Variables and Scoping¶
Until now, we have simply used variables without any explanation. Julia’s usage of variables closely resembles that of other dynamic languages, so we have hopefully gotten away with this liberty. In what follows, however, we address this oversight and provide details of how variables are used, declared, and scoped in Julia.
The scope of a variable is the region of code within which a variable
is visible. Variable scoping helps avoid variable naming conflicts. The
concept is intuitive: two functions can both have arguments called x
without the two x
‘s referring to the same thing. Similarly there are
many other cases where different blocks of code can use the same name
without referring to the same thing. The rules for when the same
variable name does or doesn’t refer to the same thing are called scope
rules; this section spells them out in detail.
Certain constructs in the language introduce scope blocks, which are regions of code that are eligible to be the scope of some set of variables. The scope of a variable cannot be an arbitrary set of source lines, but will always line up with one of these blocks. The constructs introducing such blocks are:
function
bodies (either syntax)while
loopsfor
loopstry
blockscatch
blockslet
blockstype
blocks.
Notably missing from this list are begin blocks, which do not introduce a new scope block.
Certain constructs introduce new variables into the current innermost scope. When a variable is introduced into a scope, it is also inherited by all inner scopes unless one of those inner scopes explicitly overrides it. These constructs which introduce new variables into the current scope are as follows:
- A declaration
local x
introduces a new local variable. - A declaration
global x
makesx
in the current scope and inner scopes refer to the global variable of that name. - A function’s arguments are introduced as new local variables into the function’s body scope.
- An assignment
x = y
introduces a new local variablex
only ifx
is neither declared global nor explicitly introduced as local by any enclosing scope, before or after the current line of code.
In the following example, there is only one x
assigned both inside
and outside a loop:
function foo(n)
x = 0
for i = 1:n
x = x + 1
end
x
end
julia> foo(10)
10
In the next example, the loop has a separate x
and the function
always returns zero:
function foo(n)
x = 0
for i = 1:n
local x
x = i
end
x
end
julia> foo(10)
0
In this example, an x
exists only inside the loop, and the function
encounters an undefined variable error on its last line (unless there is
a global variable x
):
function foo(n)
for i = 1:n
x = i
end
x
end
julia> foo(10)
in foo: x not defined
A variable that is not assigned to or otherwise introduced locally
defaults to global, so this function would return the value of the
global x
if there is such a variable, or produce an error if no such
global exists. As a consequence, the only way to assign to a global
variable inside a non-top-level scope is to explicitly declare the
variable as global within some scope, since otherwise the assignment
would introduce a new local rather than assigning to the global. This
rule works out well in practice, since the vast majority of variables
assigned inside functions are intended to be local variables, and using
global variables should be the exception rather than the rule,
especially assigning new values to them.
One last example shows that an outer assignment introducing x
need
not come before an inner usage:
function foo(n)
f = y -> n + x + y
x = 1
f(2)
end
julia> foo(10)
13
This last example may seem slightly odd for a normal variable, but allows for named functions — which are just normal variables holding function objects — to be used before they are defined. This allows functions to be defined in whatever order is intuitive and convenient, rather than forcing bottom up ordering or requiring forward declarations, both of which one typically sees in C programs. As an example, here is an inefficient, mutually recursive way to test if positive integers are even or odd:
even(n) = n == 0 ? true : odd(n-1)
odd(n) = n == 0 ? false : even(n-1)
julia> even(3)
false
julia> odd(3)
true
Julia provides built-in, efficient functions to test this called
iseven
and isodd
so the above definitions should only be taken
as examples.
Since functions can be used before they are defined, as long as they are defined by the time they are actually called, no syntax for forward declarations is necessary, and definitions can be ordered arbitrarily.
At the interactive prompt, variable scope works the same way as anywhere else. The prompt behaves as if there is scope block wrapped around everything you type, except that this scope block is identified with the global scope. This is especially apparent in the case of assignments:
julia> for i = 1:1; y = 10; end
julia> y
y not defined
julia> y = 0
0
julia> for i = 1:1; y = 10; end
julia> y
10
In the former case, y
only exists inside of the for
loop. In the
latter case, an outer y
has been introduced and so is inherited
within the loop. Due to the special identification of the prompt’s scope
block with the global scope, it is not necessary to declare global y
inside the loop. However, in code not entered into the interactive
prompt this declaration would be necessary in order to modify a global
variable.
The let
statement provides a different way to introduce variables.
Unlike assignments to local variables, let
statements allocate new
variable bindings each time they run. An assignment modifies an existing
value location, and let
creates new locations. This difference is
usually not important, and is only detectable in the case of variables
that outlive their scope via closures. The let
syntax accepts a
comma-separated series of assignments and variable names:
let var1 = value1, var2, var3 = value3
code
end
Unlike local variable assignments, the assignments do not occur in
order. Rather, all assignment right-hand sides are evaluated in the
scope outside the let
, then the let
variables are assigned
“simultaneously”. In this way, let
operates like a function call.
Indeed, the following code:
let a = b, c = d
body
end
is equivalent to ((a,c)->body)(b, d)
. Therefore it makes sense to
write something like let x = x
since the two x
variables are
distinct and have separate storage. Here is an example where the
behavior of let
is needed:
Fs = cell(2);
for i = 1:2
Fs[i] = ()->i
end
julia> Fs[1]()
2
julia> Fs[2]()
2
Here we create and store two closures that return variable i
.
However, it is always the same variable i
, so the two closures
behave identically. We can use let
to create a new binding for
i
:
Fs = cell(2);
for i = 1:2
let i = i
Fs[i] = ()->i
end
end
julia> Fs[1]()
1
julia> Fs[2]()
2
Since the begin
construct does not introduce a new block, it can be
useful to use the zero-argument let
to just introduce a new scope
block without creating any new bindings:
julia> begin
local x = 1
begin
local x = 2
end
x
end
syntax error: local x declared twice
julia> begin
local x = 1
let
local x = 2
end
x
end
1
The first example is illegal because you cannot declare the same
variable as local in the same scope twice. The second example is legal
since the let
introduces a new scope block, so the inner local x
is a different variable than the outer local x
.
Constants¶
A common use of variables is giving names to specific, unchanging
values. Such variables are only assigned once. This intent can be
conveyed to the compiler using the const
keyword:
const e = 2.71828182845904523536
const pi = 3.14159265358979323846
The const
declaration is allowed on both global and local variables,
but is especially useful for globals. It is difficult for the compiler
to optimize code involving global variables, since their values (or even
their types) might change at almost any time. If a global variable will
not change, adding a const
declaration solves this performance
problem.
Local constants are quite different. The compiler is able to determine automatically when a local variable is constant, so local constant declarations are not necessary for performance purposes.
Special top-level assignments, such as those performed by the
function
and type
keywords, are constant by default.
Note that const
only affects the variable binding; the variable may
be bound to a mutable object (such as an array), and that object may
still be modified.