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01262013, 01:11 PM #1
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Regex to detect power of two fractions  behaving unexpectedly
I have developed a JavaScript function to convert a decimal number of inches to a string displaying feet, inches, and fractional components  for example, 23.75 inches converts to 1' 11 3/4". (To my surprise, this appears not to be available 'precooked' anywhere I can find using Google.)
It will normally be used to convert to 'woodworking' fractions: halves, quarters, eighths, sixteenths, thirtyseconds, etc., where the denominator is a power of two. But I also want it to accept (subject to a warning and confirmation prompt) any other denominator.
The function takes two parameters: the length in inches with optional decimal, and a precision (integer number) for the denominator to use for the fractional component (if any).
I am attempting to detect whether the precision is a power of two, and prompt for confirmation if it isn't.
The code snippet looks like this:
Code:// Check if denominator is a power of two and warn if not denom = precision.toString().match(/3216842/); if(!denom) { // Denominator isn't regular woodworking fraction ok = confirm("Fraction isn't the usual woodworking half, quarter, eighth, sixteenth etc. Did you mean this?",false); if (!ok) {//code continues.... }
But it returns a value of 2 (true) for precision = 12, and therefore doesn't trigger the alert, which I don't understand. It does trigger for precision = 3 or 6.
The same expressionn in Java (not JavaScript) works as I expected, returning a match only if the string matches exactly one of the numbers in the Regex.
Am I misunderstanging how the Regex match() function is supposed to work in JS? If so, why, and how can I otherwise detect and warn when the requested precision isn't a power of two? I could (but don't want to) code it explicitly to work out mathematically if precision is a power of two between 2 and 64, but would prefer to understand and use properly a Regex to detect this.
Help welcome, if anyone can point me to a Regex tutorial specific to JavaScript, or explain what this regex is doing and why.
Thanks.
John McCLast edited by johnwmcc; 01262013 at 08:59 PM.

01262013, 01:44 PM #2
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This may (or may not) help you:
Code:<html> <head> <script type="text/javascript"> var roundTo = 16; // round to nearest 1/roundTo; function fraction() { var decimal = document.getElementById('dec').value; if ((!/^\d*\.\d+$/g.test(decimal))  (String(decimal).split('.')[1].length > 4)) { alert ("You must enter a decimal number (max 4 decimal digits)!"); document.getElementById('dec').value = ""; document.getElementById('dec').focus(); return false; } var whole = String(decimal).split('.')[0]; decimal = parseFloat("." + String(decimal).split('.')[1]); var num = "1"; for (var z=0; z<String(decimal).length2; z++){ num += "0"; } decimal = parseInt(decimal*num); num = parseInt(num); for (z=2; z<decimal+1; z++) { if (decimal%z==0 && num%z==0) { decimal = decimal/z; num = num/z; z = 2; } } var result = ((whole==0)?"" : whole+" ") + decimal + "/" + num; alert ("Fractional value = " + result); var d = num/roundTo; var n = Math.round(decimal/d); alert ("Rounded to nearest 1/" + roundTo + " inch = " + ((whole==0)?"" : whole+" ") + n + "/" + roundTo + " inch"); } </script> </head> <body> <input type="text" name="dec" id= "dec" size="6" > <button onclick = 'fraction();'>Change decimal to fraction</button> </body> </html>
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01262013, 02:00 PM #3
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Thank you for the suggestion. I can see the bit of code relevant to my issue as:
Code:decimal = parseInt(decimal*num); num = parseInt(num); for (z=2; z<decimal+1; z++) { if (decimal%z==0 && num%z==0) { decimal = decimal/z; num = num/z; z = 2; } }
Instead of denom = string.match(/regex/); I have used the more explicit version:
Code:denom = (precision == 2  precision == 4  precision == 8  precision == 16  precision == 32  precision == 64);
That solves my immediate problem and the code now works as expected.
But I still don't understand what's happening in the original code, and would like to know for future reference.
Incidentally, I also tried using variant regexs instead of the original /3216842/
Neither of these work:
/(32)(16)(8)(4)(2)/
/(32){1}(16){1}(8){1}(4){1}(2){1}/
and again I don't understand why not.
John McCLast edited by johnwmcc; 01262013 at 02:04 PM.

01262013, 02:37 PM #4

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01262013, 02:47 PM #5
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I can see why this works, but didn't think to begin with I would need the ^ or $.
Thank you.
John McC
PS. How can I mark this thread as 'Resolved'? I saw the choice when I started it, but can't find it now.Last edited by johnwmcc; 01262013 at 02:50 PM.

01262013, 03:06 PM #6

01262013, 08:58 PM #7
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Thanks. Will do.
J

01272013, 07:05 AM #8
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yeah, that's intuitive usability, NOT!
We have a resolved feature?
I've been here five years and haven't seen it.
can somebody like a mod fix that? i mean, that's just pisspoor, and there's no reason at all that 10mins of elbow grease wouldn't save thousands of folks 10 seconds a day. The 10mins would be realized personally by the modifier within a year of daily use.
the way it is, we DO NOT have a resolved feature, i don't care what anyone says; if the stupid user (like me) can't use it, it's broken, and needs fixed.
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