Empty string

Empty string is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Some languages have syntax or semantics for dealing specifically with empty strings.

The task is to:

• Demonstrate how to assign an empty string to a variable.
• Demonstrate how to check that a string is empty
• Demonstrate how to check that a string is not empty.

C

In C the strings are char pointers. A string terminates with null char '\0', which is not considered part of the string. Thus an empty string is "\0", while a null string is a null pointer which points to nothing. <lang C>/* to test a null string */ if (str) { ... } /* to test if string is empty */ if (str[0] == '\0') { ... } /* or equivelantly use strlen function */ if (strlen(str) == 0) { ... } /* or compare to a known empty string, same thing. "== 0" means strings are equal */ if (strcmp(str, "") == 0) { ... } </lang>

D

D treats null strings and empty strings as equal on the value level, but different on object level. You need to take this into account when checking for empty. <lang d>void main(){

   string s1 = null;
   string s2 = "";
   
   // the content is the same
   assert(!s1.length); 
   assert(!s2.length);
   assert(s1 == "" && s1 == null); 
   assert(s2 == "" && s2 == null);
   assert(s1 == s2);

   // but they don't point to the same memory region
   assert(s1 is null && s1 !is "");
   assert(s2 is "" && s2 !is null);
   assert(s1 !is s2);
   assert(s1.ptr == null);
   assert(*s2.ptr == '\0');
   
   assert(isEmpty(s1));    
   assert(isEmptyNotNull(s2));    

}

bool isEmpty(string s) {

   return !s.length;

}

bool isEmptyNotNull(string s) {

   return s is "";

}</lang>

J

<lang j> variable=:

  0=#variable

1

  0<#variable

0</lang>

Java

String.isEmpty() is part of Java 1.6. Other options for previous versions are noted. <lang java5>String s = ""; if(s != null && s.isEmpty()){//optionally, instead of "s.isEmpty()": "s.length() == 0" or "s.equals("")"

  System.out.println("s is empty");

}else{

  System.out.println("s is not empty");

}</lang>

Perl

In Perl, an empty string is often used to represent a false value. <lang Perl>$s = ""; if ($s) { ... } # false

1. to tell if a string is false because it's empty, or it's plain not there (undefined)

$s = undef; if (defined $s) { ... } # false; would be true on ""

1. though, perl implicitly converts between strings and numbers, so this is also false

$s = "0"; if ($s) { ... } # false; also false on "000", "0.0", "\x0", "0 with text", etc

1. but a string that converts to number 0 is not always false, though:

$s = "0 but true"; if ($s) { ... } # it's true! black magic!</lang>

Perl 6

In Perl 6 we can't just test a string for truth to determine if it has a value. The string "0" will test as false even though it has a value. Instead we must test for length.

<lang perl6>my $s = ; say 'String is empty' unless $s.chars; say 'String is not empty' if $s.chars;</lang>

Python

The empty string is printed by Python REPL as '', and is trated as boolean false, (as are most empty container types, by convention). <lang python>s = if not s:

   print('String s is empty.')

if s:

   print('String s is not empty.')</lang>

Tcl

The only special position that the empty string has in Tcl is that a great many commands return it, and the REPL of tclsh and wish doesn't print it. Otherwise, it is just like any other value. <lang tcl>set s "" if {$s eq ""} {puts "s contains an empty string"} if {$s ne ""} {phts "s contains a non-empty string"}</lang> There are other ways to check for emptiness and non-emptiness too (though the above are favored for reasons of simplicity, clarity and speed): <lang tcl>if {[string equal $s ""]} {puts "is empty"} if {[string length $s] == 0} {puts "is empty"} if {[string compare $s ""] != 0} {puts "is non-empty"}</lang>