# CFLib.org – Common Function Library Project

## TwosCompToDec(num[, bits])

##### Last updated May 31, 2002

Version: 1 | Requires: CF5 | Library: MathLib

Description:
This function will convert any 2's complement encoded base 10 value into an unencoded base 10 value, provided you specify the value and the number of bits the decimal value originally represented.

Return Values:
Returns a numeric value.

Example:

``````<cfset x = -49>
<cfoutput>
Given x=#x#<BR>
The original decimal value is: #TwosCompToDec(x, 8)#
</cfoutput>
``````

Parameters:

Name Description Required
num 2's complement you want to convert back to its original decimal value.. Yes
bits Number of bits in the original decimal value. No

Full UDF Source:

``````/**
* Decodes a 2's complement base 10 (decimal) value into an unencoded base 10 value.
*
* @param num      2's complement you want to convert back to its original decimal value.. (Required)
* @param bits      Number of bits in the original decimal value. (Optional)
* @return Returns a numeric value.
* @author Stephen Rittler (scrittler@etechsolutions.com)
* @version 1, May 31, 2002
*/
function TwosCompToDec(num, bits){
var isNegative = 0;
var varBase10 = num;
var varBase2 = 0;
var varBase2Rev = "";
var strLen=0;

// capture whether or not the input value is negative
if (varBase10 LT 0) {
isNegative = 1;
varBase10 = abs(varBase10);
}

// convert to base 2
varBase2 = NumberFormat(Right(FormatBaseN(varBase10,2), bits), RepeatString("0", bits));

// switch sign bit to 0
varBase2 = "0" & Right(varBase2, (bits - 1));

if(isNegative) {
// if the number was negative, we have to invert the value and add one

// invert; swap 0 for 1 and 1 for 0
strLen = len(varBase2);
for(i=1; i LE bits; i=i+1){
if(Mid(varBase2, i, 1) eq 0) {
varBase2Rev = varBase2Rev & 1;
} else {
varBase2Rev = varBase2Rev & 0;
}
}

// convert back to base 10
varBase10 = InputBaseN(varBase2Rev, 2);

return (varBase10 + 1);

} else {
// if number was positive, convert straight back to base 10
return InputBaseN(varBase2, 2);
}
}
``````

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