CODEWITHSUNDEEP
c++
ldog
ldog

Reputation: 12151

Validity of checking exact value of preset double

I sometimes do the following in my code when I am being lazy:

double d = 0;
 ...
if( condition1 ) d = 1.0;

...
if (d == 0) { //not sure if this is valid
}

can I always count on the fact that if I set a double to be 0 I can later check in the code for it with d == 0 reliably?

I understand that if I do computations on d in the above code, I can never expect d to be exactly 0 but if I set it, I would think I can. All my testing so far on various systems seem to indicate that this is legitimate.

Upvotes: 1

Views: 231

Answers (2)

teambob
teambob

Reputation: 2084

I would avoid it. While the comparison would almost certainly work in the example you gave, it could fall apart unexpectedly when you use values that cannot be exactly expressed using a floating point variable. Where a value cannot be exactly expressed in a floating point variable some precision may be lost when copying from the coprocessor to memory, which would not match a value that stayed in the coprocessor the whole time.

My preference would be to add a range e.g. if (fabs(d)<0.0001) or keep the information in a boolean.

Edit: From wikipedia: "For example, the decimal number 0.1 is not representable in binary floating-point of any finite precision; the exact binary representation would have a "1100" sequence continuing endlessly:"

So if you had if (d==0.1) that has the potential to create a big mess - are you sure you will never have to change the code in this way?

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477512

Yes, for most cases (i.e. excluding NaN): After the assignment a = b; the condition a == b is true. So as long as you're only assigning and comparing to the thing you assign from, this should be reliable, even for floating-point types.

Upvotes: 4

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