The lindex command accepts a parameter, list, which it treats as a Tcl list. It also accepts zero or more indices into the list. The indices may be presented either consecutively on the command line, or grouped in a Tcl list and presented as a single argument.
If no indices are presented, the command takes the form:
lindex list
or
lindex list {}
In this case, the return value of lindex is simply the value of the list parameter.
When presented with a single index, the lindex command treats list as a Tcl list and returns the index'th element from it (0 refers to the first element of the list). In extracting the element, lindex observes the same rules concerning braces and quotes and backslashes as the Tcl command interpreter; however, variable substitution and command substitution do not occur. If index is negative or greater than or equal to the number of elements in value, then an empty string is returned. The interpretation of each simple index value is the same as for the command string index, supporting simple index arithmetic and indices relative to the end of the list.
If additional index arguments are supplied, then each argument is used in turn to select an element from the previous indexing operation, allowing the script to select elements from sublists. The command,
lindex $a 1 2 3
or
lindex $a {1 2 3}
is synonymous with
lindex [lindex [lindex $a 1] 2] 3
Lists can be indexed into from either end:
lindex {a b c} 0
→ a
lindex {a b c} 2
→ c
lindex {a b c} end
→ c
lindex {a b c} end-1
→ b
Lists or sequences of indices allow selection into lists of lists:
lindex {a b c}
→ a b c
lindex {a b c} {}
→ a b c
lindex {{a b c} {d e f} {g h i}} 2 1
→ h
lindex {{a b c} {d e f} {g h i}} {2 1}
→ h
lindex {{{a b} {c d}} {{e f} {g h}}} 1 1 0
→ g
lindex {{{a b} {c d}} {{e f} {g h}}} {1 1 0}
→ g
List indices may also perform limited computation, adding or subtracting fixed amounts from other indices:
set idx 1
lindex {a b c d e f} $idx+2
→ d
set idx 3
lindex {a b c d e f} $idx+2
→ f