Question

I’m curious about an issue spotted in our team with a very large number:

var n:Number = 64336512942563914;
trace(n < Number.MAX_VALUE); // true
trace(n); // 64336512942563910

var a1:Number = n +4;
var a2:Number = a1 - n;
trace(a2); // 8  Expect to see 4
trace(n + 4 - n); // 8

var a3:Number = parseInt("64336512942563914");
trace(a3); // 64336512942563920

n++;
trace(n); //64336512942563910
trace(64336512942563914 == 64336512942563910); // true

What’s going on here?

Although n is large, it’s smaller than Number.MAX_VALUE, so why am I seeing such odd behaviour?

I thought that perhaps it was an issue with formatting large numbers when being trace‘d out, but that doesn’t explain n + 4 - n == 8

Is this some weird floating point number issue?

Answer 1

Yes, it is a floating point issue, but it is not a weird one. It is all expected behavior.

Number data type in AS3 is actually a “64-bit double-precision format as specified by the IEEE Standard for Binary Floating-Point Arithmetic (IEEE-754)” (source). Because the number you assigned to n has too many digits to fit into thos 64 bits, it gets rounded off, and that’s the reason for all the “weird” results.

If you need to do some exact big integer arithmetic, you will have to use a custom big integer class, e.g. this one.