Python math.arctan() wrong result?

Question

I am using python shell as a calculator, but I don't know why it can't handle -28/96 as an input for atan function:

My question is: Why is this wrong?

Answer 1

-28/96 is -1 in python 2 (integer division is the default for integers)

Turn that to -28/96.0 you'll get the result you want.

(or switch to python 3)

BTW: 3.14 is a very bad approximation for Pi unless you like ovoïd circles :)

Use math.pi instead.

(I just saw that you wrote 28.0/96 the first time, which was correct! Your subconscient was aware of the bug :))

Answer 2

Python handles it correctly. Python interprets "-28/96" as int divides int, so it returns the largest previous integer of "-28/96", namely, -1. What you need is "-28.0/96" and then python 2.7 will interpret the expression as float division and handle it as your wish.

Also, the same expression will return a double precise float in python 3, namely, "-0.2916666666666667".

Answer 3

Looking at the result of -28/96:

>>> -28/92
-1
>>> math.atan(-1)
-0.7853981633974483

So python uses integer division for integers, not float division.

>>> math.atan(-28.7/96)
-0.2905008632137447

gives the correct result