Prime numbers C++ program. Please help.

Geee, I swear to God I had to just remove all the info, because it simply gave me tons of errors, it just doesn't work ... I'm helpless.

>> it just doesn't work ... I'm helpless.

Can you still post what you have ? You must have gotten somewhere at least, especially since the assignment is very specific about what you have to do :

        "The outer loop can iterate from 3 to 100 while the inner loop checks to see if the counter value for the
          outer loop is prime. One way to see if number n is prime is to loop from 2 to n-1 and if any of these numbers
          evenly divides n, then n cannot be prime. If none of the values from 2 to n-1 evenly divide n, then n must be
          prime."

Which part of this gives you trouble ?

Have you written programs in C++ before ?

I've only done a simple one and all of a sudden she gave us something that she didn't even bother to explain and just wants us to figure it out ourselves. =S
I'm not a genius or can predict things or have some super powers.  That's why I need help, gosh. =[
I couldn't work it out at all.  I dunno even know what is a nested loop ... gee.  I'm not joking when I said I'm helpless.

Ok, in that case, I would start out by reading up on C++. Here are a few tutorials to get you up-to-date :

        http://www.cplusplus.com/doc/tutorial/

It's a recommended read !


Regarding your specific question :

>> I dunno even know what is a nested loop

A nested loop is one loop inside the body of another. For example two nested for loops :

        for (int i = 0; i < 10; ++i) {
            for (int j = 0; j < 20; ++j) {
                // some code
            }
        }

So, the inner for loop (the one with j as index) will be executed 10 times. The total amount of times that "some code" will be executed is thus 10 * 20 = 200 times.

Does that make sense ?

Yeah, it does.  But I kind of don't know a lot about that.
Thanks for the help really, though I don't think I'll be able to actually do my assignment, because it needs practice, and it's already night and oh well.  It's not my fault that I do not understand, it's just she gives us things that she didn't even bother to explain ....

Thanks a lot anyway ...

>> But I kind of don't know a lot about that.

That's why I posted the basic C++ tutorials ... Did you read them ?

I am working on it.  I'll try my best.

However, it would be a really great favor if you know how to do it.  You won't lose anything, and I don't really rely on others when I want to do my homeworks and stuff, that's not me, but this was urgent so I thought of asking.  But still thanks if you don't want to do it, I don't blame you. =]

I'm sure you understand, but if I give you the solution, then I've deprived you of a good learning experience, and I don't want to do that.

If you want to give it a try (don't give up ;) ), however, I will be here to help you with any problems/questions you might have. What do you think ?

Just take it one step at a time : start with the basic skeleton of a C++ application, and then add code to it, one bit at a time, until all requirements are met.

But first make sure that you understand the assignment. I assume you know what prime numbers are ?

Alright, I'll work on it with you then, and we'll see. =]
Don't you have msn messenger, it would be even easier?

I'm monitoring this thread, and I'm able to respond very fast ;)

Hehe, okay, I'll start now, and I'll see what I need.

I know that I should use a for loop because there is calculating, I hope I'm right.  But I dunno what to include in that for loop.

Take it one step at a time ... Do you know what prime numbers are ? If not, they're numbers that are divisible only by themselves and by 1.

If you're wondering about what should be in the for loops, read the assignment : it's very specific about that. Just give it a go, and write some code (one step at a time as I said earlier), and post it here ... that's the easiest way :)

for ( int n = 3, n >= 100, n++)

I feel stupid ... =S

#include <iostream>
using namespace std;
int main()
{
for (int n = 3; n <= 100; n++)

I can do this so far .. =S

>> I can do this so far .. =S

You forgot the last part ;)

You still haven't answered my question : do you know what prime numbers are ? Do you understand the assignment - what you're supposed to do ?

#include <iostream>
using namespace std;
 
int main() {
 
    for (int n = 3; n <= 100; n++) {
        // TO DO
    }
 
    return 0;
}

                                              

1:
2:
3:
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11:

I know what prime numbers are of course, but I can't do them in C++ programming, only math, lol.

Aaaah, okay, I know that I should print all the prime numbers from 3 to 100 in the first loop.  But I just don't know how.  I feel like crying.  Pfft.

Oh and by the way, I know that I was supposed to put return 0; but I didn't because the program isn't over yet hehe.  It's like I didn't even start.

>> Aaaah, okay, I know that I should print all the prime numbers from 3 to 100 in the first loop.

Indeed. Now, the loop goes from 3 to 100. If you put :

        cout << n << " ";

inside the loop, the code will print out all numbers from 3 to 100 (try it to see what I mean).

We don't want to print all numbers - we want to print ONLY the prime numbers. So, for each number, we'll have to find out whether it's a prime or not. If it's a prime, we print it, if it's not, we don't print it.

How can we know whether a number is a prime ? That's nicely explained in the assignment. Take a look at it, and try to add the code for that.

We divide by 2 to see if it's prime or not?
Umm the cout << n << " "; thing .. what do I put between "  " .. do Ihave to list the whole numbers for 3 to 100? =O
I don't get it yet ..

>> We divide by 2 to see if it's prime or not?

A prime is a number that is divisible only by itself and by 1. If it's divisible by any other value, then it's not a prime. You need to check whether the number n is divisible by any value between 2 and n-1. As it says in the assignment :

        "One way to see if number n is prime is to loop from 2 to n-1 and if any of these numbers evenly divides n,
          then n cannot be prime."


>> Umm the cout << n << " "; thing .. what do I put between "  "

Nothing. " " is just a space. Try putting it inside the for loop we had earlier, and compile it. Then run it, and see what happens. Writing a program is constantly adding code, then running it to see whether the added code does what it's supposed to do.


>> do Ihave to list the whole numbers for 3 to 100?

Not for the assignment no. For the assignment, you only need to list the prime numbers between 3 and 100.

#include<iostream>
using namespace std;
int main()
{
      int n, prime;
      
      for(int n = 3; n <=100 ; n++)
      {
            prime = 2;

You forgot the last part again ... When writing programs, it's best to be very strict. Otherwise, you'll make errors.


Can you tell me what you were trying to do in that code ?

Did you understand my last post ?

Do you understand the assignment ?

Regarding your problem with prime numbers, it's a typical homework problem and as such there are any number of tutorials and examples on the web! Try Googl'ing for "C++ Prime Numbers" and checking out a few of the links. At EE the experts cannot and will not provide you with full code solutions; however, Infinity08 is very good at maths related problems and is especially patient -- you are in good hands :)

>> I know that I was supposed to put return 0;

Actually, that's not strictly true, since (according to the standard) main() in C++ doesn't actually have to return anything (it just has to be defined to have a return type of int). It is; however, it is good practice to get into the habit of always returning a value form main. Usually, 0 is used to indicate success any other value conventionally means an error has ocurred.

"A return statement in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling exit with the return value as the argument. If control reaches the end of main without encountering a return statement, the effect is that of executing return 0;"

zainab_alkhamis, I haven't heard from you in a while now ... Are you still there ?