Sympy Piecewise expression for even and odd numbers

[Dzhelil Rufat]

The objective is to implement a Piecewise expression that gives 0 when n is even, and 1 when n is odd. One way to do it is using the floor function like below:

    from sympy import *
    from sympy.abc import n
   
    f = Lambda((n,), Piecewise((0, Eq(n, floor(n / S(2)))),
                                            (1, Eq(n, floor(n / S(2))+1))))
   
    print(f(0))
    print(f(1))
    print(f(2))
    print(f(3))

However, this returns the wrong output:

    0
    1
    1
    Piecewise()

The correct output should be:

    0
    1
    0
    1

Another way to achieve the same is to use:

    from sympy import *
    from sympy.abc import n
   
    f = Lambda((n,), Piecewise((0, Eq((-1)**n, 1)),
                                            (1, Eq((-1)**n, -1))))
   
    print(f(0))
    print(f(1))
    print(f(2))
    print(f(3))

and this returns the correct output. Is there a way to achieve this using the floor function in the original code?

PS: Question also posted on http://stackoverflow.com/questions/33289928/sympy-piecewise-expression-for-even-and-odd-numbers

[Aaron Meurer]

I answered on StackOverflow. The short answer is, use Mod.

Aaron Meurer

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