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Math Error in the .NET Framework 1.1.4322 SP1

I have discovered a math error in the .NET framework's Log function. It
returns incorrect results for varying powers of 2 that depend on whether the
program is run from within the IDE or from the command line. The amount by
which the calculation is off is very small; even though the double data type
holds the errant value, it seems to round off when printed (via ToString())
and shows the correct one. The problem is that the errant value is still used
in further calculations, which can throw off some functions.

Specific example:

The function System.Math.Log(8,2) yields a value of 3 - 4x10^-16
(2.9999999999999996) instead of 3 when run from a command line (this will not
occur if run inside the IDE). You can store the result of this computation in
a double precision variable, but if you print the variable's value to the
console by calling its ToString() method, the output will be 3. You can
verify, however, that the value is indeed off by printing the result of some
calculations using it. To see this, use Visual Studio to create a new C#
console application and paste the following code sample in its "Main" method:

---- Begin Code Sample ---
// try log function using the overloaded method that specifies the base
Console.WriteLine("Using System.Math.Log overload to specify desired
base...");
double log = System.Math.Log(8,2); // compute the log of 8 to the
base of 2 (should be 3, but will be 3 - 4x10^-16 when run from a command
prompt – note will print out as 3 regardless)
double floor = System.Math.Floor(log); // get the floor of the result
(should be 3, but will be 2 when run from a command prompt)
Console.WriteLine("Log2 8 = " + log.ToString()); // print the log of 8 to
the base of 2
Console.WriteLine("Floor(Log2 8) = " + floor.ToString()); // print the
floor of the result
Console.WriteLine("Log2 8 == 3? : " + ((bool) (log == 3)).ToString()); //
print whether the log of 8 to the base of 2 is equal to 3 (should be true,
but will be false when run from a command prompt)

Console.WriteLine();

// try the log function while doing things the "manual" way
Console.WriteLine("Using Log x / Log y method of computing the same
logarithm...");
log = System.Math.Log(8) / System.Math.Log(2); // compute the log of 8 to
the base of 2 (will be 3)
floor = System.Math.Floor(log); // get the floor of the result (will
be 3)
Console.WriteLine("Log2 8 = " + log.ToString()); // print the log of 8 to
the base of 2 computed via Log x / Log y
Console.WriteLine("Floor(Log2 8) = " + floor.ToString()); // print the
floor of the result
Console.WriteLine("Log2 8 == 3? : " + ((bool) (log == 3)).ToString()); //
print whether the log of 8 to the base of 2 is equal to 3 (will be true)

Console.Read();
--- End Code Sample ---
Note that if you calculate via log x / log y then the calculation is
correct. Using ILDASM to examine mscorlib.dll reveals that this is what is
actually happening internally. That is, the overloaded method
System.Math.Log(double a,double newBase) actually calls
System.Math.Log(double d) and divides the result by another call to
System.Math.Log(double d). That there is a difference between the results
returned by the framework when it does this internally as opposed to when you
do it is certainly interesting.

You can see the actual value of the log function by putting a statement
such as "Console.Read()" as your first line so you have a way to get the
program to stop. Compile the program and run it from the command line. Open a
new instance of Visual Studio and click on Tools > Debug Processes and select
the application. Once you have attached the debugger, break the application
and start stepping through the code. Add the variable "log" to your watch
window and you will see its value is 3 - 4x10^-16, not 3 when computed using
System.Math.Log(8,2). These steps must be taken, as the problem will not
occur if you initially start the program within the IDE.

Additionally, the problem can be verified with one line of code. Create a
new C# console application and run the following code:

Console.WriteLine(System.Math.Log(8,2) == 3);

which will print “true” when run from within the IDE and “false” when run
from the command line.

The interesting thing here is that the issue occurs on some powers of 2 if
the program is run from the command line and different powers of two if run
from within the IDE. I have verified this by computing the first 64
logarithms of 2 from both IDE and command line. You can do so by running the
following code:

for(int pow = 1;pow <= 64;pow++)
{
if(System.Math.Log(System.Math.Pow(2,pow),2) != pow)
Console.WriteLine(pow.ToString());
}

Additionally, you can compute the same logarithms in VB.NET and the answers
will still be off. This rules out the problem being isolated to a single
language and shows the issue is within the framework itself. As an
interesting side check, I computed these logarithms from VB 6.0 and all
passed the test, so the problem is isolated to .NET.

Lastly, computation using log x / log y is also off, but for different
powers of 2 than what log(power,newBase) is and it is more consistent; it
computes the incorrect value for the same powers when run from either the
command line or the IDE.

I contacted Microsoft support regarding this and they verified that they
have reproduced the problem. The last correspondence with them said, “[I am]
creating a problem report to send to the dev team now”.

The most concerning thing about all of this is that programs, at least in
this instance, behave differently when run from the command line than they do
when run inside the IDE. I find the fact that the value of calculations is
different depending on how you start your application to be alarming.
Jul 21 '05 #1
1 2687
Hi

Your problem is the way .NET processes the ==

Try this

double x = 8;
double y = 2;
double log = System.Math.Log(x,y);

if( Convert.ToDecimal(log) == Convert.ToDecimal(3) )
{
Console.WriteLine("true");
}
else
{
Console.WriteLine("false");
}
/// Output is true
Daniel Roth
MCSD.NET

limelight <li*******@discussions.microsoft.com> wrote in message news:<A4**********************************@microso ft.com>...
I have discovered a math error in the .NET framework's Log function. It
returns incorrect results for varying powers of 2 that depend on whether the
program is run from within the IDE or from the command line. The amount by
which the calculation is off is very small; even though the double data type
holds the errant value, it seems to round off when printed (via ToString())
and shows the correct one. The problem is that the errant value is still used
in further calculations, which can throw off some functions.

Specific example:

The function System.Math.Log(8,2) yields a value of 3 - 4x10^-16
(2.9999999999999996) instead of 3 when run from a command line (this will not
occur if run inside the IDE). You can store the result of this computation in
a double precision variable, but if you print the variable's value to the
console by calling its ToString() method, the output will be 3. You can
verify, however, that the value is indeed off by printing the result of some
calculations using it. To see this, use Visual Studio to create a new C#
console application and paste the following code sample in its "Main" method:

---- Begin Code Sample ---
// try log function using the overloaded method that specifies the base
Console.WriteLine("Using System.Math.Log overload to specify desired
base...");
double log = System.Math.Log(8,2); // compute the log of 8 to the
base of 2 (should be 3, but will be 3 - 4x10^-16 when run from a command
prompt – note will print out as 3 regardless)
double floor = System.Math.Floor(log); // get the floor of the result
(should be 3, but will be 2 when run from a command prompt)
Console.WriteLine("Log2 8 = " + log.ToString()); // print the log of 8 to
the base of 2
Console.WriteLine("Floor(Log2 8) = " + floor.ToString()); // print the
floor of the result
Console.WriteLine("Log2 8 == 3? : " + ((bool) (log == 3)).ToString()); //
print whether the log of 8 to the base of 2 is equal to 3 (should be true,
but will be false when run from a command prompt)

Console.WriteLine();

// try the log function while doing things the "manual" way
Console.WriteLine("Using Log x / Log y method of computing the same
logarithm...");
log = System.Math.Log(8) / System.Math.Log(2); // compute the log of 8 to
the base of 2 (will be 3)
floor = System.Math.Floor(log); // get the floor of the result (will
be 3)
Console.WriteLine("Log2 8 = " + log.ToString()); // print the log of 8 to
the base of 2 computed via Log x / Log y
Console.WriteLine("Floor(Log2 8) = " + floor.ToString()); // print the
floor of the result
Console.WriteLine("Log2 8 == 3? : " + ((bool) (log == 3)).ToString()); //
print whether the log of 8 to the base of 2 is equal to 3 (will be true)

Console.Read();
--- End Code Sample ---
Note that if you calculate via log x / log y then the calculation is
correct. Using ILDASM to examine mscorlib.dll reveals that this is what is
actually happening internally. That is, the overloaded method
System.Math.Log(double a,double newBase) actually calls
System.Math.Log(double d) and divides the result by another call to
System.Math.Log(double d). That there is a difference between the results
returned by the framework when it does this internally as opposed to when you
do it is certainly interesting.

You can see the actual value of the log function by putting a statement
such as "Console.Read()" as your first line so you have a way to get the
program to stop. Compile the program and run it from the command line. Open a
new instance of Visual Studio and click on Tools > Debug Processes and select
the application. Once you have attached the debugger, break the application
and start stepping through the code. Add the variable "log" to your watch
window and you will see its value is 3 - 4x10^-16, not 3 when computed using
System.Math.Log(8,2). These steps must be taken, as the problem will not
occur if you initially start the program within the IDE.

Additionally, the problem can be verified with one line of code. Create a
new C# console application and run the following code:

Console.WriteLine(System.Math.Log(8,2) == 3);

which will print â€Å"true” when run from within the IDE and â€Å"false” when run
from the command line.

The interesting thing here is that the issue occurs on some powers of 2 if
the program is run from the command line and different powers of two if run
from within the IDE. I have verified this by computing the first 64
logarithms of 2 from both IDE and command line. You can do so by running the
following code:

for(int pow = 1;pow <= 64;pow++)
{
if(System.Math.Log(System.Math.Pow(2,pow),2) != pow)
Console.WriteLine(pow.ToString());
}

Additionally, you can compute the same logarithms in VB.NET and the answers
will still be off. This rules out the problem being isolated to a single
language and shows the issue is within the framework itself. As an
interesting side check, I computed these logarithms from VB 6.0 and all
passed the test, so the problem is isolated to .NET.

Lastly, computation using log x / log y is also off, but for different
powers of 2 than what log(power,newBase) is and it is more consistent; it
computes the incorrect value for the same powers when run from either the
command line or the IDE.

I contacted Microsoft support regarding this and they verified that they
have reproduced the problem. The last correspondence with them said, â€Å"[I am]
creating a problem report to send to the dev team now”.

The most concerning thing about all of this is that programs, at least in
this instance, behave differently when run from the command line than they do
when run inside the IDE. I find the fact that the value of calculations is
different depending on how you start your application to be alarming.

Jul 21 '05 #2

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