Float - uint comparison sometimes succeeding?

Hello Community! First time poster here :)

I am debugging some code and am completely perplexed by a comparison between and integer and a float that sometimes evaluates true.

In the following code, a floating-point calculation is performed, typecast to a unsigned integer, which is then compared to another float. I expect that in all but the zero case, this evaluation would fail. And usually it does. But sometimes it evaluates true, and I am trying to understand what conditions lead to this.

float gA = 0.00356270161773046;

float gB = 0.00336178532868241;

[...]

float sA = 0.5 / gA;

float sB = 0.5 / gB;

[...]

const float PA = sA * gA;

if(sB == (uint16_t)(PA / gB))
    // Evaluates true.

In the above code, gA and gB are similar but different values, set elsewhere in the snipped portion, and by varying them slightly I can change the result of the evaluation. For example:

if gB = 0.00336178532868241 -> TRUE

if gB = 0.00332519446796205 -> FALSE

But I don't understand why it is ever evaluating true..!

This code was written by someone else - who is much more experienced than me - and I have little idea what they are trying to achieve through this typecast to uint16_t. Can anyone recognise what is being done here? Does this cast operation have the same (converting/rounding) function as it does in gcc etc, or is it a true typecast?

This code is compiled via C51 for a C8051 microcontroller application, in case that is relevant. I cannot debug or rebuild the code.

Thank you!

Parents Reply
  • Perhaps my terminology about typecasting is off.

    Terminology doesn't actually cover it.  Just as I guessed before, it's clear that your entire understanding of what a typecast actually is is wrong.

    And no, you won't gather such understanding from looking what particular compilers do in particular cases, regardless of whether they're GCC, C51 or something else.  The meaning of such things is not defined by implementors, but rather by the definition of the C programming language.  Which see.

    As to this compare sometimes yielding true: on what basis did you conclude that should never happen?  What exactly would prohibit "sB" from having an exact integer value which could compare equal to an uint16_t?

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