Local Variables
The following program has two variables:
def double_this num |
num_times_2 = num*2 |
puts num.to_s+' doubled is '+num_times_2.to_s |
end |
|
double_this 44 |
The variables are num and
num_times_2.
They both sit inside the method double_this.
These (and all the variables you have seen
so far) are local variables. This means
they live inside the method, and they cannot leave.
If you try, you will get an error:
def double_this num |
num_times_2 = num*2 |
puts num.to_s+' doubled is '+num_times_2.to_s |
end |
double_this 44 |
puts num_times_2.to_s |
44 doubled is 88 |
example.rb:6:in `<main>': undefined local variable or method `num_times_2' |
for main:Object (NameError) |
Undefined local variable…. In fact, we did
define that local variable, but it isn’t local to where we tried
to use it; it’s local to the method, which means it’s local
to double_this.
This might seem inconvenient, but it’s actually quite nice.
Although it does mean you have no access to variables inside
methods, it also means they have no access to your
variables and thus can’t screw them up, as the following example shows:
tough_var = 'You can't even touch my variable!' |
|
def little_pest tough_var |
tough_var = nil |
puts 'HAHA! I ruined your variable!' |
end |
|
little_pest tough_var |
puts tough_var |
HAHA! I ruined your variable! |
You can't even touch my variable! |
In fact, two variables in that little program are
named tough_var: one
inside little_pest and one outside of
it. They don’t communicate. They aren’t related. They aren’t
even friends. When we called little_pest
tough_var, we really just passed the string from
one tough_var to the other (via the
method call, the only way they can even sort of communicate) so
that both were pointing to the same string.
Then little_pest pointed its
own local tough_var
to nil, but that did nothing to
the tough_var variable outside the
method.
