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ARopo wrote: in the finance sector
Then he should have also checked for parentheses and red ink.
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what I really admire about this is the somewhere it was called and tested like this
if (IsNegative(flVal))
instead of
if (flVal < 0)
though you could argue that the IsNegative function makes it clear to read at the calling end but then you could write the function like this
BOOL IsNegative(float flVal) { return (flVal < 0); }
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Doesn't this relate back to the discussion on floating point numbers and epsilon in another thread? My C is a bit rusty (and was never good to start off with: it is the language of Satan and has driven better men than me insane) but I have a feeling that if you just compare a float to 0, you may not get the answer you expect.
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Sometime you can get odd rounding errors when dealing with high precision, but sprintf a float into a buffer and you're more likely to introduce rounding errors, %f usually defaults to 6 decimial places
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No, most often it does not. There shouldn't be much discussion about the sign of a number. If you want to know whether it is negative, just use x < 0. That is quite different from the question "have we reached our target yet?", which does require some slack ("epsilon") to cope with rounding errors and other minor inaccuracies.
There is one exception: the result of some operation could be an extremely small number (say -1.E-500), which is mathematically negative, but can not be represented in a 64-bit floating-point number, and then gets stored as zero (or maybe "NotANumber").
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
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Reminds me of my computer science class in high school lol
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double A;
A = ...some calculations...
if (A == 1 || String.Compare(A.ToString(), "1") == 0)
At what point does A become a string and needs a string comparison 
I know the language. I've read a book. - _Madmatt
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Mark Nischalke wrote: At what point does A become a string and needs a string comparison
When you don't understand how floating point comparisons work. I guess.
Chris Meech
I am Canadian. [heard in a local bar]
In theory there is no difference between theory and practice. In practice there is. [Yogi Berra]
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While that code looks ludicrous and is hiding its probable intent very well, it does make some sense, although I would prefer to tackle the issue differently. He probably is looking for values of A that are either one, or pretty close to one, so close that the default numeric format just shows "1".
Here is a little test illustrating that:
double c=1;
double b=1;
for (int i=0; i<1000; i++) {
double a=c+b;
string s=a.ToString();
if (s=="1") {
log("i="+i+" b="+b+" a="+a+"="+a.ToString("E30")+" s="+s);
break;
}
b=b/2;
}
It generates this output:
i=48 b=3.5527136788005E-15 a=1=1.000000000000003600000000000000E+000 s=1
It is always risky to use equality tests on floating-point numbers. The normal approach would be something along these lines:
if ( Math.Abs(a-1) < epsilon ) ...
where epsilon would be a small positive constant, say 1.0E-15
[EDIT] fixed a little a/c confusion [/EDIT]
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).
modified on Thursday, May 13, 2010 10:50 AM
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Understandable. Not having context I know it's difficult to make a proper analysis. In other places there is this
if(A == 1 || A == 2 || A == 3)
which make the first example even more ludicrous. Why a double was being used for something that could only be an int is another mystery.
I know the language. I've read a book. - _Madmatt
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When I was learning BASIC way back in high school we only used reals (and strings), and we who noticed that putting a percent sign on a variable name (e.g. A%) would make it an integer wondered why anyone would want to use an integer when reals could do so much more... 
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Mark Nischalke wrote: if(A == 1 || A == 2 || A == 3)
Damn C# compiler. A clever C novice would write:
if(A == 1 || 2 || 3) ...
which C# does not like.
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).
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but COBOL does.
+1 COBOL.
you can also do multiple assignments :
set A to B,C,D
+2 COBOL.
which brings COBOL up to a score of -32.
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Great. What is keeping you from using it? there are a couple of .NET Cobol compilers. Fujitsu has one, ...
and CP could sure use some Cobol articles.
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).
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Luc Pattyn wrote: What is keeping you from using it?
who said i wanted to ?
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In Pascal, one could code:
If (A in [1,2,3]) Then
.. do something
The time and space efficiency of doing that varies considerably by compiler, though. Incidentally, Pascal has variables of type "Set of (whatever)" and includes operations to compute intersection, union, test for inclusion or subset, etc.
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Luc Pattyn wrote:
log("i="+i+" b="+b+" a="+a+"="+a.ToString("E30")+" s="+s);
It generates this output:
i=48 b=3.5527136788005E-15 c=1=1.000000000000003600000000000000E+000 s=1
That's clever, you put in the letter a and it prints out the letter c.
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Yeah, I switched a and c to better resemble the OP; and of course I forgot one instance.
I'll fix it!
BTW: you don't want to know what else my log() method is capable of...
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).
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Luc Pattyn wrote: where epsilon would be a small positive constant, say 1.0E-15
Or even Double.Epsilon - thats why it's there.
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I don't think so; MSDN says "The value of this constant is 4.94065645841247e-324.", as it is the smallest non-zero value a double can hold.
However what the OP probably needed is the smallest value you can add to one to make it different from one (that is about 1.E-15), or even a multiple thereof, as testing against the smallest possible value would be risky itself, as a lot of floating-point operations aren't accurate up to the last bit.
IMO they should not have called their value Epsilon.
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).
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Luc Pattyn wrote: where epsilon would be a small positive constant, say 1.0E-15
You can't reliably use a constant for comparisons. The value of epsilon has to be related to the magnitude of the numbers involved. For example (VC++ 6 compiler for reference)
double d1 = 1e299;
double d2 = 1e299 - 1;
double diff = d1 - d2;
gives a result with diff being zero, not one.
double d1 = 1e10;
double d2 = 1e10 - 1;
double diff = d1 - d;
gives a result with diff being 1, as expected.
Basically, epsilion has to be larger than the minimum precision of the double data type given the values contained in it, and the precision, as an absolute, varies with the values.
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that is elementary.
the example was about a value of 1, that is why 1.E-15 was appropriate.
In general, for a non-zero positive target value, one would typically use:
if (Math.Abs(currentValue-targetValue) < targetValue*epsilon) ...
Luc Pattyn [Forum Guidelines] [Why QA sucks] [My Articles]
I only read formatted code with indentation, so please use PRE tags for code snippets.
I'm not participating in frackin' Q&A, so if you want my opinion, ask away in a real forum (or on my profile page).
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It might be obvious to you, but the "absolute value of difference less than very small constant" idiom is very common, and leads to some really hard to find bugs.
I only mentioned it because your original post used the constant, and the more that gets published, the more the bug hets promuglated in code all over the place. You even see it in CS academic texts, which is really concerning.
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Luc Pattyn wrote: In general, for a non-zero positive target value, one would typically use:
The minimum amount of error one should accept would depend not on the target value, but upon the magnitude of the largest value from which something was subtracted to yield the target value. For example, when subtracting 281474976710655.1 from 281474976710656.1, the final result will be about 1.03125 which is much larger than a typical 'epsilon' one would expect for a number around one.
The real question, of course, should be how close does the number need to be to be identical from a 'practical' perspective. That's going to depend entirely on the application. If one is trying to produce a list of boards one needs to cut, and two boards differ by 0.1", that may or may not be a difference worthy of producing a separate line item. If boards have to fit within 0.05", the 0.1" difference would be significant. If the boards are going to have some mounting slop, even a 0.5" difference might be insignificant. Obviously the computer isn't going to magically know what's meaningful and what isn't.
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At the A.ToString() part. 
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