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Programming-Idioms

  • C
  • Java

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.)

import static java.lang.System.out;
import java.lang.reflect.Field;
try {
    Class<?> c = getClass();
    Field f = c.getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
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";
}

The string is turned into its equivalent as an identifier using a mixin then we statically check if the code that displays the variable will be compiled. If so then write the variable for real.

In D this doesn't makes much sense since everything in this function is prepared at compile-time.
New implementation...
< > ⌨
programming-idioms.org 
try {
    Class<?> c = getClass();
    Field f = c.getDeclaredField("x");
    out.println(f.get(this));
} catch (NoSuchFieldException e) {

} catch (IllegalAccessException e) {

}
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);
implicit none

print *,x
if (typeof x !== 'undefined') {
    console.log(x);
}
try {
	console.log(x);
} catch (e) {
	if (!e instanceof ReferenceError) {
		throw e;
	}
}
if x then print(x) end
$x = 'foo';

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