getMaxInput.html

<html>

<head>
	<script type="text/JavaScript">
		function get(eid) {return document.getElementById(eid);};
		function getMax( idPrefix, cnt ){
			for (var i = 0; i < cnt; i++) {
				// read value of input with id of [idPrefix + i] and
				// convert to numeric, if possible
				var num = parseFloat(get(idPrefix + i).value);
				// if conversion fails, advise user and exit immediately
				if (isNaN(num)) {
					alert('all inputs must be numeric');
					return;
				}
				//...otherwise, if first iteration, keep number - for
				// later iterations keep biggest
				if (0 == i) max = num;
				else max = (num < max)?max:num;
			};
			get( 'max' ).value = max;
		}
	</script>
</head>

<body>
	<form>
		<p>
			Instructions:<br />
			Enter numbers into fields and click "go" button.<br />
			The maximum value of your inputs will appear below.<br /><br />
			Input values:<br />
			<!--
				inputs all have ids like ["in" + integer] - "go" button sends
				prefix "in" and total count of inputs to getMax() function
			-->
			<input id="in0" type="text" /> = Input 1<br />
			<input id="in1" type="text" /> = Input 2<br />
			<input id="in2" type="text" /> = Input 3<br />
			<input id="in3" type="text" /> = Input 4<br />
			<input id="in4" type="text" /> = Input 5<br />
			<input type="button" value="go" onclick="getMax( 'in', 5 )" /><br />
			Output value:<br />
			Maximum: <input type="text" id="max" />
		<p>
	</form>
</body>

</html>