# Writing a function to find prime numbers

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Daniel Kingsley on 19 Aug 2015
Commented: Irfan Turk on 22 Jul 2019
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

#### 1 Comment

David Young on 19 Aug 2015
Please format your code - it's not readable as you can see. (There's a "{} Code" button.)

James Tursa on 19 Aug 2015
Edited: James Tursa on 19 Aug 2015
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

#### 1 Comment

Daniel Kingsley on 19 Aug 2015
@James, Thank you. I am still getting an error for the number 3 as an input. Maybe I should use rem instead of mod?

Pranash Azrot on 19 Aug 2015
%%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

#### 1 Comment

Daniel Kingsley on 20 Aug 2015
@Pranash, I don't understand the comments. I thank you for your help.

Irfan Turk on 21 Jul 2019
Edited: Irfan Turk on 21 Jul 2019
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 on 21 Jul 2019
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 on 21 Jul 2019
It would make more sense to break after assigning 0 to C.
Irfan Turk on 22 Jul 2019
Thank you Walter. That's a good way to make the code better...