html - I can't get the onclick attribute on radio buttons to execute javascript functions

Well, as my title suggests, I'm having a little difficulty in executing javascript functions using radio buttons. I would love to have an alert box pop-up wen I select a radio button, but my code seems to ever be working against my "love(s)". Please help me out.

Here's my HTML, it isn't much.

    
    

    


    

        
Question One
        
Calculate the area of a circle or a cube
        

            

                
Choose shape to find area
                    Circle

                    
                    Cube
            
        
    
    

    

and here is the javascript

var radios = document.getElementsByName("shape");
var len = radios.length;

function checkRadios()
{
    for(int i = 0; i < len; i++)
    {

        if(radios[i].checked)
        {
            alert(radios[i].getAttribute("value") + " selected.");
        }
    }
}

I feel like I need to add that, i am just a beginner in js and will appreciate if the help isnt too complicated. i.e, i just want basic javascript without any libraries such as jquery, as those will be too difficult for me to understand at the moment.