The append Command
The append command takes a variable name as its first argument and concatenates its remaining arguments onto the current value of the named variable. The variable is created if it does not already exist:
set foo z
append foo a b c
The append command is efficient with large strings.
The append command provides an efficient way to add items to the end of a string. It modifies a variable directly, so it can exploit the memory allocation scheme used internally by Tcl. Using the append command like this:
append x " some new stuff"
is always faster than this:
set x "$x some new stuff"
The lappend command described on page 61 has similar performance benefits when working with Tcl lists.