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Strings and Lists
Strings in Tcl are simply concatenated characters. If the string contains white-space it must be delimited by quotes:
Tcl Strings
| No Format | ||||
|---|---|---|---|---|
| ||||
# Legal string syntax in Tcl
set x hello
set y "hello"
set z "hello world"
{noformat}
|
If
...
the
...
string
...
contains
...
variables
...
these
...
are
...
resolved
...
immediately:
...
Variables
...
Resolved
...
in
...
a
...
String
...
| No Format | ||||
|---|---|---|---|---|
| ||||
set x 4
puts "He ate $x apples" ; # prints 'He ate 4 apples'
{noformat}
|
Lists
...
are
...
elements
...
delimited
...
by
...
'
...
{...}'
...
or
...
created
...
using
...
the
...
list
...
command.
...
The
...
elements
...
may
...
be
...
any
...
variable
...
or
...
data
...
structure
...
allowed
...
in
...
Tcl,
...
including
...
other
...
lists.
...
Creating
...
Lists
...
| No Format | ||||
|---|---|---|---|---|
| ||||
set l { He ate 4 "green apples" }
set l [list He ate 4 "green apples" ]
{noformat}
|
This
...
lists
...
contains
...
4
...
elements,
...
the
...
last
...
element
...
being
...
"green
...
apples."
...
Evaluation
...
of
...
Variables
...
Lists
...
created
...
with
...
braces
...
differ
...
from
...
strings
...
in
...
that
...
variables
...
are
...
not
...
resolved
...
when
...
the
...
list
...
is
...
declared.
...
Simple
...
List
...
| No Format | ||||
|---|---|---|---|---|
| ||||
set x 4
puts "He ate $x apples" ; # prints 'He ate 4 apples'
puts { He ate $x apples } ; # prints 'He ate $x apples'
{noformat}
|
The
...
use
...
of
...
braces
...
to
...
preserve
...
variables
...
is
...
useful
...
when
...
statements
...
are
...
to
...
be
...
evaluated
...
at
...
a
...
later
...
point
...
of
...
execution.
...
This
...
mode
...
of
...
programming
...
is
...
greatly
...
used
...
when
...
creating
...
interface
...
menus,
...
when
...
so-called
...
callback
...
functions
...
are
...
assigned
...
to
...
various
...
elements
...
of
...
the
...
menu.
...
The
...
callback
...
functions
...
will
...
be
...
called
...
in
...
response
...
to
...
a
...
window
...
event,
...
which
...
may
...
happen
...
at
...
any
...
time.
...
Callback
...
Functions
...
| No Format | ||||
|---|---|---|---|---|
| ||||
set x 4
qpushbutton "Immediate" "qinformation Information $x" ; # displays '4'
qpushbutton "Delayed" { qinformation Information $x } ; # displays '5'
set x 5
{noformat}
|
In
...
current
...
example
...
showing
...
Callback
...
Functions
...
,
...
two
...
buttons
...
are
...
created
...
which
...
both
...
display
...
the
...
value
...
of
...
x
...
when
...
activated.
...
The
...
first
...
button
...
is
...
given
...
a
...
callback
...
string
...
which
...
immediately
...
resolves
...
to
...
'qinformation
...
4'.
...
This
...
string
...
is
...
stored
...
together
...
with
...
the
...
button
...
and
...
is
...
implemented
...
at
...
the
...
same
...
time
...
the
...
user
...
activates
...
the
...
button.
...
By
...
this
...
time
...
x
...
has
...
been
...
set
...
to
...
'5',
...
but
...
that
...
no
...
longer
...
affects
...
the
...
contents
...
of
...
the
...
callback
...
string.
...
The
...
second
...
button
...
is
...
given
...
a
...
brace-delimited
...
list
...
as
...
the
...
callback
...
function.
...
The
...
variable
...
x
...
is
...
not
...
resolved
...
and
...
the
...
list
...
{
...
qinformation
...
$x
...
}
...
is
...
assigned
...
as
...
callback
...
function
...
regardless
...
of
...
the
...
value
...
of
...
x.
...
When
...
the
...
user
...
later
...
activates
...
the
...
button
...
however,
...
x
...
holds
...
the
...
value
...
'5',
...
and
...
this
...
is
...
displayed
...
on
...
the
...
output
...
channel.
...
Command
...
Substitution
...
A
...
special
...
form
...
of
...
command
...
evaluation
...
involves
...
the
...
square
...
brackets
...
'
...
[ref:
...
...
...
]'.
...
When
...
encountering
...
an
...
expression
...
in
...
square
...
brackets,
...
Tcl
...
will
...
evaluate
...
the
...
expression
...
inside,
...
and
...
replace
...
the
...
expression
...
in
...
brackets
...
with
...
the
...
result
...
before
...
evaluating
...
the
...
outer
...
statement.
...
Command
...
Substitution
...
| No Format | ||||
|---|---|---|---|---|
| ||||
set result [expr 2 + 2 ] puts $result {noformat} |
In
...
the
...
above
...
example
...
the
...
command
...
'expr
...
2
...
+
...
2'
...
is
...
evaluated
...
and
...
replaced
...
with
...
it's
...
result,
...
which
...
is
...
'4'.
...
Any
...
valid
...
Tcl
...
statement
...
that
...
returns
...
a
...
value
...
can
...
be
...
placed
...
inside
...
square
...
brackets.
...
The
...
list
...
command,
...
for
...
example,
...
returns
...
a
...
new
...
list
...
formed
...
by
...
concatenating
...
the
...
elements
...
provided
...
as
...
arguments
...
:
...
Nested
...
List
...
Expression
...
| No Format | ||||
|---|---|---|---|---|
| ||||
set l [list 1 2 3]
{noformat}
|
Here
...
the
...
variable
...
'l'
...
is
...
assigned
...
and
...
the
...
value
...
returned
...
by
...
the
...
bracketed
...
expression,
...
which
...
in
...
this
...
case
...
creates
...
a
...
list.
...