Programming-Idioms

Idiom

# 108 Determine if variable name is defined

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Lead paragraph

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.)

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Other implementations

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";
}

Demo

implicit none

print *,x

if (typeof x !== 'undefined') {
    console.log(x);
}

Demo
Origin

try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}

if x then print(x) end

$x = 'foo';

if (isset($x)) {
    echo $x;
}

{$if DECLARED(x)}
writeln(x);
{$endif}

use 5.012;

print $x if $x; # won't compile, see explanation

if 'x' in locals():
	print x

Doc
Origin

puts x if defined?(x)

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