Output error when input to factorial is not a non-negative integer

[ghost]

The standard definition of factorial only deals with non-negative integers. If you decided to implement only it then you should give errors when the users tried to calculate factorial of real numbers. The current behavior is you rounded the real number to the nearest integer and returned the factorial of this integer instead. For example, pi! will be calculated as 4!, e! will be calculated as 3!. This is wrong. Either implement the extended definition of factorial for real numbers using Gamma function or simply spit out an error if the input is not a non-negative integer.

[sharkdp]

Either implement the extended definition of factorial for real numbers using Gamma function

we have that already: gamma(…).

or simply spit out an error if the input is not a non-negative integer.

That's probably a good idea, yes.

[sharkdp]

This is now fixed in Numbat. You can try it online here: https://numbat.dev/?q=%28-3%29%21