]

function [z1] = pfact(Z)
if Z>=2
    p = 2:Z;
    q = mod(Z,p);
    a=find(q==0);
    z1=p(a)
elseif Z==1
    z1 = 1
elseif Z==0
    z1 = []
    disp("error")
end


pfact(20)

This gives me all the factors, can someone help me manipulate this code so it gives only the prime factors that when mutiplied together it gives you the original number.

like 20 should give output of [2,2,5]

Answer 1

[Dyuman Joshi]

The code you wrote gives the divisors of Z.

This should work well unless you are dealing with extremely large numbers -

out1=pfact(20)

out1 = 1×3

2 2 5

out2=pfact(prod(primes(15)))

out2 = 1×6

2 3 5 7 11 13

function [z1] = pfact(Z)
if Z>=2
    p = primes(sqrt(Z));
    z1 = [];
    while Z>1
        d = p(rem(Z,p)==0);
        if isempty(d)
            z1=[z1 Z];break
        end
        z1=[z1 d];
        Z = Z/prod(d);
    end
    z1=sort(z1);
elseif Z==1
    z1 = 1;
elseif Z==0
    z1 = [];
    disp("error")
end
end