Golang浮点精度float32 vs float64

Question

I wrote a program to demonstrate floating point error in Go:

func main() {
    a := float64(0.2) 
    a += 0.1
    a -= 0.3
    var i int
    for i = 0; a < 1.0; i++ {
        a += a
    }
    fmt.Printf("After %d iterations, a = %e
", i, a)
}

It prints:

After 54 iterations, a = 1.000000e+00

This matches the behaviour of the same program written in C (using the double type)

However, if float32 is used instead, the program gets stuck in an infinite loop! If you modify the C program to use a float instead of a double, it prints

After 27 iterations, a = 1.600000e+00

Why doesn't the Go program have the same output as the C program when using float32?

Answer 1

Agree with ANisus, go is doing the right thing. Concerning C, I'm not convinced by his guess.

The C standard does not dictate, but most implementations of libc will convert the decimal representation to nearest float (at least to comply with IEEE-754 2008 or ISO 10967), so I don't think this is the most probable explanation.

There are several reasons why the C program behavior might differ... Especially, some intermediate computations might be performed with excess precision (double or long double).

The most probable thing I can think of, is if ever you wrote 0.1 instead of 0.1f in C.
In which case, you might have cause excess precision in initialization
(you sum float a+double 0.1 => the float is converted to double, then result is converted back to float)

If I emulate these operations

float32(float32(float32(0.2) + float64(0.1)) - float64(0.3))

Then I find something near 1.1920929e-8f

After 27 iterations, this sums to 1.6f