TCL/OTCL

Arrays and Lists

Arrays

Just like variables, you don't need to define arrays in Tcl.
Array elements are created when you first use the element.

Array elements are denoted by brackets:

ArrayName ( ArrayIndex )

Example:

set color(0) purple set color(1) red set color(2) red puts "$color(0) $color(1) $color(2)" // Prints: purple ... NOTES: puts $color(0) $color(1) $color(2) // Error: puts needs 1 arg ! puts {$color(0) $color(1) $color(2)} // Prints: $color(0)...

Simple iteration through of elements in an array
- We have seen that the command
  returns the number of elements in the array ARRAY-NAME
- Under normal circumstance (like C++, Java), if we use consecutive numeric array indices, we can use a simple for-loop to iterate through the elements in an array.
- Example:
Associative Arrays
- In Tcl, you can use anything as array index, not just integers
  (you can ofcourse use integers as array index if you so desire !).
- When the array index is a string, the array works like a dictionary
  Such an array is called an associative array
- Example: using strings as indices
- Obviously, the traditional for loop CANNOT be used to iterate through an associate array
array names: make a list of the indices of an (associative) array
- To iterate through all elements in an associate array, we need the names of all the indices used.
- The array names command returns a list that contains all the indices of an array:
- Example:
- Next, we will learn how to manipulate list in Tcl...
Lists in Tcl
- The list is a basic data structure in Tcl
- There are many Tcl commands that manipulate lists
- A list is nothing nore than an ordered collection of stuff; numbers, words, strings, or other lists.
- You can put items of different types in one list !

Creating lists in Tcl

List can be created by setting a variable to an explicit list of items:
A list item can be composite !
List can also be created by the an list command:
In this case, you should save the return value of the command in a variable:

Examples:

set list1 {abc def klm xyz} set list2 { {jan 1} {feb 2} {march 3} }

Accessing lists: "lindex" (list index)

A list in Tcl is NOT created through links as in programming languages like C++ or Java.
A list in Tcl is a serie of items (very much like an array of dynamic size).
An interesting feature of Tcl's list is that list elements can be accessed in a similar manner as arrays, i.e., using an index (a list index)
The list index command lindex can be used to access individual elements in the list:
- Returns the index^th item from the list.
- NOTE: list indices start from 0 (not 1), so the first item is at index 0, the second item is at index 1, and so on.

Examples:

lindex { abc def klm xyz } 1 Returns: def
set list1 { abc def klm xyz } lindex $list1 1 Returns: def
lindex { {jan 1} {feb 2} {march 3} } 1 returns: feb 2
set list2 { {jan 1} {feb 2} {march 3} } lindex $list2 1 returns: feb 2

Number of elements in list: llength

Using llindex we can find all the names used as index in an associative array.
We still need to know how many elements are in the list so that we can use a for-loop to iterate through all elements.
The list length command "llength" returns the number of elements in a list:

Examples:

llength { abc def klm xyz } Result: 4
llength { {jan 1} {feb 2} {march 3} } Result: 3
set list1 {abc def klm xyz} llength $list1 Result: 4
set list2 { {jan 1} {feb 2} {march 3} } llength $list2 Result: 3

Iterating through the list using foreach statement

The plain way to iterate through every element in a list is using the foreach command:

The foreach statement is executed as follows:

The variable VARIABLE_NAME will take on every element in the list LIST
The foreach command will execute the BODY code one time for each list item in list.
On each pass, the code in BODY can use the variable VARIABLE_NAME to access of the current list item.

Examples:

foreach i { abc klm xyz } { puts "Hello $i" } Output: Hello abc Hello klm Hello xyz
set list2 { {jan 1} {feb 2} {march 3} } foreach i $list2 { puts $i } Outputs: jan 1 feb 2 march 3

Iterating through the list using for statement

You can also use the for command to iterate through a list as follows:
- You know how many elements are in the list through: [llength LIST]
- You can access the i^th elements in the list through: [lindex LIST $i]
Hence, the following loop construct will access all elements in the list:

Examples:

set list1 { abc def klm xyz } for {set i 0} {$i < [llength $list2]} {incr i} { puts [lindex $list1 $i] } Output: abc def klm xyz
set list2 { {jan 1} {feb 2} {march 3} } for {set i 0} {$i < [llength $list2]} {incr i} { puts [lindex $list2 $i] } Output: jan 1 feb 2 march 3

Iterating through an associative array using foreach statement

We know how to obtain a list of names that is used as array indices:

set color(myHouse) purple set color(myCar) red set color(myChair) red array names color Output (list): myCar myChair myHouse

The array names command returns the name and the corresponding index

We can use this list in the foreach command to iterate through the elements in the array:

set LIST [array names color]
foreach { name } $LIST { puts "name is $name" } Output: name is myCar name is myChair name is myHouse
foreach { name } $LIST { puts "Color of item \"$name\" is $color($name)" } Output: Color data stored in entry 0 on item "myCar": red Color data stored in entry 1 on item "myChair": red Color data stored in entry 2 on item "myHouse": purple

Iterating through an associative array using for statement

We can also use a for statement to iterate through elements in an associative array.

We know how to obtain a list of names that is used as array indices:

set color(myHouse) purple set color(myCar) red set color(myChair) red set LIST [array names color] LIST = myCar myChair myHouse

The number of elements in the list is given by:

set color(myHouse) purple set color(myCar) red set color(myChair) red set LIST [array names color] llength $LIST

We can use the list LIST in a for command to iterate through the elements in the array:

set color(myHouse) purple set color(myCar) red set color(myChair) red set LIST [array names color]
for {set i 0} {$i < [llength $LIST]} { incr i } { set name [lindex $LIST $i] ;# get i-th name puts "name is $name" } Output: name is myCar name is myChair name is myHouse
for {set i 0} {$i < [llength $LIST]} { incr i } { set name [lindex $LIST $i] ;# get i-th name puts "Color of item \"$name\" is $color($name)" } Output: Color of item "myCar" is red Color of item "myChair" is red Color of item "myHouse" is purple

Further reading
- Intro to List: click here
- Intro to Asscoiative Arrays: click here
- Foreach on associative arrays: click here