Trimming (computer programming)

In computer programming, trimming (trim) or stripping (strip) is a string manipulation in which leading and trailing whitespace is removed from a string.

For example, the string (enclosed by apostrophes)

'  this is a test  '

would be changed, after trimming, to

'this is a test'

Variants

Left or right trimming

The most popular variants of the trim function strip only the beginning or end of the string. Typically named ltrim and rtrim respectively, or in the case of Python: lstrip and rstrip. C# uses TrimStart and TrimEnd, and Common Lisp string-left-trim and string-right-trim. Pascal and Java do not have these variants built-in, although Object Pascal (Delphi) has TrimLeft and TrimRight functions.^[1]

Whitespace character list parameterization

Many trim functions have an optional parameter to specify a list of characters to trim, instead of the default whitespace characters. For example, PHP and Python allow this optional parameter, while Pascal and Java do not. With Common Lisp's string-trim function, the parameter (called character-bag) is required. The C++ Boost library defines space characters according to locale, as well as offering variants with a predicate parameter (a functor) to select which characters are trimmed.

Special empty string return value

An uncommon variant of trim returns a special result if no characters remain after the trim operation. For example, Apache Jakarta's StringUtils has a function called stripToNull which returns null in place of an empty string.

Space normalization

Space normalization is a related string manipulation where in addition to removing surrounding whitespace, any sequence of whitespace characters within the string is replaced with a single space. Space normalization is performed by the function named Trim() in spreadsheet applications (including Excel, Calc, Gnumeric, and Google Docs), and by the normalize-space() function in XSLT and XPath,

In-place trimming

While most algorithms return a new (trimmed) string, some alter the original string in-place. Notably, the Boost library allows either in-place trimming or a trimmed copy to be returned.

Definition of whitespace

The characters which are considered whitespace varies between programming languages and implementations. For example, C traditionally only counts space, tab, line feed, and carriage return characters, while languages which support Unicode typically include all Unicode space characters. Some implementations also include ASCII control codes (non-printing characters) along with whitespace characters.

Java's trim method considers ASCII spaces and control codes as whitespace, contrasting with the Java isWhitespace() method,^[2] which recognizes all Unicode space characters.

Delphi's Trim function considers characters U+0000 (NULL) through U+0020 (SPACE) to be whitespace.

Usage

Following are examples of trimming a string using several programming languages. All of the implementations shown return a new string and do not alter the original variable.

Other languages

In languages without a built-in trim function, it is usually simple to create a custom function which accomplishes the same task.

AWK

In AWK, one can use regular expressions to trim:

 ltrim(v) = gsub(/^[ \t]+/, "", v)
 rtrim(v) = gsub(/[ \t]+$/, "", v)
 trim(v)  = ltrim(v); rtrim(v)

or:

 function ltrim(s) { sub(/^[ \t]+/, "", s); return s }
 function rtrim(s) { sub(/[ \t]+$/, "", s); return s }
 function trim(s)  { return rtrim(ltrim(s)); }

C/C++

There is no standard trim function in C or C++. Most of the available string libraries^[4] for C contain code which implements trimming, or functions that significantly ease an efficient implementation. The function has also often been called EatWhitespace in some non-standard C libraries.

In C, programmers often combine a ltrim and rtrim to implement trim:

#include <string.h>
#include <ctype.h>

void rtrim(char *str)
{
  char *s;
  s = str + strlen(str);
  while (--s >= str) {
    if (!isspace(*s)) break;
    *s = 0;
  }
}

void ltrim(char *str)
{
  size_t n;
  n = 0;
  while (str[n] != '\0' && isspace((unsigned char)str[n])) {
    n++;
  }
  memmove(str, str + n, strlen(str) - n + 1);
}

void trim(char *str)
{
  rtrim(str);
  ltrim(str);
}

The open source C++ library Boost has several trim variants, including a standard one:^[5]

#include <boost/algorithm/string/trim.hpp>
trimmed = boost::algorithm::trim_copy("string");

Note that with boost's function named simply trim the input sequence is modified in-place, and does not return a result.

Another open source C++ library Qt has several trim variants, including a standard one:^[6]

#include <QString>
trimmed = s.trimmed();

The Linux kernel also includes a strip function, strstrip(), since 2.6.18-rc1, which trims the string "in place". Since 2.6.33-rc1, the kernel uses strim() instead of strstrip() to avoid false warnings.^[7]

Haskell

A trim algorithm in Haskell:

 import Data.Char (isSpace)
 trim      :: String -> String
 trim      = f . f
    where f = reverse . dropWhile isSpace

may be interpreted as follows: f drops the preceding whitespace, and reverses the string. f is then again applied to its own output. Note that the type signature (the second line) is optional.

J

The trim algorithm in J is a functional description:

     trim =. #~ [: (+./\ *. +./\.) ' '&~:

That is: filter (#~) for non-space characters (' '&~:) between leading (+./\) and (*.) trailing (+./\.) spaces.

JavaScript

There is a built-in trim function in JavaScript 1.8.1 (Firefox 3.5 and later), and the ECMAScript 5 standard. In earlier versions it can be added to the String object's prototype as follows:

String.prototype.trim = function() {
  return this.replace(/^\s+/g, "").replace(/\s+$/g, "");
};

Perl

Perl 5 has no built-in trim function. However, the functionality is commonly achieved using regular expressions.

Example:

$string =~ s/^\s+//;            # remove leading whitespace
$string =~ s/\s+$//;            # remove trailing whitespace

or:

$string =~ s/^\s+|\s+$//g ;     # remove both leading and trailing whitespace

These examples modify the value of the original variable $string.

Also available for Perl is StripLTSpace in String::Strip from CPAN.

There are, however, two functions that are commonly used to strip whitespace from the end of strings, chomp and chop:

• chop removes the last character from a string and returns it.
• chomp removes the trailing newline character(s) from a string if present. (What constitutes a newline is $INPUT_RECORD_SEPARATOR dependent).

In Perl 6, the upcoming major revision of the language, strings have a trim method.

Example:

$string = $string.trim;     # remove leading and trailing whitespace
$string .= trim;            # same thing

Tcl

The Tcl string command has three relevant subcommands: trim, trimright and trimleft. For each of those commands, an additional argument may be specified: a string that represents a set of characters to remove—the default is whitespace (space, tab, newline, carriage return).

Example of trimming vowels:

set string onomatopoeia
set trimmed [string trim $string aeiou]         ;# result is nomatop
set r_trimmed [string trimright $string aeiou]  ;# result is onomatop
set l_trimmed [string trimleft $string aeiou]   ;# result is nomatopoeia

XSLT

XSLT includes the function normalize-space(string) which strips leading and trailing whitespace, in addition to replacing any whitespace sequence (including line breaks) with a single space.

Example:

<xsl:variable name='trimmed'>
   <xsl:value-of select='normalize-space(string)'/>
</xsl:variable>

XSLT 2.0 includes regular expressions, providing another mechanism to perform string trimming.

Another XSLT technique for trimming is to utilize the XPath 2.0 substring() function.

See also

• Comparison of programming languages (string functions)

References

External links

• Tcl: string trim
• Faster JavaScript Trim - compares various JavaScript trim implementations
• php string cut and trimming- php string cut and trimming