Double rounding fails in some cases while evaluating Expressions
It seems that addition of doubles can produce values slightly different than they should to be.
Please take a look at attached workflow (simple_error.xml).
Condition that should be satisfied to escape the loop is: 1.7 > 1.5 + 0.1
However, loop is interrupted sooner, because of incorrect calculation of doubles. Value of "p" is set to: 1.6000000000000003
This, of course, makes it impossible to use doubles as check points for the loops.
However, it seems that casting to string and back works fine (take a look at second workflow - simple.xml)
Updated by Christopher Brooks almost 9 years ago
- Status changed from New to Closed
- Assignee changed from Derik Barseghian to Christopher Brooks
- % Done changed from 0 to 100
It is almost never a good idea to compare doubles without using an epsilon factor. Because of rounding, doubles are unlikely to precisely represent a value.
One workaround in the expression language is to use
which is defined as:
Return true if the first argument
is close to the second (within
EPSILON, where EPSILON is
a static public variable of this
Tokens also have an isCloseTo() method:
/** Test whether the value of this Token is close to the argument
* Token. The argument and this token are converted to
* equivalent types, and then compared. Generally, this is the
* higher of the type of this token and the argument type.
* Subclasses should implement the protected _isCloseTo() method
* to perform the correct type-specific operation.
* @see #isEqualTo
* @param rightArgument The token to test closeness of this token with.
* @param epsilon The value that we use to determine whether two
* tokens are close.
* @return A boolean token that contains the value true if the
* units of this token and the argument token are the same, and their
* values are close.
* @exception IllegalActionException If the argument token is not
* of a type that can be compared with this token, or the units
* are not the same.
public final BooleanToken isCloseTo(Token rightArgument, double epsilon)
See 13.4.1 Invoking Methods in http://ptolemy.eecs.berkeley.edu/books/Systems/chapters/Expressions.pdf