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The difference between lists and strings - Tcl

There's not much difference between lists and strings in Tcl. In fact a list is just a string in a particular format, and if you split a string that contains just letters and spaces into a "list" you may not change anything AT ALL. Which means that if you make a coding error you may not spot it until some of the characters which form a part of the list syntax to handle specials are used within the string.

Here's a listish setup:

  set john {{tom simon ann sophie cindy}}

But when I split that and iterate through the elements,I don't get what I want - strange things happen with the inner { and }. Code:

  set mitems [split $john]
  foreach thing $mitems {
    puts $thing
  }


When I run that, the results I get are:

  {tom
  simon
  ann
  sophie
  cindy}


As my handling of the list of lists wasn't correct .... what would have been better is to have written


  foreach thing $john {
    foreach name $thing {
      puts $name
    }
  }


which would give

  tom
  simon
  ann
  sophie
  cindy


For Perl, Python, Ruby and PHP programmers, who are used to split doing a considerable amount of work, the small step taken by Tcl's split comes as a bit of a surprise, and even people who've been writingTcl for a while can end up scratching their head at extra curly braces which they hadn't expected from code like that above.

I'm teaching Tcl in a couple of weeks ... starting on Tuesday 30th August for delegates new to programming, and the following day for delegates with prior experience of another language - see [here] for a description of that course.
(written 2011-08-16)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T206 - Tcl/Tk - Lists
  [4455] Working out distance between places, using OS grid references and a program in Tcl - (2015-03-11)
  [4454] Everything is a string - even a list - (2015-03-11)
  [4209] Lists in Tcl - fundamentals in a commented source code example - (2013-11-16)
  [3618] lists and struct::list in Tcl - Introduction to struct::list and examples - (2012-02-18)
  [3583] Expanding a list of parameters in Tcl - {*} and eval - (2012-01-17)
  [3582] Tcl collections - lists, dicts and array - (2012-01-16)
  [3415] User defined sorting and other uses of callbacks in Tcl and Tk - (2011-09-02)
  [3285] Extracting data from a string / line from file - Tcl - (2011-05-10)
  [2472] split and join in tcl and expect - (2009-10-23)
  [2468] What are Tcl lists? - (2009-10-22)
  [1601] Replacing the last comma with an and - (2008-04-04)
  [1405] Sorting in Tcl - lists and arrays - (2007-10-24)
  [1402] Tcl - append v lappend v concat - (2007-10-23)
  [1334] Stable sorting - Tcl, Perl and others - (2007-09-06)
  [1283] Generating traffic for network testing - (2007-07-29)
  [1282] Stringing together Tcl scripts - (2007-07-29)
  [781] Tcl - lappend v concat - (2006-06-27)
  [463] Splitting the difference - (2005-10-13)
  [144] Tcl sandwich - lists in Tcl - (2004-12-08)


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