Consider this program
float a = 0.7f;
if (a < 0.7)
{
Console.WriteLine("Less");
}
The output is Less. Why??
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2take a look: stackoverflow.com/questions/618535/…Ric– Ric2013-11-18 14:07:30 +00:00Commented Nov 18, 2013 at 14:07
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2the explanation is Very Long. you should read: for basic explanation: stackoverflow.com/questions/618535/… for full explanation: docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.htmlNahum– Nahum2013-11-18 14:07:57 +00:00Commented Nov 18, 2013 at 14:07
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1Probably because the binary floating point representation cannot exactly represent a decimal number like 0.7Baldrick– Baldrick2013-11-18 14:07:59 +00:00Commented Nov 18, 2013 at 14:07
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Not sure why the downvotes though. Would not have considered it a bad question. Just a duplicate...Baldrick– Baldrick2013-11-18 14:13:11 +00:00Commented Nov 18, 2013 at 14:13
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2 Answers
Because 0.7 does not have an exact representation as a float or a double: it is not an exact sum of negative powers of 2.
It happens that the closest representation of 0.7 as a is float approximately 0.69999998807907104492, while the closest double representation is 0.69999999999999995559. As you can see, double is slightly greater, which explains the behavior of your program.
Here is a small demo that you could run to see the values on your system:
printf("%20.20f %20.20f\n", 0.7, (float)0.7);
The takeaway lesson here is that you should not expect double and float representations of mathematically equal numbers to compare for equality correctly. Only a small subset of fractional numbers are representable in floating point system as exact numbers.
Since the overwhelming majority of fractions would be approximated, it is a good idea to do the comparisons with some level of tolerance. For example, instead of writing if (a == 0.7) you should write if (abs(a - 0.7) < 1E-8)
answered Nov 18, 2013 at 14:13
Sergey Kalinichenko
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You're unknowingly comparing apples and potatoes in your code.
float a = 0.7f; // "0.7f" is a float
if(a< 0 .7) // "0.7" is a double
{
Console.WriteLine("Less"); //You'll see it because of different representations
}
Your check will work as you expect if you match the number types:
float a = 0.7f;
if(a < 0.7f)
{
Console.WriteLine("Less"); // You won't see this
}
This is why numbers should never be hard-coded. Best way to fix your code:
float check = 0.7f;
float a = 0.7f;
if(a < check)
{
Console.WriteLine("Less"); // You won't see this either
}
