Thread: c wont run my

Originally Posted by ajl913

hi forum
i've been trying to solve this problem:
[I]The following iterative sequence is defined for the set of positive integers:

n → n/2 (n is even)
n → 3n + 1 (n is odd)

This one intrigues me... I ran your code and here's a screen snip of what I got...

Attachment 10356

This is happening right at the end of your while (j <= i) and it's being caused by
accessing the array in seq += a[j]; although I don't see exactly why you're doing that.

Aren't you supposed to be stopping at j == 1?

Originally Posted by whiteflags

Except that's not the problem.

The problem with invoking undefined behavior is that you could "fix" something entirely unrelated, and get positive results. Commontater's access violation message is more typical of out of bounds array access.

What do we have in the program that looks like that?

Code:

 a[1]=1;
    for(i=2; i<1000000; i++){

The acceptable range for an array is from 0 to n-1, inclusive. In this case, n is one million.

In this case if he starts the *calculations* at 0 or 1 it just hangs.

As for the array... totally unnecessary... I elminated that from the program and got this...

Attachment 10357

Code:

#include "global.h"
#include <errorx.h>
#includ <windows.h>

int _cdecl main(int argc, char *argv[])
  { 

    unsigned int seq, maxseq=0, i, j, num=0;
    unsigned int ticks;     

    ticks= GetTickCount();

    for(i=2; i<1000000; i++){
        seq=0;
        j = i;
        while(j >= i){
            if(j & 1)
                j=(j * 3) + 1;
            else
                j /= 2;
            seq++;
       }
        if(seq>maxseq){
            maxseq=seq;
            num=i;
        }
    }
    ticks = GetTickCount() - ticks;
    printf("\nThe longest sequence results from %d, taking %d iterations\n",num,maxseq);
    printf("Runtime = %d milliseconds\n\n",ticks);
    return 0; }

Ok one more pass, this time altering the code to correctly bring j down to 1...

Attachment 10358

Code:

#include "global.h"
#include <errorx.h>
#include <windows.h>

int _cdecl main(int argc, char *argv[])
  { unsigned int ticks;     
    unsigned int seq, maxseq=0, i, num=0;
    register unsigned int j;

    ticks= GetTickCount();

    for(i=2; i<1000000; i++)
      { seq=0;
        j = i;
        while(j > 1)
          { if(j & 1)
              j=(j * 3) + 1;
            else
              j /= 2;
            seq++; }
        if(seq>maxseq){
            maxseq=seq;
            num=i;
        }
    }
    ticks = GetTickCount() - ticks;
    printf("\nThe longest sequence results from %d, taking %d iterations\n",num,maxseq);
    printf("Runtime = %d milliseconds\n\n",ticks);
    return 0; }

With j as a register variable it took nearly 200ms off the time.
Using & instead of % to check for odd numbers saved an additional 80ms.

Originally Posted by CommonTater

With j as a register variable it took nearly 200ms off the time.

That is surprising since conventional wisdom is that the register keyword is useless because modern compilers are able to determine very well what variables should take advantage of registers.

Originally Posted by laserlight

That is surprising since conventional wisdom is that the register keyword is useless because modern compilers are able to determine very well what variables should take advantage of registers.

I was surprised too... However, there may have been a secondary effect here... In PellesC before the register keyword is used, you have to enable other optimizations ("Increase Speed More", in the IDE Ox and Ot at the command line) I didn't check, but it's entirely possible the gain was more due to those flags than the keyword.

EDIT: I checked with the optimizations on then with and without the register keyword... about 12ms faster with the keyword, so apparently it does matter (a little bit) on PellesC. But your compiler's mileage may vary




That was fun... Thanks to the OP for giving me a challenge today!