Updated 2015-01-28 23:58:43 by aspect

split ,a built-in Tcl command, splits a string into a list

Synopsis  edit

split string ?splitChars?

Documentation  edit

official reference

Description  edit

SplitChars defaults to the standard white-space characters.

The result of split is a list of substrings of string that are delimitied by splitChars, which is a list of characters. If adjacent characters in string are also in splitChars, the result will the empty substring between those adjacent characters. If the first character of string is in splitChars, the result will include the empty substring before the first character, and if the last character of string is in splitchars, the result will include the empty string after the last character.

See also  edit

Additional string functions
Arts and crafts of Tcl-Tk programming
provides string delimit, which splits strings on string match-style patterns or regexp-style patterns
Splitting strings with embedded strings
Splitting a string on arbitrary substrings
Counting characters in a string
where split was pretty good...
to split as Tcl would split a command into words prior to evaluation.

Examples  edit

Splitting by dots

split comp.unix.misc .
comp unix misc

Splitting into characters

split {Hello world} {}
H e l l o { } w o r l d

Splitting on the empty string is an optimized case, and is an efficient operation.

Splitting by whitespace: the pitfalls

split { abc def  ghi}
{} abc def {} ghi

Usually, if you are splitting by whitespace and do not want those blank fields, you are better off doing:
regexp -all -inline {\S+} { abc def  ghi}
abc def ghi

Defininition of White-Space Characters  edit

ulis: where in the doc are defined the standard white-space characters?

DKF: I believe there's a standard (ANSI? POSIX?) somewhere. But the answer includes "space", "tab", and "newline".

escargo: By "tab" do you mean both horizontal tab (ASCII 9) and vertical tab (ASCII 11)? (See http://www.asciitable.com/) Arguments could be made for most of the ASCII characters under 33.

Strick: Let's ask Tcl what it thinks are white:
$ env | grep en_
$ cat what-chars-does-split-think-are-white.tcl
for {set i 0} {$i<65536} {incr i} {
  if {[llength [format "/%c/" $i]] > 1} { puts -nonewline "$i " }
$ tclsh what-chars-does-split-think-are-white.tcl
9 10 11 12 13 32 $

escargo 2005-04-01 :

9 = ASCII TAB, 10 = ASCII LF (line feed), 11 = ASCII VT (vertical tab), 12 = ASCII FF (form feed), 13 = ASCII CR (carriage return), and of course 32 = ASCII Space.

I would have thought that the separator characters would count as white space (28-31, FS, GS, RS, US), but I guess they are regarded as "nonprinting" characters.

DKF: I actually mean "what does isspace() think is whitespace". :^)

Strick: Oops, i forgot to actually use split in my script above. So now I test four different notions of white, and get three different answers. I understand why Tcl's builtin list-splitting rules must be fixed, regardless of locale. But it seems 'split' should use the list-splitting rule or the the string is space rule, but it uses its own (pre-unicode?) rule:
$ cat what-chars-does-split-think-are-white.tcl
puts "tcl=[info patch] LANG=$env(LANG)"

puts -nonewline {according to llength: }
for {set i 0} {$i<65536} {incr i} {
  if {[llength [format "/%c/" $i]] > 1} { puts -nonewline "$i " }
puts {} 

puts -nonewline {according to split: }
for {set i 0} {$i < 65536} {incr i} {
  if {[llength [split [format /%c/ $i]]] > 1} { puts -nonewline "$i " }
puts {} 

puts -nonewline {according to 'string is space': }
for {set i 0} {$i < 65536} {incr i} {
  if {[string is space [format %c $i]]} { puts -nonewline "$i " }
puts {} 

puts -nonewline {according to regexp {\s}: }
for {set i 0} {$i < 65536} {incr i} {
  if {[regexp {\s} [format "%c" $i]]} { puts -nonewline "$i " }
puts {}

$ tclsh what-chars-does-split-think-are-white.tcl
tcl=8.4.7 LANG=en_US.UTF-8
according to llength: 9 10 11 12 13 32
according to split: 9 10 13 32
according to 'string is space': 9 10 11 12 13 32 160 5760 8192 8193 8194 8195 8196 8197 8198 8199 8200 8201 8202 8203 8232 8233 8239 12288
according to regexp {\s}: 9 10 11 12 13 32 160 5760 8192 8193 8194 8195 8196 8197 8198 8199 8200 8201 8202 8203 8232 8233 8239 12288

escargo 2006-01-27: If split used chars 9 10 11 12 13 32 then there would be only two sets, with the smaller set as a proper subset of the larger set. The two characters that would have to be added are the vertical tab and form feed.

Splitting on Substrings  edit

splitChars is a series of 0 to n individual characters. However, if you want to split on a specific sequence of 2 or more characters together, or if you want to split on a regular expression, split will not work for you. See Tcllib's textutil::splitx, or ycl::string::delimit for that functionality.

SS 2004-01-31: Or you can use the following function:
proc wsplit {string sep} {
    set first [string first $sep $string]
    if {$first == -1} {
        return [list $string]
    } else {
        set l [string length $sep]
        set left [string range $string 0 [expr {$first-1}]]
        set right [string range $string [expr {$first+$l}] end]
        return [concat [list $left] [wsplit $right $sep]]

This version is recursive, so it may be better to rewrite it if you plan to use the function against very long strings with many separators. The difference between wsplit and splitx is that splitx uses regexp, so it may create problems with unknown separators.

IL 2005-01-03: on the near anniversary of this proc, the iterative version, quick-n-dirty since I'm in a hurry to parse some html...
proc wsplit { str sepStr } {
   set strList   {}
   set sepLength [string length $sepStr]

   while {[set index [string first $sepStr $str]] != -1} {
       set left [string range $str 0 [expr {$index + $sepLength - 1}]]
       set str  [string range $str [expr {$index + $sepLength + 1}] end]
       lappend strList $left
   return $strList

hmm use this version instead, the string first doesn't catch strings sepstrs connected to the ones you want
proc wsplit {str sepStr} {
    if {![regexp $sepStr $str]} {
        return $str}
    set strList {}
    set pattern (.*?)$sepStr
    while {[regexp $pattern $str match left]} {
        lappend strList $left
        regsub $pattern $str {} str
    lappend strList $str
    return $strList

RS writes recently:

Note that the wsplit can be done simpler:

  1. map the separating string to a single char that cannot appear in the string
  2. split on that single char
proc wsplit {str sep} {
  split [string map [list $sep \0] $str] \0
% wsplit This<>is<>a<>test. <>
This is a test.

Sarnold 2006-06-21: Sarnold Here is my version of wsplit:
proc wsplit {str sep} {
    set out {} 
    set sepLen [string length $sep]
    if {$sepLen < 2} {
        return [split $str $sep]
    while {[set idx [string first $sep $str]] >= 0} {
        # the left part : the current element
        lappend out [string range $str 0 [expr {$idx-1}]]
        # get the right part and iterate with it
        set str [string range $str [incr idx $sepLen] end]
    # there is no separator anymore, but keep in mind the right part must be
    # appended
    lappend out $str

escargo: So what should you use when you don't care how many spaces were between tokens, you just want the non-blank tokens in the list and none of the separators?

RS: Easy, just use a filter:
proc filter {cond list} {
    set res {}
    foreach element $list {
        if {[$cond $element]} {
            lappend res $element
    set res
% filter llength [split "a   list   with many   spaces"]
a list with many spaces

... or use
% split [regsub -all {[ \t\n]+} "a   list   with many   spaces" { }] 

to eliminate the excess white space ...

... or use
% lreplace "a   list   with many   spaces" 0 -1

to force reinterpretation as a list ...

Caveat: Byte Arrays  edit

RS 2006-07-04: When you split on {} on a byte array, it may be surprising that the result may contain unscannable characters for \x00 bytes. I had to work around like this:
proc hexdump str { 
    set res {} 
    foreach c [split $str {}] { 
        set i [scan $c %c] 
        if {$i eq {}} {set i 0} ;#<--------------------- here 
        lappend res [format %02x $i] 
    return $res

Protecting Separators  edit

Sometimes you want to be able to insert one of the separators anyway, but still split on all "unprotected" separators. The following procedure will do that.
proc psplit { str seps {protector "\\"}} {
    set out [list]
    set prev ""
    set current ""
    foreach c [split $str ""] {
        if { [string first $c $seps] >= 0 } {
            if { $prev eq $protector } {
                set current [string range $current 0 end-1]
                append current $c
            } else {
                lappend out $current
                set current ""
            set prev ""
        } else {
            append current $c
            set prev $c
    if { $current ne "" } {
        lappend out $current

    return $out

So splitting the string I intend to use the character \. to separate between sentences. And can demonstrate it! on . would return a list with two elements only:
set str {I intend to use the character \. to separate between sentences. And can demonstrate it!}
puts [psplit $str .]

would print out
{I intend to use the character . to separate between sentences} { And can demonstrate it!}

PYK 2014-03-02: There was previously a discussion by escargo here that made no sense to me, so I've removed it. If someone sees the point of that discussion, please bring it back!