MAPLE - Simple procedure to test primality

[dkotschessaa]

This is technically a "homework" type question so I just need a bit of help.

I'm just learning maple and writing a very basic procedure. I'm testing primality exhaustively by using the mod function. (Yes, maple has a built in procedure for this, but this is just an exercise).

ISPRIME := proc (n)
local i;
local s;
if n < 2 then return false end if;
if n = 2 then return true end if;
for i from 2 to n do if n mod i = 0 then print (false) else print(true);
end do;
end proc;

I kind of know why this isn't working but I don't know how to fix it. If I find an i such that n mod i is zero, then the number n is not prime - so that's all. Print (false) in that case.

If it is not the case that n mod i = 0 then I want to continue testing all the different i's up to n. (really n-1) if I don't get anything, print (true). But it seems to not be looping the procedure for all the possible i's.

Help/hints appreciated. Thanks.

-Dave K

 

[.Scott]

Start by answering these questions:
Do you want ISPRIME to return true or false or do you want it to print true or false?
What exactly do you want ISPRIME to do if it discovers the number is no prime? (clue: your program isn't doing that)

http://www.maplesoft.com/support/help/Maple/view.aspx?path=break

 

[.Scott]

Since this isn't a homework problem, I can just give you the answer.

I would have your "IsPrime" function return true or false and have the calling program do the printing.

You should set a "FoundDivisor" variable to false. Then inside the loop, if the Mod function returns zero, set FoundDivisor to true and "break" from the loop. If you drop through the loop without finding a divisor, "FoundDivisor" will still be false. So then just return not FoundDivisor.

 

[dkotschessaa]

I figured something else out, but thanks for your reply!

 

[alphy]

Hello Dave K,

Thank you for reaching out for help with your maple procedure. It seems like you have a good understanding of the concept of testing primality, but there are a few errors in your code that may be causing it not to work as intended.

Firstly, in your for loop, you are starting at i=2 and going up to n, which means that you are only checking if n is divisible by 2 and n itself. This is not enough to determine if a number is prime. You need to check for divisibility by all numbers from 2 to n-1.

Secondly, your if statement is not correctly structured. You have it set up to print false if n mod i is 0, but you also need to have an else statement to print true if n mod i is not 0. As it is currently written, your code will only print false if n is not divisible by 2.

Here is a revised version of your code that should work correctly:

ISPRIME := proc (n)
local i;
if n < 2 then return false end if;
if n = 2 then return true end if;
for i from 2 to n-1 do
if n mod i = 0 then return false else return true
end do;
end proc;

I hope this helps you with your assignment. If you have any further questions, please don't hesitate to ask. Good luck with your studies!