Why Does Floating-Point Multiplication Produce Different Results with Variables and Literals in Go?

Variable vs. Literal Floating-Point Multiplication in Go: A Precision Puzzle

In Go, comparing floating-point literals to their variable counterparts may yield surprising results. Consider the following code:

x := 10.1
fmt.Println("x*3.0 == 10.1*3.0:", x*3.0 == 10.1*3.0)

Although the multiplication operations are identical, the result is false. Why is there a discrepancy?

Constants and Precision

Floating-point constants and literals in Go possess unlimited precision. However, upon assignment to a typed variable, the value must conform to the type's constraints. In this case, x is assigned to a float64, which has limited precision.

Preserving Precision

When explicitly specifying a float literal, as in 10.1*3.0, the full precision is maintained before the operation is performed. In contrast, when assigning a float literal to a variable, precision is lost during conversion to the target type.

Implications

This behavior is by design and has implications for floating-point comparisons. When comparing a typed variable to its literal equivalent, precision differences may cause unexpected results. As documented in the Go blog post on constants, numerical constants exist in an unconstrained numeric space, but face limits when assigned to specific types.

Example

Consider the following constant declared in Go:

const Huge = 1e1000

While this constant can be used within expressions, it cannot be assigned to a float64 variable due to its excessive precision. Hence, the statement fmt.Println(Huge) will fail to compile.

Conclusion

The discrepancy in floating-point multiplication between literals and variables stems from the loss of precision during assignment to typed variables. Understanding this behavior is crucial for avoiding unexpected outcomes and ensuring accurate comparisons in Go floating-point operations.

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