Writing a function to find prime numbers

This is homework. I already turned it in. I just don't know why this function doesn't work. I just want to learn what I missed. Thanks so much. This is the problem:

Write a function myprime that takes n, a positive integer,as an input and returns true if n is prime or returns false otherwise. Do not use the isprime or primes or factor built-­‐in functions.

Here is my code:

function result = myprime(n)
%%initially set output flag to true
 result = true;
%%iterate over all positive integers 2,3,...,n-1
%%if n is not divisible by any of these factors....it is prime
 if (n == 1)
     result = 'false';
 elseif (n == 2)
     result = 'true';
 else 
    for i=2:n-1,
        if (mod(n,i)==0)
           result = 'false';
        end
    end
 end
%%return "true" or "false" instead of 1 or 0  
 if (result)
    result = 'true';
 else
    result = 'false';
 end

[David Young]

Please format your code - it's not readable as you can see. (There's a "{} Code" button.)

Answer 1

[James Tursa]

The basic problem is that you are mixing character variables (with the single quotes ' ') with logical variables (without single quotes). E.g.,

 result = true;  <-- LOGICAL
%%iterate over all positive integers 2,3,...,n-1
%%if n is not divisible by any of these factors....it is prime
 if (n == 1)
     result = 'false'; <-- CHARACTER
 elseif (n == 2)
     result = 'true'; <-- CHARACTER
 else 
    for i=2:n-1,
        if (mod(n,i)==0)
           result = 'false'; <-- CHARACTER
        end
    end
 end
%%return "true" or "false" instead of 1 or 0  
 if (result) <-- Intent of test is that variable is LOGICAL
    result = 'true';
 else
    result = 'false';
 end

So stick to one or the other. E.g., to use logical variables in the first part of your code:

 if (n == 1)
     result = false;
 elseif (n == 2)
     result = true;
 else 
    for i=2:n-1,
        if (mod(n,i)==0)
           result = false;
        end
    end
 end

[Daniel Kingsley]

@James, Thank you. I am still getting an error for the number 3 as an input. Maybe I should use rem instead of mod?

Answer 2

[Pranash Azrot]

 %%Yaar meh edah kitaah see. meerah answer teek see. menu 100% meliah see. try karrey. 

 function x = myprime(n)
if (rem(n,2)~=0 || n==2)
    k = fix(n/2);

 else
    x = false;
    return;
end
for i = 1:k
    w(i) = rem(n,i);
end
t=w(w==0);
[m n] = size(t);
if n<=1
    x = true;
else 
    x = false;
    return;
end

[Daniel Kingsley]

@Pranash, I don't understand the comments. I thank you for your help.

Answer 3

[Irfan Turk]

You can find all prime numbers upto a certain number with the following code

%This code find all prime numbers
%upto the entered number 
clear all;
N=input('Prime Numbers until:');
if N<2
    return;
elseif N==2
    disp(2);
    return;
end
Pr(1)=2;Pr(2)=3;Count=3;
for i=4:N
    C=Check(i);
    if C==1
        Pr(Count)=i;
        Count = Count +1;
    end
end
disp(Pr);
function C=Check(i)
C=1;
for k=2:(ceil(sqrt(i)))
    if mod(i,k)==0
        C=0;
    end        
end
end

[John D'Errico]

How many times will you post this poor code to find prime numbers? I see that you have resurrected several old, long dead questions, only to post the same novice code.

[Walter Roberson]

It would make more sense to break after assigning 0 to C.

[Irfan Turk]

Thank you Walter. That's a good way to make the code better...

Answer 4

[Farah Salman]

function y = isPrime(N)

for i=2: floor(N/2)

if mod(N,i)==0

y=false;

return

end

y= true;

end

[Steven Lord]

This code has at least one bug.

z = isPrime(2)

Output argument "y" (and maybe others) not assigned during call to "solution>isPrime".

function y = isPrime(N)
for i=2:floor(N/2)
if mod(N,i)==0
    y=false;
    return
end
y= true;
end
end