function validateAddToCart() {
    var valid = true;
    
    for( var i = 0; i < document.forms.length; i++ ) {
    	var currentValue = document.forms[ i ].quantity.value;
		var availableQuantity = document.forms[ i ].available_quantity.value;
		
    	if( currentValue == "" || !isInteger( currentValue ) ) {
        	alert ( "Please enter an integer quantity greater than zero." );
        	valid = false;
    	}
    	else if( currentValue > availableQuantity ) {
    		alert( "There are only " + availableQuantity + " available." );
    		valid = false;
    	}
	}
	
    return valid;
}

function isInteger( str ) {
	var valid = true;

    for (var i = 0; i < str.length; i++) {   
        // Check that current character is number.
        var c = str.charAt(i);
        if (((c < "1") || (c > "9"))) valid = false;
    }
    
    return valid;
}