Quote:Original post by agisler
You need to use a loop that checks within the loop if the current number is 100. Example:

for (int i=100;i<=100;i++)
{

//check if "i" is a prime number}

I realize it's a typo, but just to avoid any confusing for OP: you start counting at one (and you can skip the even numbers obviously). [smile]

Quote:Original post by AngleWyrm
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All that remains is to write the function called is_prime().

That algorithm will miss the prime 2.

Thanks for everything, you guys. But I don't think that I'm at that level just yet.

Can you describe a procedure for determining if a number is prime or not?

This is the heart of programming; procedural thinking.

Forget about the language for a minute and try to describe a procedure. If I give you the number 42, can you describe a process that proves it is or is not a prime?

It is not prime because:

42 | 2
21 | 3
7 | 7
1
42 is a composite number.

I'm such a dunce, I wrote a post expanding on all of that and how to check if a number is evenly divisible and completely forgot about the modulus operator. That little % sign in AngleWyrm's example is called the modulus operator and is used to check if a number is evenly divisible by another number. It is a critical step in finding primes. Go look it up to better understand the sample.

I think it might also be worth explaining casting. In Wyrm's example:

int divisor = (int) sqrt(test_value);

sqrt() takes and returns a double, which is like a float but more precise. It lets you store non-whole numbers. sqrt(42) would yield 6.48. But what he wants is 6, not 6.48. So it needs to be converted somehow...

Recall than the int datatype cannot store the number 6.48. If you wrote code like this:

int divisor = sqrt(test_value);

It would take the value 6.48 and shove it into an int slot. (The computer will convert it on the fly.) The value 6 will be stored, which is what's wanted.

But the compiler will give you a warning about an implicit cast. It gives you this warning because in that conversion, data is thrown away, and if you wrote such code on accident, it can break your program. Under some circumstances it could also misunderstand what you want. So...

You must communicate to the compiler that you specifically want to cast it from float to integer.

By putting the target data type in parenthesis before a value, you perform an explicit cast. Here's the code again with the cast highlighted:

int divisor = (int) sqrt(test_value);

Putting that before the value tells the compiler "I want to convert this from its current data type to an int; I understand what this does and have made a conscious decision".

And that will get rid of the warning.