How to evaluate the following prefix expression? Can you please help in evaluating following prefix expression? Thank you.
– ^ / + + 4 6 2 – 4 1 2 9
 A: The standard way to evaluate prefix expression is a stack. If you have not heard of it before, it is easy to understand anyways, as I will show.
We will process your expression – ^ / + + 4 6 2 – 4 1 2 9 in reverse order, while maintaining a list of numbers. Every time we see a number, we append it to the end of the list. If we see an operand (+ - * / ^), we will calculate the result using the last two numbers in our list, remove the two numbers, and append the result instead.

Below, I will process each token of your expression one by one, while keeping track of the list.

*

*Token = 9, append to list, list = [9]

*Token = 2, append to list, list = [9, 2]

*Token = 1, append to list, list = [9, 2, 1]

*Token = 4, append to list, list = [9, 2, 1, 4]

*Token = -, calculate result using $4$ and $1$, giving us $4 - 1 = 3$, and we append 3: list = [9, 2, 3]

*Token = 2, append to list, list = [9, 2, 3, 2]

*Token = 6, append to list, list = [9, 2, 3, 2, 6]

*Token = 4, append to list, list = [9, 2, 3, 2, 6, 4]

*Token = +, calculate 6 + 4, list = [9, 2, 3, 2, 10]

*Token = +, calculate 2 + 10, list = [9, 2, 3, 12]

*Token = /, calculate 12 / 3, list = [9, 2, 4]

*Token = ^, calculate 4 ^ 2, list = [9, 16]

*Token = -, calculate 16 - 9, list = [7]
As you can see, the result is 7. If at any point the list is empty (i.e. "adding one number"), then the expression is invalid.
Hope this helps.
A: I put in brackets everywhere first, starting with each operation and enclosing two arguments
   – ^ / + + 4 6 2 – 4 1 2 9 
= (– (^ (/ (+ (+ 4 6) 2) (– 4 1)) 2) 9)

Generally the arguments are single digit numbers, apart from where the minus sign appears; in this case the result of the minus operation is an argument.
Hopefully from here it should be straight-forward to see what to do. One way would be to convert everything into infix notation;
 (– (^ (/ (+ (+ 4 6) 2) (– 4 1)) 2) 9)
= (((4+6)+2)/(4-1))^2 - 9
= 7

Or you could go through and evaluate the prefix operations one by one, like Gareth Ma's answer.
