if statement has no effect

This is a discussion on if statement has no effect within the C++ Programming forums, part of the General Programming Boards category; Here is the algorithm for an audio delay. I want a slight amount of noise to be added to fedback ...

  1. #1
    Budding Synth Programmer samGwilliam's Avatar
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    if statement has no effect

    Here is the algorithm for an audio delay. I want a slight amount of noise to be added to fedback signals.

    Code:
    void samDelay::processReplacing (float **inputs, float **outputs, long sampleFrames)
    {
        float *inLeft = inputs [chLeft];
        float *inRight = inputs [chRight];
    
        float *outLeft = outputs [chLeft];
        float *outRight = outputs [chRight];
    
        float fedBack [nChannels];
    
        while(--sampleFrames >= 0)
        {	
    		fedBack [chLeft] = xFade (bufRetrieve (chRight), bufRetrieve (chLeft), crossover) * feedback;
    		fedBack [chRight] = xFade (bufRetrieve (chLeft), bufRetrieve (chRight), crossover) * feedback;
    
    		if (fedBack [chLeft])
    			fedBack [chLeft] += noise (0.2f);
    
    		if (fedBack [chRight])
    			fedBack [chRight] += noise (0.2f);
    
    		// Output = input + output from delay lines * feedback:
    		*outLeft = (*inLeft++ + fedBack [chLeft]);
    		*outRight = (*inRight++ + fedBack [chRight]);
    
    		// Feed it back into the delay line:
    		bufInsert (chLeft, *outLeft++);
    		bufInsert (chRight, *outRight++);
    
    		bufMove ();		// Next indices.
    	}
    }
    So I use an if statement to make sure to only add noise if there is a signal, or the plugin would just constantly generate noise. But that's just what it does. It seems to completely ignore the if statement and just indiscriminately add noise.

    The delay lines are initialised to 0 at the start, so there's no possibility of any non-zero values causing noise to be added when not wanted.

    What could be the cause?

  2. #2
    C/C++ homeyg's Avatar
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    Well, I don't see 'chRight' or 'chLeft' declared anywhere in there, but it wouldn't compiled if that was the case.

    And another problem: can a float value be converted to a bool value? You are trying to make this conversion with 'fedBack[chRight]' and 'fedBack[chLeft]' in the if statements and I don't even think that is possible.

  3. #3
    Budding Synth Programmer samGwilliam's Avatar
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    It is possible - I tested it in a console application beforehand. Even if I explicitly state: if (fedBack [chLeft] != 0.0f); it still does the same thing.

    And this is just an excerpt - chLeft and chRight are enums of 0 and 1.

  4. #4
    C/C++ homeyg's Avatar
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    Another problem could be that the variable 'sampleFrames' is causing the array to go past its limits in the if test. It may be converting null characters to float (0), and in effect, causing the if statements to fail.

  5. #5
    Sweet
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    You should not check to see if a float is equal to zero it's not going to work. You are going to want to check if it's greater than or check if it's less than.
    Woop?

  6. #6
    and the hat of wrongness Salem's Avatar
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    A recent post on floating point comparisons.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  7. #7
    Budding Synth Programmer samGwilliam's Avatar
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    Quote Originally Posted by prog-bman
    You should not check to see if a float is equal to zero it's not going to work. You are going to want to check if it's greater than or check if it's less than.
    Tried it. I even gave it a bit of leeway with:

    Code:
    if ((fedBack [chLeft] > 0.01f) || (fedBack [chLeft] < -0.01f))
        fedBack [chLeft] += noise (0.2f);
    
    if ((fedBack [chRight] > 0.01f) || (fedBack [chRight] < -0.01f))
        fedBack [chRight] += noise (0.2f);
    It seems to work now, but that was strange. Cheers!

    Does anyone recommend making it more elegant with:

    Code:
    if (fabs (fedback [chLeft]) > 0.01f)
        fedBack [chLeft] += noise (0.2f);
    
    if (fabs (fedback [chRight]) > 0.01f)
        fedBack [chRight] += noise (0.2f);

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