217: e to the pi Minus pi

Explain xkcd: It's 'cause you're dumb.
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e to the pi Minus pi
Also, I hear the 4th root of (9^2 + 19^2/22) is pi.
Title text: Also, I hear the 4th root of (9^2 + 19^2/22) is pi.


Computers use "floating point" numbers to store decimals. As noted in the comic, e^pi - pi is 19.999099979. However, Hat Guy's teammates' algorithms truncate to 3 decimal digits — giving a result of 19.999. Yet the programmers thought that 19.999 should come out to 20 unless they had errors in their algorithms (they did not; 19.999 would be the correct result). ACM is the Association for Computing Machinery; it sponsors the International Collegiate Programming Contest.

In the title text, another mathematical coincidence is presented. The 4th root of (9^2 + 19^2/22) is 3.1415926525, which is extremely close to pi (≈3.1415926535).


Cueball: Hey, check it out: e^pi-pi is 19.999099979. That's weird.
Black Hat: Yeah. That's how I got kicked out of the ACM in college.
Cueball: . . . what?
Black Hat: During a competition, I told the programmers on our team that e^pi-pi was a standard test of floating-point handlers--it would come out to 20 unless they had rounding errors.
Cueball: That's awful.
Black Hat: Yeah, they dug through half their algorithms looking for the bug before they figured it out.

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Asserting that the programmers' algorithms truncated to three decimal digits is an unsupported and unnecessary extrapolation. Most floating-point implementations use binary, not decimal, and 19.999099979 looks very much like a rounding error in binary floating-point that has accumulated over several operations. Daddy (talk) 12:39, 29 April 2013 (UTC)

Fixed. Xhfz (talk) 22:57, 16 August 2013 (UTC)

The third bullet-point above needs changing... (9^2+(19^2/22))=97.4090909091 which is close to pi to the fourth power, so it should be (as noted in the text) (9^2+(19^2/22))^1/4 Squirreltape (talk) 19:27, 25 February 2014 (UTC)

Actually, in-case you didn't notice, it says "∜(9² + 19²/22)", not just the sum on its own. I checked the sum on my calculator, and it is equal to what the page is saying. "∜(9² + 19²/22)" means "4th root of (9^2+19^2/22)" (What the title text is saying), or on Windows Calculator, "(9^2+19^2/22) yroot(4)" (Basically what the sum is saying). So, the 3rd bullet point is correct. --Katavschi (talk) 22:48, 23 April 2014 (UTC)

It says above that (π + 20)^i ≈ -i, but this should be (π + 20)^i ≈ -1. Proof: π + 20 ≈ e^π => (π + 20)^i ≈ (e^π)^i = e^(πi) = -1.

The ACM competitions are famous for being under tight time pressure. Making your own team waste time would absolutely get you kicked out (and make enemies) Mountain Hikes (talk) 04:40, 23 September 2015 (UTC)

"If they thought about the mathematics"

hm, are you saying it is obvious that e^ pi - pi is not 20? How would you know without approximating it? The sum of two irrationals is not necessarily irrational. 01:58, 26 October 2015 (UTC)

approximate e^pi using slightly bigger numbers than e and pi (say e: 2.7183 and pi: 3.1416) and subtract a value that is slightly smaller than pi (say 3.1415). The result is less than 20 and a upper limit for e^pi - pi 19:59, 22 August 2016 (UTC)