MATLAB: Writing a function to find prime numbers

homeworkMATLAB

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

Best Answer

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
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