Edit in JSFiddle

function forFloor() {
    document.getElementById('floorOut').value = Math.floor(document.getElementById('floor').value);
}

function forRound() {
    document.getElementById('roundOut').value = Math.round(document.getElementById('round').value);
}

$(document).ready(function() {
    $("#floor, #round").keydown(function (e) {
        // Allow: backspace, delete, tab, escape, enter and .
        if ($.inArray(e.keyCode, [46, 8, 9, 27, 13, 110, 190]) !== -1 ||
             // Allow: Ctrl+A
            (e.keyCode == 65 && e.ctrlKey === true) || 
             // Allow: home, end, left, right
            (e.keyCode >= 35 && e.keyCode <= 39)) {
                 // let it happen, don't do anything
                 return;
        }
        // Ensure that it is a number and stop the keypress
        if ((e.shiftKey || (e.keyCode < 48 || e.keyCode > 57)) && (e.keyCode < 96 || e.keyCode > 105)) {
            e.preventDefault();
        }
    });
    forRound();
    forFloor();
});

<h1>Enter the Left Hand Side text box value to get Result on Right hand side text box.</h1>
<div id="floorDiv">Result of Math.floor(
    <input type="text" id="floor" onkeyup="forFloor()" value="2.6"> ) is
    <input type="text" id="floorOut" readonly>.
</div>
<div id="roundDiv">Result of Math.round(
    <input tyep="text" id="round" onkeyup="forRound()" value="2.6"> ) is
    <input type="text" id="roundOut" readonly>.
</div>