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Programming-Idioms

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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.)

implicit none

print *,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";
}
import std.stdio;
static if (is(typeof(x = x.init)))
    writeln(x);
if (typeof x !== 'undefined') {
    console.log(x);
}
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
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}
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


try:
    x
except NameError:
    print("does not exist")
if 'x' in locals():
	print(x)
puts x if defined?(x)
New implementation...
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programming-idioms.org 
implicit none

print *,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";
}
static if (is(typeof(x = x.init)))
    writeln(x);
if (typeof x !== 'undefined') {
    console.log(x);
}
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
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}
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


try:
    x
except NameError:
    print("does not exist")
if 'x' in locals():
	print(x)
puts x if defined?(x)