How to create an indexed variable in sympy?

Question

x,i,n = symbols("x i n")
summation(x,(i,1,n))

How do I make x indexed by i?

Answer 1

Without a numerical upper limit, it won't do anything, but otherwise you can use a function-like expression or an indexed variable:

>>> Sum(Indexed('x',i),(i,1,3))
Sum(x[i], (i, 1, 3))
>>> _.doit()
x[1] + x[2] + x[3]

>>> x = Function('x')
>>> Sum(x(i),(i,1,3)).doit()
x(1) + x(2) + x(3)

>>> Sum(x(i),(i,1,n)).doit()
Sum(x(i), (i, 1, n))