Evaluating logical AND expression

Hello friends,
I need one confirmation.
Please look at the following code:

Code:

---

#include <stdio.h>
#include <stdlib.h>

void sort (int[], int);

int main ( void )
{
        int i;
        int array[10];

        for (i = 0; i < 10; i++)
        {
                array[i] = rand() % 100;
                printf("%d ", array[i]);
        }
        putchar('\n');
        sort (array, 10);
        for (i = 0; i < 10; i++)
        {
                printf("%d ", array[i]);
        }

        return 0;
}

void sort (int array[], int len)
{
        int i, j, save_var;

        for (i = 1; i < len; i++)
        {
                save_var = array[i];
                j = i - 1;
                while (j >= 0 && array[j] > save_var)
                {
                        array[j + 1] = array[j];
                        j --;
                }
                array[j + 1] = save_var;
        }
}

---

please look at the sort function. At some point in program execution "j" will
become -1. In that case condition in while loop is false. However, since
in condition there is logical AND with other condition array[j] >save_var, at this
point this will become array[-1], and of course it would cause crash of the program.
I think that this doesn't matter because in C AND is short circuited and therefore
since j is -1 and j >= 0 is false, other part of condition will not be evaluated
and everything will work fine.
Am I right here?
Thanks

>Am I right here?
Yes.

Thanks Prelude, BTW when we can expect more good stuff (tutorials) from you?

The next tutorial should be my rewrite of splay trees, but it's somewhat stalled at the moment as I'm looking for a way to distinguish it from other resources. I'm also working on a few topics focusing on assembly language (my low level fever has kicked in again), but my free time is more often than not being spent doing other things, so I can't give you a time frame.

you are right, at some point of time j becomes -ve. But after j becomes -ve,it come back to the while loop to check the condition. since j>=0 , so while loop will not run. so there is no condition when array[-ve] will occure.

>at some point of time j becomes -ve
Okay, just for future reference, abbreviations like -ve will confuse people. If you use proper English, you'll be properly understood.

Yes, I saw -ve but I understand it like "minus something". Never mind. I have to say that I think it's not good practice to rely on the fact that C will short circuit AND expression. I had very rough time recently because I programmed a Siemens S7-300 PLC in SCL (language which is very similar to Pascal) that same algorithm and every time I tried to execute programm, CPU went to fault mode. It took me a while to understand that in SCL (and probably in Pascal) AND expression is not short circuited and can introduce a lot of unexpected behavior.
I solved problem using another temp variable:

Code:

---

temp = array[j];
/*....*/
while (j >= 0 && temp >save_var)
/*...*/

---

And in future, I'll always try to avoid short circuit conditions.

- Micko

>>abbreviations like -ve will confuse people

Too true, I had a lecturer earlier this year who used it a lot. From
what I gather, the notation corresponds to postive and negative
thus giving +ve and -ve

Quote:

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And in future, I'll always try to avoid short circuit conditions.

---

What problem? In this case the short-circuiting is what should be making it work without a workaround. array[j] will still be checked if j >= 0 evaluates as true.

>And in future, I'll always try to avoid short circuit conditions.
Why? Just because some other language uses full boolean evaluation doesn't mean you can't take advantage of short circuiting in C. That's just silly.

Quote:

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Originally Posted by Micko

I have to say that I think it's not good practice to rely on the fact that C will short circuit AND expression.

---

I'd say its practice is just fine in C, it's other languages that have issues.

But to each his own. Perhaps rewrite the loop in an equivalent manner.

Code:

---

      for ( j = i - 1; j >= 0; --j )
      {
        if ( array[j] <= save_var )
        {
            break;
        }
        array[j + 1] = array[j];
      }

---

But then, I'm not sure about these other languages.

Hehe, that's right. It's not problem in C, but I used to use it all time, and eventually without thinking I rewrote C code in Pascal like SCL and had a lot of problems, because there was a lot of other code and CPU goes to fault indicating error with description from which I couldn't conclude what's wrong. It took me about two hours to find out what is worng. Everything is OK when "thinking in C" but things get complicated if that logic is used "mechanicaly".

Thanks for replies.