Float arithmetics

[abdu adil]

Im writing a program which uses simple neural network perception algorithm, I have problem with doing negative float arithmetic, when I do this

summ = x1*w1+x2*w2-t

summ gets the value of -2.77555756156e-17

this happens only when x1 = 1.0 w1 = 0.2, x2 = 0.0 w2 = 0.1 t=0.2. If i do this in the python console everything is fine but in the program it gives me that strange value.

here is the full code http://tinypaste.com/1b8f9dd6 the calculation is done at the top, in the first function.

Btw Im using python 2.7, Im confused as hell

[abdu adil]

I found whats wrong, basically this is normal for float, what should I have done is to round the summ to precision 1, so round(summ,1) it will give me 0.0 in sted of that 'small' zero value

[bvdet]

This phenomenon is explained in the Python documentation on Floating Point Arithmetic.