Tcl string concatenation

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• Two string variables can easily be concatenated as follows:

  set alpha "Hel"
  set beta "lo"
  puts stdout $alpha$beta

  This gives:

  Hello

• Another way is to use append:

  set alphabeta "Hel"
  append alphabeta "lo"
  puts stdout $alpha$beta

  This gives:

  Hello

• However, when mixing a string variable and a literal string, the result is not what would be expected:

  set alpha "Hel"
  puts stdout $alpha"Hel"

  This gives:

  Hel"lo"

• It is important to note that the concat tcl function applies on lists. It is possible to concat some strings by using a list but this is not the good way:

  set alphabeta [concat "Hel" "lo"]
  puts stdout $alphabeta

  This gives a list of two members, displayed with a space between them:

  Hel lo

  A list can be merged to give a string (here without spaces between members) as follows:

  set alphabeta [join [concat "Hel" "lo"] ""
  puts stdout $alphabeta

  This gives:

  Hello

  However this is not efficient.

• Finally, the simplest way is by using curly braces to separate the variable name:

  set alpha "Hel"
  set alphabeta ${alpha}lo
  puts stdout $alphabeta

  This gives:

  Hello

• Conclusion:
  • With append:

    set alphabeta "Hel"
    append alphabeta "lo"

  • With curly braces:

    set alpha "Hel"
    set alphabeta ${alpha}lo

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This page is part of the Tcl/Tk page of the Airplug documentation.
Back to the Tcl/Tk page / Back to Airplug documentation page.