Bug Report

[Barış Bulut]

x,y = symbols('x,y')

y = x**3

y.subs(x,5)

125

x = 5

y.subs(x,x)

x**3

y.subs(x,10)

x**3

y.subs(x,3)

x**3

x = 3

y.subs(x,125)

x**125

y.subs(x,10)

x**10

[David Bailey]

Hi Barış,

It isn't a bug!

Unfortunately as soon as you execute x=5 you destroy the symbol in x, replacing it by 5 .

That means that y.subs(x,10)  becomes

(x**3).subs(5,3)

No replacements are possible, so this return x**3

Then you set x to 3 so that y.subs(x,125) becomes

(x**3).subs(3,125) which yields

x**125

etc.

The correct code would be

x=symbols('x')

y=x**3

y.subs(x,5)

y.subs(x,10)

etc.

Notice that y never needed to be a variable, because you never used it - y got overwritten just as x did.

If you want to store an expression like x**3 in a python variable, I find it useful to use a longer name that wouldn't normally be used in a mathematical expression. If you set up a variable as a symbol, then it is best not to assign to it later until perhaps you understand what is going on.

Cheers,

David

[Barış Bulut]

Hi David,

Thank you for your answer and suggestions. It was really helpful.

10 Şubat 2020 Pazartesi 02:12:34 UTC+3 tarihinde David Bailey yazdı: