Logo

Programming-Idioms

This language bar is your friend. Select your favorite languages!

Idiom #108 Determine if variable name is defined

Print the value of variable x, but only if x has been declared in this program.
This makes sense in some languages, not all of them. (Null values are not the point, rather the very existence of the variable.)

no strict 'vars';  # the default
print $x;  # compile error

print $x if $main::{x}; # prints $x if it was declared


use strict 'vars';
# or
use v5.11; # or higher

print $x if $main::{x};  # unavoidable compile error


import std.stdio;
static if (is(typeof(x = x.init)))
    writeln(x);
int x = 42;

void printIfDefined(alias name)()
{
    import std.stdio: writeln;
    static if( __traits(compiles, writeln(mixin(name))))
        writeln(mixin(name));
}

void main(string[] args)
{
    printIfDefined!"x";
    printIfDefined!"Foo.bar";
}
implicit none

print *,x
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
if (typeof x !== 'undefined') {
    console.log(x);
}
import static java.lang.System.out;
import java.lang.reflect.Field;
try {
    Field f = getClass().getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
if x then print(x) end
$x = 'foo';

if (isset($x)) {
    echo $x;
}
if(@$foo) print($foo);
{$if DECLARED(x)}
writeln(x);
{$endif}
try:
    x
except NameError:
    print("does not exist")
if 'x' in locals():
	print(x)
puts x if defined?(x)
New implementation...
< > ⌨
programming-idioms.org 
print $x;  # compile error

print $x if $main::{x}; # prints $x if it was declared


use strict 'vars';
# or
use v5.11; # or higher

print $x if $main::{x};  # unavoidable compile error


static if (is(typeof(x = x.init)))
    writeln(x);
int x = 42;

void printIfDefined(alias name)()
{
    import std.stdio: writeln;
    static if( __traits(compiles, writeln(mixin(name))))
        writeln(mixin(name));
}

void main(string[] args)
{
    printIfDefined!"x";
    printIfDefined!"Foo.bar";
}
implicit none

print *,x
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
if (typeof x !== 'undefined') {
    console.log(x);
}
try {
    Field f = getClass().getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
if x then print(x) end
$x = 'foo';

if (isset($x)) {
    echo $x;
}
if(@$foo) print($foo);
{$if DECLARED(x)}
writeln(x);
{$endif}
try:
    x
except NameError:
    print("does not exist")
if 'x' in locals():
	print(x)
puts x if defined?(x)