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Tcl - append v lappend v concat

Should you use append, lappend or even concat to add to variable in Tcl?

append puts one string directly on the end of another, without adding any extra characters beyond those in the incoming variables.

lappend treats the target string as a list, and will usually add an extra space on the end of it prior to appending the new item so that the result is also a list with ONE new item added ... the added item being protected using additional characters such as { and } to turn it into a list element if necessary.

concat (and note the different syntax!) takes two LISTS and joins them together - it differs from lappend in that lappend takes the second element as a single item and not as a list. Concat is something that many occasional Tcl users seem to forget about ... and then waste time writing a loop of lappends!

Example - source code

set people "John Joan Jane"
set gatecrashers "Bill Ben Boris"
#
# append adds to string, lappend adds to list
puts $people
append people t
lappend people Julian
puts $people
#
# concat connects two lists, lappend adds one item to a list
set everyone [concat $people $gatecrashers]
puts $everyone
lappend people $gatecrashers
puts $people


In operation:

Dorothy:~/csr1 grahamellis$ tclsh alc
John Joan Jane
((append will add "t" to Jane, lappend adds new item Julian))
John Joan Janet Julian
((concat adds a whole new list))
John Joan Janet Julian Bill Ben Boris
((but lappend adds a single item to the list))
John Joan Janet Julian {Bill Ben Boris}
Dorothy:~/csr1 grahamellis$

(written 2007-10-23)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T206 - Tcl/Tk - Lists
  [144] Tcl sandwich - lists in Tcl - (2004-12-08)
  [463] Splitting the difference - (2005-10-13)
  [781] Tcl - lappend v concat - (2006-06-27)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [1283] Generating traffic for network testing - (2007-07-29)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1405] Sorting in Tcl - lists and arrays - (2007-10-24)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [2468] What are Tcl lists? - (2009-10-22)
  [2472] split and join in tcl and expect - (2009-10-23)
  [3285] Extracting data from a string / line from file - Tcl - (2011-05-10)
  [3394] The difference between lists and strings - Tcl - (2011-08-16)
  [3415] User defined sorting and other uses of callbacks in Tcl and Tk - (2011-09-02)
  [3582] Tcl collections - lists, dicts and array - (2012-01-16)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [3618] lists and struct::list in Tcl - Introduction to struct::list and examples - (2012-02-18)
  [4209] Lists in Tcl - fundamentals in a commented source code example - (2013-11-16)
  [4454] Everything is a string - even a list - (2015-03-11)
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)


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