var df, dc, cb; // declare variables

$(document).ready(function() { // once the document and all of it's elements are loaded and ready to go
    // obtain elements as jQuery objects
    df = $('#degrees_fahrenheit');
    dc = $('#degrees_celcius');
    cb = $('#convert');

    $(':input').keydown(function(e) { // anytime the keydown event fires on an input on the page
        unicode = e.keyCode ? e.keyCode : e.charCode; // get the unicode of the key pressed
        return (String.fromCharCode(unicode).match(/^[0-9]+$/) != null) || // is it a number pressed OR
        (unicode == 109 && this.selectionStart == 0 && this.value.indexOf('-') == -1) || // is minus key and is caret in first position, and there are no minus signs in the field already? OR
        (unicode == 8 || unicode == 37 || unicode == 39 || unicode == 46); // is backspace left right or delete key?
    });

    cb.click(setValues);
    $(':input').keyup(setValues);

    $(':radio').change(function() { // any time a radio element changes state this is fired with the element as context
        df.set(this.id != "to_celcius");
        dc.set(this.id == "to_celcius");
    });
});

$.fn.set = function(bool) {
    this.attr('disabled', bool);
    this.val('');
};

function calc() {
    x = $('#to_celcius').attr('checked') == 'checked' ? df.val() : dc.val();
    x = x == "-" || x == "" ? 0 : x; // assumes 0 if the input field is empty or a minus sign
    return $('#to_celcius').attr('checked') == 'checked' ? Math.round((x - 32) * 5/9) : Math.round(x * 9/5 + 32); // depending on f, select proper equation and calculate for x
}

function setValues() {
    if ($('#to_celcius').attr('checked') == 'checked') { // Are you converting F to C?
        dc.val(calc());
    } else {
        df.val(calc());
    }
}