Help me with my basic program, nothing I create will run

I have tried to attempt 10 challenges from a book I am reading (obviously a c coding book (= )
But each time i have tried one I fail... For some reason, to my knowledge I have done exactly what the book shows me to do. I have attempted to go over my latest attempt (Create a program that produces prime numbers). After several attempts my program actually compiles, but of course it doesn't do anything, except count and for some reason it makes my screen flicker. My first question is, what am I doing wrong? And second question is how can anyone stand going over a program over and over again and not seeing any result?
My code is as follows, it is very basic:

Code:

---


#include <stdio.h>

int[/COLOR] main(void)
 {
int test_number;                /* This variable shall be tested whether or not it is prime, although at this point it does not seem to want to */
int tester;               
 test_number = 3;
 tester = 2;

  while (1 == 1)
    {

          if ((test_number % tester == 0) && (test_number != tester))
                {
                  test_number = test_number + 1;
                    tester = 2;
                    continue;
                }
       
       
       
        else if ((test_number % tester != 0) && (test_number != tester))
                {
                    tester = tester + 1;
                    continue;
                }



        else
                {
                  printf("%d is prime.\n", test_number);
                    test_number = test_number + 1;
                    tester = tester + 1;
                    continue;
                }
    }

 return 0;

}

---

Also if anyone can recommend a tutorial that they know for certain is up to date and all code in it is correct PLEASE let me know, I have given up on trying to learn anything from the book I have and at this time I can't run out and get another one )=
I see there is a sticky post, but I'm wondering if you can recommend a tutorial that you started out on and didn't come across later.

Now I must get some sleep (= 4 a.m. here (=

Thanks for any help.

a. It kind of goes for ever, a while-true loop without a 'break'.
b. 1 == 1 evaluates to true or 1... so while(1) (restructure your program so it's not infinite!)
c. Add some debugging lines so you know whats happening, eg. while(1) { puts("Loop iteration"); ...

d. Do a rewrite, your logic and program design is horrid ;)

Quote:

---

Originally Posted by zacs7

a. It kind of goes for ever, a while-true loop without a 'break'.
b. 1 == 1 evaluates to true or 1... so while(1) (restructure your program so it's not infinite!)
c. Add some debugging lines so you know whats happening, eg. while(1) { puts("Loop iteration"); ...

d. Do a rewrite, your logic and program design is horrid ;)

---

The idea is that It is an infinite loop )= That way it will continue to search for prime numbers.
I attempted to add debugging lines, i placed printf functions every line to see what the program is doing, it goes through the loop until where it supposed to print the prime numbers, there it continues to repeat the error.
I would do a rewrite but I'm not going to get a different result, I don't know what I did wrong so I would assume I would continue the error again and again... Kinda like my loop there....
Thanks for your response =)

What is the error?

Your going to get overflow since it goes forever... So your signed int is going to loop around and around infinitly. For arguments sake, say the range of a signed int is -10,000 to 10,000. That means, on your 10,000th iteration, 10,000 + 1 = -10,000, and so on an so forth.

Writting your algorithm on paper, with a NS or FlowChart would probably benifit you. Your example shows your not lacking knowledge of C, but your logic & design is the problem. Bare in minde program flow is an important factor.

Quote:

---

Originally Posted by zacs7

What is the error?

Your going to get overflow since it goes forever... So your signed int is going to loop around and around infinitly. For arguments sake, say the range of a signed int is -10,000 to 10,000. That means, on your 10,000th iteration, 10,000 + 1 = -10,000, and so on an so forth.

Writting your algorithm on paper, with a NS or FlowChart would probably benifit you. Your example shows your not lacking knowledge of C, but your logic & design is the problem. Bare in minde program flow is an important factor.

---

That would be a problem wouldn't it... (=
So infinite loops shall be avoided from now on, okay, thanks (=
Not sure what you mean by flowchart though, I guess I must make a stop at google. I shall redesign it again. If you don't see a working version put on soon I probably went crazy (=

Thanks again for your help

As suggested by others the program design is definitely terrible.
And use a very large number as your limit (of course within bounds of the type you're defining)
and then execute your program instead of using an infinite loop.
An infinite loop will definitely be very irritating to debug if something's wrong.

Not to mention the infinite loop is most likely the source of the problem itself ;)

To answer you other question, many C books today are pretty much up to date. I have one that was written in 1994 and although the syntax it uses is pretty dated, it still works and runs as it should.

There are many tutorials on C on the internet. Google "C tutorial" and you will see what I mean. The only problem is these are quite limioted in explanation, and dont cover the important topics of the langauge in that much detail. Get a book as suggested on the sticky thread :)

Also some tutorials might be written by those not entirely 'qualified'. Consider using tutorials on this very website?

Thanks for your responses. I shall go buy an updated book, I'll get rid of my infinite loop (even though I favor them ): ) and make another attempt at making the program run. When I try to create it again is there anything I should not do (besides the loop like that)? And I'm open to any hints at designing better (=

Thanks again.

Infinite loops have their uses (although it should never really be infinite).

Consider more meaningful variable names ;)

Quote:

---

Originally Posted by zacs7

Infinite loops have their uses (although it should never really be infinite).

Consider more meaningful variable names ;)

---

Ok I shall do that (=
Curious what do you mean by infinite loops that aren't infinite, lol, maybe it has magical qualities :D

I managed to buy a book today (had to go 3 book stores to find a book on C, found lots of C++ though )=
The book I found is called C The complete Reference, 4th edition by Herbert Schildt. I noticed this book/author did not get very good reviews any read this book, is it really that bad?

Also if anyone has any advice on how I can make better programs let me know, I'm interested in any opinions.

Thanks again.

By "finite" infinite loops, he's talking about loops, whose condition are always true and run forever but where in the loop body is somewhere a condition that could break out of the loop.

Code:

---

while (1)
{
  // do some fancy stuff
  if (some_condition)
    break;
}

---

In a very simple case, the condition could of course be written inside the loop condition, but there are more advanced cases where you can't possibly write all the conditions in the loop condition or where you simply don't know if you should continue at that point in the code. A common example for such constructions are game engines, that infinitely display something on the screen (until the program exits).

Another method to write infinite loops that break at some point are:

Code:

---

int cond = 1;
while (cond)
{
  //do some stuff here
  if (some_other_condition)
    cond = 0;
}

---

Quote:

---

Herbert Schildt

---

Arrrrg!!! Run and hide from his bad practices! ;)

Quote:

---

Originally Posted by swgh

Arrrrg!!! Run and hide from his bad practices! ;)

---

Yeah Ive seen quite a bit of bad reviews for this author, to bad all I had to choose from was this or a book from the "For Dummies" line, I hope he's not as bad as everyone says he is.


Question about the infinite loop would it still be ok to use the infinite loop if I could guarantee that the variable would stay within its limits (of what it can contain), or if I was able to stop it at the point where that limit were to be reached?

Also can someone give me a hint at why the program doesnt run...I've looked but nothing jumps out at me )=

Thanks for the help

If you want the loop to break at a certain point, just place an if with a break.

Code:

---

int test = 1; /*1 is true hence infinite true while loop*/

while ( test )
{
  test++;

  if ( test > 10 )
  {
      break;
  }
}

printf("Loop broke as test reached 10\n");

---

Oh one more question, from the book I'm reading I get the following code:

Code:

---

int main(void)
{
  int x;
  x = 10;
if (x == 10) {
  int x; /* this x hides the outer x */

  x = 99;
  printf("inner x: %d\n", x);
}
  printf("outer x: %d\n", x);
return 0;
}

---

Does this mean that X takes the function replaces the x with a new value and then it returns to the previous value of x when the function is over with. Also does this mean that functions can affect other functions (without the second int x; x would have the same value as it would outside right?) So... I'm not sure if I asked that right and I think I confused myself writing that...

What do you want to actually do with the original block of code? Just search for prime numbers?

And btw, in your last example, a new x is created. To you, the programmer, it appears that you have two x's, but to the compiler, they are separate variables, with separate addresses. Local variables only last from their declaration until the ending brace of whatever block they are in.

Quote:

---

Originally Posted by swgh

If you want the loop to break at a certain point, just place an if with a break.

Code:

---

int test = 1; /*1 is true hence infinite true while loop*/

while ( test )
{
  test++;

  if ( test > 10 )
  {
      break;
  }
}

printf("Loop broke as test reached 10\n");

---

---

Okay I shall break out of all infinite loops. (= I suppose i can do that from now on.

Question: when my terminal fills up by a program, does my computer have to store all the information on the screen or does it need to just print it to the screen. I would immediatly think the computer must save all the info, but I let my computer run up for quite some time (with the previously seen loop) and the computer seemed to run as it did before, maybe at some point it deletes or replaces some of the old things it stored?

Quote:

---

Originally Posted by MacGyver

What do you want to actually do with the original block of code? Just search for prime numbers?

And btw, in your last example, a new x is created. To you, the programmer, it appears that you have two x's, but to the compiler, they are separate variables, with separate addresses. Local variables only last from their declaration until the ending brace of whatever block they are in.

---

I just want the program to run, and search for prime numbers yes, after I get through this problem I'll try something more advanced with it.

By not putting in the second variable, and throwing x somewhere in that function would the computer just assume to use the variable x from the last function, and why is that if so?

Quote:

---

Originally Posted by Ravens'sWrath

I just want the program to run, and search for prime numbers yes, after I get through this problem I'll try something more advanced with it.

---

Find a simple algorithm for finding prime numbers. Don't overcomplicate it.

Quote:

---

Originally Posted by Ravens'sWrath

By not putting in the second variable, and throwing x somewhere in that function would the computer just assume to use the variable x from the last function, and why is that if so?

---

Well, what should the compiler assume?

Code:

---

int main(void)
{
        int y;
        y = 5; /* there is only 1 y, so the compiler knows which one to use. */

        return 0;
}

---

Note the giant braces. When you say, y = 5, it looks for y inside the current braces. If there was a global variable also named y, then the compiler would still use the one inside main(). It assumes that you mean the most inner-declared variable.

It won't look inside other functions, though, for a variable. A valid variable is either inside the current set of braces, inside the current function, or declared globally. That's about it.

Quote:

---

Originally Posted by MacGyver

Find a simple algorithm for finding prime numbers. Don't overcomplicate it.



Well, what should the compiler assume?

Code:

---

int main(void)
{
        int y;
        y = 5; /* there is only 1 y, so the compiler knows which one to use. */

        return 0;
}

---

Note the giant braces. When you say, y = 5, it looks for y inside the current braces. If there was a global variable also named y, then the compiler would still use the one inside main(). It assumes that you mean the most inner-declared variable.

It won't look inside other functions, though, for a variable. A valid variable is either inside the current set of braces, inside the current function, or declared globally. That's about it.

---

Ok I can grasp that (=
I'm thinking searching for an existing algorithm for finding prime numbers would be cheating myself but if thats all theres left to do... (=

Thanks again,
-Ravens'Wrath

Of course, there are many examples of algorithms that find prime numbers. If you don't know how to search for prime numbers with a pencil and paper (and maybe a calculator), then you will need to use somebody else's algorithm.

You can't expect to program a computer to do something that you don't know how to do, yourself.

If you do know how to search for prime numbers by hand, then programming a computer to do it is an excellent challenge, and you'll learn a lot, as well.

I won't let you fail, so don't worry. :)

If you want to work with me on this, let me know, and we'll move along, smartly. There's been several good posts here, but they've been geared to a more C refinement to your code, rather than a C "let's solve this problem, learn a bit, and have some fun" kind of approach.

If you want to go with this, just jot down the steps you use to find a prime, by hand, in either plain English or pseudo code. We'll flush it out from there.

Adak

First, do you know what a prime number is? If not, I'll give a hint in terms of actual prime numbers: 2, 7, 23, and 31 are some small prime numbers.
If you do know what prime numbers are, do you know how to find out whether a number is prime or not, that meets the definition? If not, you should research it then play a little game of "factors" to practice it (it's a game I made up long ago).
If you know how to find prime numbers, then write a program that determines how to tell if a number is prime or not. As a treat, try writing a program where you can the game "factors" through merely inputting numbers and totaling up your score while you play.

I DO believe you're the kind of student I always wanted to sit next to during a difficult math test! :D

Now, why are you holding back on 1, 3, 11, 13, & 17?

He's a math geek, AND he's cagey - watch out for this guy! :)

OMG, warn people before you're posting such big images of yourself: http://www.ulillillia.us/aboutme/aboutmefavorites.html

I was only giving some examples of prime numbers. 1 is not prime though. It's not even composite either.

That link is to a very old file. This is the newer version, but still over a year old (unlike 2 or so years from that HTML version). Also, please stay on topic as well.

Been a way for one day and so many posts (=
Thanks for your time in trying to help. And to answer everyones question, yes I know what a prime number is :P Im just having trouble setting up a program that will find them...

I have made another attempt to try do this but I have had the same problem as before, I can get a program to compile but thats about it, the following is how I'm attempting to solve the problem:

Code:

---


/* program to find first 100 prime numbers */

#include <stdio.h>
int main(void)
{

int counter, test_n, divisor;
counter = 100;
test_n = divisor = 2;

while (counter)

{

        if (test_n == divisor)
                {
                printf("%d is prime", test_n);
                  counter = counter - 1;
                test_n = test_n + 1;
                divisor = 2;
                }
        else
                while (test_n % divisor)
                        {
                        divisor = divisor +1;
                        }
        if (test_n != divisor)
                {
                divisor = 2;
                test_n = test_n + 1;
                }
}


return 0;
}

---

My program seems to be nearly right... But still doesn't give me all of the first 100 primes (although it does seem to list only primes, haven't checked all though).

Any ideas why this isn't working...
by the way, my trash has 76 copies of a.out and they're all attempts at this silly program...

so any help would be greatly appreciated.

Basically, you are attempting to find the first 100 prime numbers by trial division, right? If so, the first part that you need to get right is: how to determine if an integer greater than 1 is prime using trial division?

Quote:

---

Originally Posted by laserlight

Basically, you are attempting to find the first 100 prime numbers by trial division, right? If so, the first part that you need to get right is: how to determine if an integer greater than 1 is prime using trial division?

---

Yes thats what I thought I had done, maybe I should go over my code to show what I think it should do...

Code:

---


while (counter) /* This should keep my program going as long as long as my counter is above zero (in other words until my program gets 100 primes */

{

        if (test_n == divisor) /* when my program gets to this point and finds the divisor and the number testing to be equal, the program shall reconginze this number as a prime */
                {
                printf("%d is prime", test_n);
                  counter = counter - 1; 
                test_n = test_n + 1;
                divisor = 2;
                }
        else
                while (test_n % divisor)  /* this should test whether or not the number can be a prime, if the while statement produces a value that is not true then the program should move on to the second part of the test.
                        {
                        divisor = divisor +1;
                        }
        if (test_n != divisor)  /* this will prove the number to be prime or not going on results from the while loop above. */
                {
                divisor = 2;
                test_n = test_n + 1;
                }
}

---

Is there something that I am over looking or am I simply doing something completely wrong here?

It looks like your algorithm almost works, except that you are off by one prime, because 3 is not listed as a prime. To see why 3 is left out, consider what happens when test_n = 2. First, it is printed, then test_n=3 and divisor=2. Now, test_n != divisor, so test_n=4 and divisior=2. On the next iteration, 3 has been skipped entirely.

This may be a more typical algorithm for trial division:

Code:

---

if number is 2
    number is prime
else if number is divisible by 2
    number is composite
else
    calculate square root of number
    loop over the odd integers starting from 3 until the square root calculated
        if number is divisible by current odd integer
            number is composite (can end loop)
    if loop ends without finding a divisor
        number is prime

---

Quote:

---

My program seems to be nearly right... But still doesn't give me all of the first 100 primes (although it does seem to list only primes, haven't checked all though).

Any ideas why this isn't working...
by the way, my trash has 76 copies of a.out and they're all attempts at this silly program...

so any help would be greatly appreciated.

---

I like your logic statements, but I dislike the way they're joined together. :) It's sort of like a magician. He's NOT actually doing magic, but it seems like he might be (or is), because of the artful way he misdirects your eye, so you can't see what he's really doing, even though he's right in front of you.

That's what you code does for me - it misdirects me just enough to miss some logic actions that we need.

I've tried to leave out some syntax items you'll need to add, and I haven't run this program yet, so expect some bugs.

Your previous program was the "I sat down at the keyboard and typed up some code w/o having worked out how to find prime numbers, step by step, by hand, first".

This program I wrote is the same - and dangerous because I don't work with square roots and prime's, and haven't written a program to do this in a VERY long time.

When you craft code on paper first, you get the quality product you're capable of. When you sit down and pound keys first, you get only as good as your previous knowledge extends, and little improvement.

Humans are very lazy and find efficient ways to do repetitive work with the tools they can get: Quicksort is a fine example.

Computers are very dumb, and will happily do a Bubblesort on a 10million records sized database, on a regular basis.

/* program to find first 100 prime numbers */

Code:

---

#include <stdio.h>
#include <math.h>  /* for square root function */
int main(void)
{

  int counter, test_n, divisor;
  counter = 100;
  divisor = 3;
  printf("\n\n")
  for (test_n = 1; test_n < 4; test_n++)
      printf("Our prime numbers are &#37;7d ", test_n)

  while (counter)

  {
     
      max = sqr(test_n)      /* this is the max number we have to test for primeness */
      while (divisor < max)  /* main inner working loop */
      {
            if (test_n % divisor == 0) {
                is_prime = 0  /* test_n is not prime */                     
                break
            }
            else 
                is_prime = 1  /* OK, maybe it is a prime after all */

            divisor = divisor +1;  /* this should be changed to improve the program. How'd you change it? */
      }
   
      if (is_prime) {  /* it's a prime number */
          counter--      /* print the prime and continue countdown to 100 primes  */
          printf("Our prime numbers are %7d ", test_n)
      }
           

      /* reset divisor and is_prime, increment test_n,  */
      divisor = 3
      test_n++ 
      is_prime = 0

  }



  return 0;
}

---