Get value of dynamic input element in form object

My question is: How do I reference a dynamic 'name' of an input element in a form?

For instance, with the following HTML:

<form>
    <input type="text" name="qty1" value="input1" />
     <input type="text" name="qty2" value="input2" />
     <input type="text" name="qty3" value="input3" />

    <input type="submit" value="Submit" onClick="checkVal(this.form); return false;" />
</form>

Javascript:

function checkVal(form) {
    for (var i = 1; i <= 3; i++) {
        alert(form.qty+i.value);  // Here's where my problem is..
    }
}

The above javascript does not work. The alert is outputting NaN.

How do I reference qty1, qty2, and qty3 in a for loop using i variable?

Here's a jsfiddle: http://jsfiddle.net/MRzWf/

Use Bracket notation

 form["qty" + i].value

function checkVal(form) {
    for (var i = 1; i <= 3; i++) {
        console.log("qty" + i, form["qty" + i].value);
    }
}

<form>
    <input type="text" name="qty1" value="input1" />
    <br/>
    <input type="text" name="qty2" value="input2" />
    <br/>
    <input type="text" name="qty3" value="input3" />
    <br/>
    <input type="submit" value="Submit" onClick="checkVal(this.form); return false;" />
</form>

Simply login as a dictionary

http://jsfiddle.net/MRzWf/2/

 alert(form["qty"+i].value);