TIP #224: Add New [array] Subcommands 'incr' and 'value'
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TIP:	224
Title:	Add New [array] Subcommands 'incr' and 'value'
Version:	$Revision: 1.12 $
Authors:	Peter MacDonald <peter@pdqi.com>
	Robert Seeger <rseeger00@aol.com>
State:	Draft
Type:	Project
Tcl-Version:	8.7
Vote:	Pending
Created:	Tue Sep 28 00:00:00 UTC 2004
Abstract
The TIP proposes the addition to two new subcommands to the array command: incr and value. The 'incr subcommand would increment an element in an array. The value subcommand would query, reference, and/or initialize an array element.
Rationale
[215] proposed modifying the incr command to auto-initialize variables which do not exist, rather than throwing an error. After some debate, it was identified that frequency counting in arrays was the primary area of interest. In particular, the excessive use of if/info exists constructs in such code is inefficient, verbose and rather difficult to read. The addition of these two subcommands could substantially reduce the complexity of much Tcl code.
Specification
Two new subcommands will be added to array, being incr and value. Both commands take an array name and an elem parameter, but, no string match is performed on elem.
array incr
array incr var elem ?value? ?init?
The array incr subcommand would increment var(elem), if it exists, by value, otherwise, initializes it to init. The default for value is 1, and the default for init is 0. The subcommand will return the new value and will support both integer and double values (based on the rules for addition in expr).
array value
array value var elem ?value? ?init?
The array value would just return the current contents of var(elem) if it exists, or value otherwise. If the init parameter resolves to true (as determined by Tcl_GetBooleanFromObj() of course), the variable is initialized to value if it doesn't already exist. The default for value is the empty string, and the default for init is false.
Reference Implementation
Following is a Tcl only implementation of the incr/value subcommands:
proc Array cmd var elem amt  init 0 
   # Implement the Array subcmds incr and value on var(elem):
   #  - incr   increment a variable by amt or initialize
   #           to init if undefined.
   #  - value  return the value if defined, else return amt
   #           initializing if init.
   upvar $var uvar
   if [string match $cmd* incr] 
      if $amt ==   set amt 1 
      if [info exists uvar($elem)] 
         return [set uvar($elem) [expr $uvar($elem)+$amt]]
      
      return [set uvar($elem) $init]
    elseif [string match $cmd* value] 
      if [info exists uvar($elem)] 
         return $uvar($elem)
      
      if $init 
         return [set uvar($elem) $amt]
      
      return $amt
    else 
      error "usage: Array incr|value var elem ?amt? ?init?"

Discussion
Wangnic notes:
array value var elem "" false can be written as array get var elem
Array get returns name/value pairs. Array value returns just the value. Also if elem has a * in it, there may be multiple matches.
Hobbs notes:
[200] already rejected array values
A single item is returned from array value, not a list, as there is no string match on elem.
The problem domain is not list processing (which is expected to be relatively expensive), but rather frequency counting and set matching.
RHSeeger notes:
It would seem more consistant to push for the incr command to include a way to initialize a variable if it doesn't exist (as per [215] or the discussed options contained therein), rather than add an incr subcommand to array.
Being a fan or reintroducing the array values TIP (since the core mailing list indicates it was never fully rejected), I think array value might be a bit confusing, being only off by the lack of plurality. (I was one of the people pushing for [200] though, so take that aspect with a grain of salt)
Copyright
This document has been placed in the public domain.
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