Thread: post vs pre increment

why does my linux c program only like preincrement and not postincrement? is this a programmer error or a system dependent issue?

i can only get post increment to work in loops.
outside of loops it has been forcing me to use the preincrement.

i already have the program working with the preincrement.
It isn't causing compiler errors. It's more like when I run it switching the incrementation style, it just sits there and doesn't do a single thing. I'm assuming it might be logic error, but I do this in c++ and it work. i'm using slackware 9.1 and gcc 3.4.0

If you feel like helping me with another error I'm lost on how to fix. I'm trying to have the program print all even numbers from 2-50. in a format of 2=1+1. all even numbers must be a sum of two primes.

I have it working for every number but 2,4,6 and 10.
I hardcoded the answers, but this will not satisfy my teacher.
I just can not find why it will not work for those numbers.
I have traced this program 3 times and I still have no clue.

And is this in the incrememt part of the for loop?

Code:

for(r=0;r<primecountcp;r++) // And other loops like this one

Or are we talking about your use of increment in a more complex expression as in

Code:

primecp[++*primecountptr]=w;

If it's the latter then pre increment and post increment with a dereference mean something different. For post increment you need to use parentheses to force evaluation as you want it:

Code:

primecp[(*primecountptr)++]=w;

If it's the former then your compiler has issues. The two styles are equivalent.

both issues involve the pointer references.
that is good to know.
That fixed the number 10.
Now I'm having problems with the system recognizing the ones that would are 1+1=2 2+2=4
I can not get the program to notice 1+5=6 either.
This is making my brain melt.
thanks.

Originally posted by xviddivxoggmp3 Now I'm having problems with the system recognizing the ones that would are 1+1=2 2+2=4
I can not get the program to notice 1+5=6 either.
This is making my brain melt.
thanks. [/B]

What? None of those are valid C statements. You'll have to clarify your meaning. (IE: As people have said over and over, provide a sample of how you're trying to use it.) "1+5" is not an lvalue, and if that's what you're trying to do, your compiler will be telling you as much.

In reference to the above, provide a short example of exactly what you're doing.

Quzah.

output only should look like the following.

2=1+1
4=2+2
6=3+3
8=3+5
10=3+7
12=5+7
14=3+11
16=3+13
16=5+11
18=5+13
18=7+11
20=3+17
20=7+13
22=3+19
22=5+17
24=5+19
24=7+17
24=11+13

I have my loops testing for every occurance of the problem.
"Every even number must be represented as a sum of two primes"
I have written the code and debugged everthing except for 2,4, and 6.

My code below finds the primes and inputes it into the array.

Code:

void prime(int primecp[],int evencountcp,int *primecountptr)
{
  int w,b;/*loop control variables*/

  for(w=2;w<=evencountcp;w++)/*loop1 used to find the primes */
  {
    for(b=2;b<=w;b++)/*loop2 used to find the primes*/
    {
      if(w%b)/*modulation test for a possible prime*/
      {
        if(b==w-1)/*final test for a possible prime*/
        {
          primecp[(*primecountptr)++]=w;/*inserts prime into array*/
          break;/*breaks loop2*/
        }
        continue;/*continues loop2*/
      }
      else
      {
        break;/*breaks loop2*/
      }
    }
  }
}

my code below prints it.

Code:

void sum(int primecp[],int evencountcp,int primecountcp,int *countptr)
{
  int r,h;/*loop variables for the sum test*/

  if(primecountcp!=0)/*checks for an empty array*/
  {
    for(r=0;r<primecountcp;r++)/**/
      for(h=r+1;h<primecountcp;h++)
      {
        if(primecp[r]+primecp[h]==evencountcp)
        {  
          if(*countptr==0)
          {
            printf("2=1+1\n");
            printf("4=2+2\n");
            printf("6=3+3\n");
          }

          printf("%i=%i+%i\n",evencountcp,primecp[r],primecp[h]);

          (*countptr)++;

          if(!(*countptr%15))
          {
                printf("press any key to see more");
                getchar();
                system("clear");
          }
          }

          break;
        }
        else
        {
          continue;
        }
    }
  }
}

I'm trying to have every print as a result of my loops and not due to the red outlined printf statements. I'm missing something and have been unable to pen point it.

1 isn't prime therefore 2 cannot be written as the sum of two primes.

Looks like 2 is not being added to your list of primes.

isn't 1 devisable by 1 and itself(1)?
is that not the rule of prime?