TIP 343: A Binary Specifier for [format/scan]

Login
Bounty program for improvements to Tcl and certain Tcl packages.
Tcl 2017 Conference, Houston/TX, US, Oct 16-20
Send your abstracts to tclconference@googlegroups.com
by Aug 21.
Author:		Alexandre Ferrieux <alexandre.ferrieux@gmail.com>
State:		Final
Type:		Project
Vote:		Done
Created:	03-Dec-2008
Post-History:	
Keywords:	Tcl,binary
Tcl-Version:	8.6

Abstract

This TIP proposes to add a %b specifier to the format and scan commands for working with integers in base-2 representation.

Background

The format and scan commands already have decimal, hexadecimal, and octal support, but people wanting binary today resort to a hack, namely going hexadecimal first and then string mapping the hex digits to their four-bits binary representations. We also already have the "0b" notation as input for expr. This lack is inelegant.

Proposed Change

This TIP proposes to reuse the existing binary representation machinery to expose a new %b specifier:

	format %b 5
	=> 101
	scan 000101 %b x;set x
	=> 5

The good properties of the existing code make the addition really painless, in that it automagically combines with the size (l,ll), width (digits), and prefix (#) modifiers:

	format %#06b 5
	=> 0b0101
	format %llb [expr {(2**72-1)/7}]
	1001001001001001001001001001001001001001001001001001001001001001001001

Rationale

That is really low-hanging fruit. All the pieces are in place, it's just a matter of exposition. The binary representation is a nice pedagogic tool, and having it handy (rather than with a hack) is a bonus.

Reference Implementation

See Patch 2368084 https://sourceforge.net/support/tracker.php?aid=2368084 .

Copyright

This document has been placed in the public domain.

History