## lua "for" loop broken

Case number: | 845813-991146 |

Topic: | Game: Tools |

Opened by: | Rav3n_pl |

Status: | Open |

Type: | Bug |

Opened on: | Friday, November 25, 2011 - 22:22 |

Last modified: | Saturday, November 26, 2011 - 08:53 |

code:

for i=1,10,1 do print(i) end

output: (...)7, 8, 9, 10

for i=0.1,1,0.1 do print(i) end

output: (...)0.7, 0.8, 0.9, 1

for i=0.1,1,0.01 do print(i) end

output: (...)0.97, 0.98, 0.99 !!!! NO 1 !!!

When step is 0.08 or lower it is not doing "final" count.

Sat, 11/26/2011 - 08:53

#2

(...)

Thats why in some langs "for" can be uses only on int variables... :)

This isn't actually a bug in either lua or Foldit - it has to do with how real numbers are represented in computers. (Google "What Every Computer Scientist Should Know About Floating-Point Arithmetic")

The short answer is that just like 1/3 and 1/7 can't be exactly represented as a finite-length decimal, 1/10 can't be represented as a finite length value in the binary representation that computers use. When you write "0.01" in a computer program, the computer converts this to a binary number that is almost-but-not-quite equal to 0.01. (It also "helpfully" converts the almost-but-not-quite value back to the "simple" decimal representation when printing.) So when you add them together multiple times, the errors accumulate, and you don't actually hit 1.0 on the last time, but rather a number slightly higher, which then doesn't get printed.

You can test this by using a value that can be exactly converted to binary, e.g. 1/64 = 0.015625

for i=0,1,0.015625 do print(i) end

output: (...) 0.984375, 1

The work around is to stick with iterating over integers, which can be exactly converted to binary, and then do the division afterward. That way you don't get the accumulation of small errors:

for i=10,100,1 do print(i/100) end

output: (...) 0.97, 0.98, 0.99, 1