Talk:Trimming (computer programming)

C/C++ example needs actual code

The C/C++ example isn't too illustrative of the theory behind trimming a string in C. Perhaps an example that contains the actual code would be more appropriate. (I want to see some loops in there!) --indil 20:55, 19 July 2006 (UTC)[reply]

I went for a brief overview and stuck with the simplest functions/libraries/implementations I could find. Please feel free to add some code :) —Pengo ^{talk · contribs} 13:02, 23 July 2006 (UTC)[reply]

I've changed the code for the C example. I believe the old example would have leaked memory - for processing large amounts of data, this could cause problems! On a side note, could someone check to make sure I haven't done something dumb while writing this (it compiled and worked in VC++) --Portej 07:29, 26 September 2007 (UTC)[reply]

in AWK better to use those command:

function ltrim(s) { sub(/^ */, "", s); return s }

function rtrim(s) { sub(/ *$/, "", s); return s }

function trim(s) { return rtrim(ltrim(s)); }

Keep the functions. Remove just code

Hi, I think you should remove the "other languages which don't have trim functions" section. They don't have trim or a close analogue, so they should be trimmed. It is superfluous text at the end of the article that happens to take up more than half the article that is supposed to be on trim rather than what is not trim. It is quite easy to define trim-like and, by omision what is not trim-like. That section becomes a tutorial on how to write trim in other languages and should be removed or moved to this talk page. Please refer to this.

Trim in other languages

In languages without a built-in trim function, a custom function may need to be written, or a library found.

AWK

AWK uses regular expressions to trim[1] :

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

or:

function ltrim(s) { sub(/^ */, "", s); return s }
function rtrim(s) { sub(/ *$/, "", s); return s }
function trim(s) { return rtrim(ltrim(s)); }

C/C++

There is no standard trim function in C or C++. The equivalent function has also often been called EatWhitespace in non-standard C libraries.

The open source C++ library Boost has several trim variants, including a standard one: [2]

trimmed = boost::algorithm::trim_copy(string);

Note that with boost's function named simply trim the input sequence is modified in-place[3], and does not return a result.

The Linux kernel also includes a strip function, strstrip(), since 2.6.18-rc1, which trims the string "in place".

The open source and portable C and C++ library The Better String Library has support for trimming as well:

btrimws (b = bfromcstr ("  string  "));

The following C code snippet can be used to remove white spaces from a string:

char* trimWhitespace(char* inString);
int isWhitespace(char c);

int 
isWhitespace(char c)
{
   return c == ' ' || c == '\t' || c == '\v' || c == '\f';
}

char* 
trimWhitespace(char* iS)
{
   char *ret = 0;
   int iSLength = 0, retLength = 0, sStart = 0, sEnd = 0;
   int i = 0, n = 0;

   iSLength = (int)strlen(iS);

   // Find first non-whitespace character
   while (sStart < iSLength && isWhitespace(iS[sStart]))
      sStart++;

   // Find last character before all whitespace
   // Starting point is the last character
   sEnd = iSLength - 1;
   while(sEnd > 0 && isWhitespace(iS[sEnd]))
      sEnd--;

   // Create a new string and allocate the memory
   retLength = (sEnd-sStart + 1) + 1; // Extra 1 is for the NULL terminator
   ret = (char*)malloc(retLength * sizeof(char));
   for (i = sStart; i <= sEnd; i++)
      ret[n++] = iS[i];
   ret[n] = `\0`;

   return ret;
}

Haskell

A trim algorithm in Haskell [4]

import Data.Char
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.

JavaScript

There is no built-in trim function, but it can be added to the String class [5]:

To add a trim function to all strings:

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

This allows the same syntax as Java to be used for JavaScript.

Perl

Perl has no built-in function, and a trimming is usually achieved through 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 from a string if present.

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: 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 has the function normalize-space(string) which strips leading and trailing whitespace and also replaces any sequence of whitespace characters (including linebreaks) with a single space.

Example:

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

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