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(function() {
        setValues();
    });

    $(':input').keyup(function() { // anytime the keyup event fires on an input in the page
        setValues();
    });

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

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

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

function setValues() {
    to_c = ($('#to_celcius').attr('checked') == 'checked'); // Are you converting F to C?
    dc.val(to_c ? calc(false, df.val()) : dc.val());
    df.val(to_c ? df.val() : calc(true, dc.val()));
}