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

Author:		Alexandre Ferrieux <[email protected]>
State:		Final
Type:		Project
Vote:		Done
Created:	03-Dec-2008
Keywords:	Tcl,binary
Tcl-Version:	8.6
Tcl-Ticket:		2368084


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


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}]


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 .


This document has been placed in the public domain.