ifactors Test Problem

[toml_12953]

I'm trying to create an is_prime function:

Code:

EXPORT is_prime(X)
begin
  if dim(ifactors(X))=={2} then
    return true;
  else
    return false;
  end;
end;

dim(ifactors(7))

returns {2} but when I call is_prime(7), I get Error: Bad argument type.

I tried with both upper and lowercase X and I tried the 2 both in and out of braces.

What am I doing wrong?

[Helge Gabert]

Try storing DIM(ifactors(X)) into a list, say L1, and then try

IF L1(1) == 2 THEN . . .

Works here.

(This is because, in Home, a statement like {2}=={2} returns {1}, not 1).

But aside from that, why do you test DIM(ifactors(X))? That won't tell you if X is prime.

[toml_12953]

(09-06-2017 05:16 PM)Helge Gabert Wrote:  Try storing DIM(ifactors(X)) into a list, say L1, and then try

IF L1(1) == 2 THEN . . .

Works here.

(This is because, in Home, a statement like {2}=={2} returns {1}, not 1).

But aside from that, why do you test DIM(ifactors(X)? That won't tell you if X is prime.

I thought that if the length of the ifactors vector returns 2 then there are only two factors, the number itself and 1. I was wrong. The list doesn't include 1 so dim(ifactors(8)) would return 2 as well.

[Tim Wessman]

(09-06-2017 07:03 PM)toml_12953 Wrote:  If the length of the ifactors vector returns 2 then there are only two factors, the number itself and 1. That means it's prime.

Are you doing this as a learning exercise? (I'll leave alone the issue with this method to figure prime for someone else)

The is already a function specifically for primes. Quite many in fact under the toolbox->CAS->Integer->Prime menu.

[Didier Lachieze]

(09-06-2017 05:16 PM)Helge Gabert Wrote:  Try storing DIM(ifactors(X)) into a list, say L1, and then try

IF L1(1) == 2 THEN . . .

You can also use EQ to compare two lists :

IF EQ(DIM(ifactors(X)),{2}) THEN....

...but ifactors(8) returns [2 3]: factor 2, multiplicity 3, so it won't help you to identify prime numbers.

[Helge Gabert]

Yeah, well, I'm not the OP and I never claimed that. Indeed,there are infinite cases of composite numbers which return {2} with this method.

I merely pointed out how to change the program to make it run, instead of coming up with the error message encountered by the OP.

EQ()is a good suggestion, however.

[toml_12953]

(09-06-2017 07:10 PM)Tim Wessman Wrote:  

(09-06-2017 07:03 PM)toml_12953 Wrote:  If the length of the ifactors vector returns 2 then there are only two factors, the number itself and 1. That means it's prime.

Are you doing this as a learning exercise? (I'll leave alone the issue with this method to figure prime for someone else)

The is already a function specifically for primes. Quite many in fact under the toolbox->CAS->Integer->Prime menu.

Yes, I was just trying to come up with a function that didn't use the one from the menu. Thanks to all who answered my question!

[Helge Gabert]

I forgot to mention:

You could use TYPE(ifactor(number to test)).

For primes, should return 0. For composites, 8.

(Of course, ifactor could return 0 for certain integers in error).