`subs` change the value of an expression

[newptcai]

The following code in REPL is bit strange

julia> n = symbols("n")
n

julia> ans = 4^n-3^n-n*3^(n-1)
   n    n - 1      n
- 3  - 3     ⋅n + 4 

julia> println(subs(ans, (n,4)))
67

julia> println(ans)
nothing

How can substitute n by 4 turns ans into nothing?

[newptcai]

This is different from this behaviour

julia> ex = (x-y)*(x+2y)
(x - y)⋅(x + 2⋅y)

julia> println(subs(ex, (x, 3)))
(3 - y)*(2*y + 3)

julia> println(subs(ex, (y, 3)))
(x - 3)*(x + 6)

julia> println(ex)
(x - y)*(x + 2*y)

[mzaffalon]

println returns nothing and the variable ans is bound to the last result printed to the REPL. You should remove the println statement to get what you want, or even better, assign the result to a variable with a different name.

[newptcai]

println returns nothing and the variable ans is bound to the last result printed to the REPL. You should remove the println statement to get what you want, or even better, assign the result to a variable with a different name.

:astonished: Oh the ans is a special variable! I didn't know that.