MOD function fails to give correct answer

Hi y'all.

I've just run into this problem. While trying to update a signal every 4 seconds, the following strange results showed up which prevented me from correct updates.

simu_step = 0.0100, t_current = 3.9700, timeinterval_agc = 4.0000, mod(t_current, timeinterval_agc) = 3.9700 ======================================================

simu_step = 0.0100, t_current = 3.9800, timeinterval_agc = 4.0000, mod(t_current, timeinterval_agc) = 3.9800 ==============================================================

simu_step = 0.0100, t_current = 3.9900, timeinterval_agc = 4.0000, mod(t_current, timeinterval_agc) = 3.9900 ==============================================================

simu_step = 0.0100, t_current = 4.0000, timeinterval_agc = 4.0000, mod(t_current, timeinterval_agc) = 4.0000 ==============================================================

simu_step = 0.0100, t_current = 4.0100, timeinterval_agc = 4.0000, mod(t_current, timeinterval_agc) = 0.0100 ==============================================================

simu_step = 0.0100,t_current = 4.0200, timeinterval_agc = 4.0000, mod(t_current, timeinterval_agc) = 0.0200 ==============================================================

As you can see, everything seems to be all right, except that mod(4, 4) = 4 and not 0. Though I am sure I can find a way to get around it, I still cannot understand why MATLAB would give such a strange answer.

Any ideas?

Thanks --Laurentiu

Answer 1

[Walter Roberson]

4.0000 does not mean "exactly 4": it means "between 3.99995 and 4.00005"

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F

[Walter Roberson]

Advice: do not increment t_current by 0.01 each time. Instead, increment a variable by 1 each time, and compute t_current as the variable / 100

[Laurentiu]

Thanks a lot, Walter. Will try your suggestion.

Funny thing about it is that the operation fails for t_current = 4, 8, 16, but is successful for t_current = 20. That's what actually made me think initially that my code was working, but surprised me when I went to check it in details.

Thanks a lot for your prompt answer.