c++ returns 7.45058 instead of 0

Question

I'v ran into another anomaly:

float T = 19.0 / 99.0;
float moo = (99.0 * T) - 19.0;

as expected, T = 0.191919, however 'moo' is meant to be 0 but instead is 7.450586. I can only presume some form of casting is once again required somewhere.

Try to use `double` instead of `float`! Also a good read: [what every computer scientist should know about floating-point arithmetic](http://perso.ens-lyon.fr/jean-michel.muller/goldberg.pdf) — Rahul Tripathi, May 12 '15 at 12:10
I get 7.45058e-08, a very small number. You probably did, too. — molbdnilo, May 12 '15 at 12:14
Are you sure it's `7.450586`, and not `7.450586e-008` or some other really small exponent? — Benjamin Lindley, May 12 '15 at 12:14
I also get 7.45058e-08: https://ideone.com/lMJqDK but 0 if you use double instead of float: https://ideone.com/spuUn8 — drescherjm, May 12 '15 at 12:15

score 0 · Accepted Answer · answered May 12 '15 at 12:17

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Change your code to this and you will get 0 on moo

float T = 19.0f / 99.0f; float moo = (99.0f * T) - 19.0f;

Also the value on moo without this is not 7.45, it is 7.45058e-08 which is almost zero.

answered May 12 '15 at 12:17

exs

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I guess that is some kind of compiler optimization. On ideone I still get 7.45058e-08 – Vinz May 12 '15 at 12:26
I am using Apple LLVM 6.1 – exs May 12 '15 at 12:29

score 0 · Answer 2 · answered May 12 '15 at 12:22

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Since any floating point numbers are represented in 2's complement notation in c++.you will usually get approximate values , while manipulating floating point values .So 7.450586e-008 is exact behaviour . If you need more precision use double.

answered May 12 '15 at 12:22

mystic_coder

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c++ returns 7.45058 instead of 0

2 Answers2