Weird problem -- negative doubles not being processed by my code

I'm trying to write a very basic if() conditional statement, which in its ideal/simplest form, should look like this:

Code:

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double Lq[5] = {0.0};
double Lq_sign = 0.0;

/* call function to assign values to Lq[ ] array */

for(j = 0; j < 5; j++){
    Lq_sign = 0.0;

    if(Lq[j] >= 0.005) Lq_sign = 1.0;
    else if(fabs(Lq[j]) < 0.005) Lq_sign = 0.0;   
    else if(Lq[j] <= -0.005) Lq_sign = -1.0;

}

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When I print the resulting Lq_sign values for each iteration, the values of 1.0 and 0.0 are correctly assigned with no problems. However, the assignment of -1.0 never happens (the Lq values that *should* be -1.0 show up as 0.0).

I've done some troubleshooting, to determine why the negative Lq values aren't being recognized. Below are the various troubleshooting statements that I've performed:

Code:

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...
else if(fabs(Lq[j]) > 0.005) printf("abs val is > 0.005.\n"); /* A */
else if(Lq[j] < 0.0) printf("Lq[%d] is < 0.\n", j);  /* B */
else if((fabs(Lq[j]) > 0.005) && (Lq[j] < 0)) Lq_sign = -1.0; /* C */

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Statement A above always executes when it should, whether the Lq value in question is + or --. However, statement B *never* executes, although there are plenty of negative Lq values that should trigger this statement. And, of course, statement C never executes.

Does anyone have a suggestion about why my negative Lq[ ] array values aren't being recognized as negative?? Thanks in advance for your input.

Do you know what abs does? (Other than make me look ripped?) It makes a number not negative! Why would you ever reach B?

Code:

---

if number > 0.0005 then
    ...
else
if( FABS < 0.0005 )
...

---

The only thing you have not covered here is an exact match for 0.005. I think the only way you could possibly reach B is if you enter -0.0005. Have you tried that?


Quzah.

Aha! I hadn't been aware that calling fabs( ) was actually *changing* the value!

Here is my revised code, which now does work:

Code:

---

if(Lq[j] >= 0.005) Lq_sign = 1.0;
else if((Lq[j] > -0.005) && (Lq[j] < 0.005)) Lq_sign = 0.0;
else if(Lq[j] < 0) Lq_sign = -1.0;

---

Thanks for pointing that out, quzah.

I trust you also realize Lq_sign being a single variable will only ever reflect the sign of the last element in your Lq[] array?

Quote:

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I trust you also realize Lq_sign being a single variable will only ever reflect the sign of the last element in your Lq[] array?

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Yes, that is intentional, Tater, but thanks for pointing it out. I clear and reuse that variable each time through the loop.

Quote:

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Originally Posted by CodeKate

Aha! I hadn't been aware that calling fabs( ) was actually *changing* the value!

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What is it you think it actually does? If you give it -1.3, it returns 1.3. But it doesn't actually change what you pass it into something else:

Code:

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#include<math.h>
#include<stdio.h>
int main( void )
{
    float f = -1.3;
    if( fabs( f ) > 1.0 )
        printf( "f is still %f\n", f );

    return 0;
}

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It doesn't actually change what f is.


Quzah.

Quote:

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It doesn't actually change what f is.

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Ok, that was my initial belief about the action of calling fabs(). However, I still don't understand why my third condition in my OP wasn't executing:

Code:

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if(Lq[j] >= 0.005) Lq_sign = 1.0;     
else if(fabs(Lq[j]) < 0.005) Lq_sign = 0.0;         
else if(Lq[j] <= -0.005) Lq_sign = -1.0;

---

The negative values had absolute values greater than 0.005, and yet the 3rd condition never executed...

Quote:

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Originally Posted by CodeKate

Yes, that is intentional, Tater, but thanks for pointing it out. I clear and reuse that variable each time through the loop.

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You're original code sample did not depict any other processing than a rather tight loop scanning the array. With what you showed us, the only reasonable conclusion is that on exiting the loop Lq_sign is going to be set to whatever the last value in the array is...

I know I'm nit picking... but for clarity, it would help if you would indicate other processing, even as a comment, when posting code.

Quote:

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I know I'm nit picking... but for clarity, it would help if you would indicate other processing, even as a comment, when posting code.

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Sorry about that. I'll try to avoid such omissions in future posts. :)