Find the amount of prime numbers between two numbers by using only length and primes

Is there any possible way to find the amount of prime numbers between two numbers by using only the commands length and primes? So far I have tried this, but whenever I change the number in the if statements to something larger than 2, the code suddenly won't work.

P = primes(20000);

p=[];

if P >= 10000

p = [p, P];

end

amount = length(p)

Answer 1

[Cris LaPierre]

Yes, it is. Consider what primes returns: Prime numbers less than or equal to input value.

If I can find out the number of primes less than or equal to two different values, then I can use a little math to figure out the number of primes between those two values.

[iseinas]

Does my code look okay to you?

[Cris LaPierre]

This is clearly a homework assignment, so I'm intentionally not commenting on your code. Perhaps pick a smaller range and test your code to see if it works. Perhaps between 6 and 20?

You can accomplish this without using a for-loop, though.

[iseinas]

Hey again! I figured it out by using setdiff. I'm pretty new to Matlab so didn't know there was so many functions you could use. Have only been coding with Javascript so far. Thanks a lot though :)

[Cris LaPierre]

Can you use setdiff? That means you are solving the problem using a function other than primes and length.

[iseinas]

Ah.. well then :/ Now I'm not really sure what I'm supposed to do. Is there a way to do the same function without using setdiff?

[Cris LaPierre]

Yes. If I know there are 3 prime numbers <=6, and 8 prime numbers <= 20, then I can figure out the number of primes between 6 and 20 by taking the difference: 8-3 = 5.

[iseinas]

OMG it was really easy, I totally forgot about that,, thank you so much. My math skills really dropped right there :c You really helped a lot.