Finding the number of digits on the iPhone Calculator

[TheKaneB]

Hi,
I'm trying to figure out the number of internal digits used in the iPhone standard Calculator App but I'm not sure I'm doing it right.
By calculating 1/3 and removing all the visible digits I start seeing weird results after the 20th digit or so. I guess it's an artifact caused by the binary representation? Is there a way to infer the exact number of binary digits used? I guess it's 64 bits but I want to be able to test my guess.

Any help appreciated!
Thanks!

[Albert Chan]

Try to find eps, such that eps + 1 - 1 = 0

For example, my ipad Sci:Pro Calc apps has 128 bits precision

Code:

2 ^ -127 + 1 - 1 =
5.877471754111438e-39

2 ^ -128 + 1 - 1 =
0

Update: it is safer to double check around found eps, since 1+eps is a half-way case.
3e-39 + 1 - 1 → 5.877471754111438e-39
2e-39 + 1 - 1 → 0

[TheKaneB]

nice trick, thank you!

Using this method it turns out it's 64 bit precision