Does anyone know why this code:
sum=0
def get_sum n
return sum if n<1
sum=sum+n
get_sum(n-1)
end
get_sum 10
gives me this?
rb:3:in `get_sum': undefined local variable or method `sum' for main:Object (NameError)
from 1.rb:8:in `<main>'
The code makes perfectly sense, and you can understand what it is doing.
Normal variables declared outside a function are not accessible inside the function.
You could prefix sum with $ to make it a global variable (not usually a good idea.)
$sum=0
def get_sum n
return $sum if n<1
$sum=$sum+n
get_sum(n-1)
end
get_sum 10
#= 55
If you have a real test case where you want to do this, I can suggest a suitable approach if you want.
Rather than using a global variable, another method using recursion would be like this (limited by stack depth, I guess):
def get_sum(n)
return n if n < 1
n + get_sum(n - 1)
end
sum = get_sum(10)
If you ever use a Ruby implementation that offers tail call optimization (I don't know of any implementations that do), ProGNOMmers's method would be a bit nicer on the stack, but as is a quick test has both exceeding the maximum stack level around n = 9000 or so. Not that you should be recursing 9000 times or anything.
As @Dogbert wrote, normal variables declared outside a function are not accessible inside the function.
This is an approach which doesn't use global variables (which are not suited for recursion):
def get_sum(n, sum = 0)
return sum if n<1
get_sum(n-1, sum+n)
end
get_sum(10) #=> 55
You never initialized the variable sum before using it.
You can totally avoid using that variable, since it does nothing:
def get_sum n
return 0 if n<1
get_sum(n-1) + n
end
get_sum 10
You might want to try this :
def get_sum n
return n if n == 0
return n + get_sum(n-1)
end
In this version you don't need to instanciate any global variable ( which is not a good idea ), and you actually perform a regressive sum :)
n==0 is not adequate. What will your code do if it receives -1? Use < or <=.