Editing Icon (programming language) (section)

===Collections===
Icon includes several [[Collection (abstract data type)|collection types]] including [[List (abstract data type)|lists]] that can also be used as [[Stack (abstract data type)|stacks]] and [[Queue (abstract data type)|queues]], [[associative array|tables]] (also known as maps or dictionaries in other languages), [[Set (abstract data type)|sets]] and others. Icon refers to these as ''structures''. Collections are inherent generators and can be easily called using the bang syntax. For instance:

<syntaxhighlight lang="icon">
 lines := []                    # create an empty list
 while line := read() do {      # loop reading lines from standard input
   push(lines, line)            # use stack-like syntax to push the line on the list
 }
 while line := pop(lines) do {  # loop while lines can be popped off the list
   write(line)                  # write the line out
 }
</syntaxhighlight>

Using the fail propagation as seen in earlier examples, we can combine the tests and the loops:

<syntaxhighlight lang="icon">
 lines := []                    # create an empty list
 while push(lines, read())      # push until empty
 while write(pop(lines))        # write until empty
</syntaxhighlight>

Because the list collection is a generator, this can be further simplified with the bang syntax:

<syntaxhighlight lang="icon">
 lines := []
 every push(lines, !&input)
 every write(!lines)
</syntaxhighlight>

In this case, the bang in {{code|write}} causes Icon to return a line of text one by one from the array and finally fail at the end. {{code|&input}} is a generator-based analog of {{code|read}} that reads a line from [[standard input]], so {{code|!&input}} continues reading lines until the file ends.

As Icon is typeless, lists can contain any different types of values:

<syntaxhighlight lang="icon">aCat := ["muffins", "tabby", 2002, 8]</syntaxhighlight>

The items can included other structures. To build larger lists, Icon includes the {{code|list}} generator; {{code|code=i := list(10, "word")}} generates a list containing 10 copies of "word". Like arrays in other languages, Icon allows items to be looked up by position, e.g., {{code|code=weight := aCat[4]}}. [[Array slicing]] is included, allowing new lists to be created out of the elements of other lists, for instance, {{code|aCat :{{=}} Cats[2:4]}} produces a new list called aCat that contains "tabby" and 2002.

Tables are essentially lists with arbitrary index keys rather than integers:

<syntaxhighlight lang="icon">
 symbols := table(0)
 symbols["there"] := 1
 symbols["here"] := 2
</syntaxhighlight>

This code creates a table that will use zero as the default value of any unknown key. It then adds two items into the table, with the keys "there" and "here", and values 1 and 2.

Sets are also similar to lists but contain only a single member of any given value. Icon includes the {{code|++}} to produce the union of two sets, {{code|**}} the intersection, and {{code|--}} the difference. Icon includes a number of pre-defined "Cset"s, a set containing various characters. There are four standard Csets in Icon, {{code|&ucase}}, {{code|&lcase}}, {{code|&letters}}, and {{code|&digits}}. New Csets can be made by enclosing a string in single quotes, for instance, {{code|vowel :{{=}} 'aeiou'}}.