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Rounding Down in Tcl

Posted by eyarrow (eyarrow), 26 January 2004

Is there any package I can call.... so that I can round down in Tcl as opposed to rounding up?

Many thanks


Posted by admin (Graham Ellis), 26 January 2004
Hmm.  I thought Tcl rounded down ...

% expr 10/3
% expr 11/3
% expr 12/3
% expr 10./3
% expr 11./3
% expr -11/3

All of these are rounding down except the 11./3 which has rounded up in about the 12th significant figure.

The -4 answer to the last test might be a suprise, but this is still rounding down.

Can you post a more specific example of the sort of behaviour that you want to change, and I'll answer further

Posted by eyarrow (eyarrow), 26 January 2004

Many thanks.. what I am refering to is as follows:-

If we wish to round to 3 DP.... then normal rounding is as follows

1.64354          =>   1.644

If we were to round down to 3Dp... then it would be as follows

1.64354          =>  1.643

I believe, this is called ROUND_HALF_DOWN in Java.

Many thanks


Posted by admin (Graham Ellis), 26 January 2004
Easiest way is to use the floor function, but that rounds down to the nearest whole number.   Here's a definition of a proc that does want you want (shifts your value, floors it, shifts it back)

proc round_down {val rounder} {
       set nval [expr floor($val*$rounder) /$rounder]
       return $nval

Demo of that in use:

% source ndp.tcl
% round_down 123.456789 1000

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