jQuery dynamic selector

You should use the val() function:

var myValue = $("#new_grouping_"+i).val(); // to get the value

$("#new_grouping_"+i).val("something");    // to set the value 

$("#new_grouping_"+i).val() gets you the value of a form.
$("#new_grouping_"+i).text() gets you the text of an html element.
$("#new_grouping_"+i).html() gets you the html of an html element.

$("#new_grouping_"+i).val('value') sets the value of a form.
$("#new_grouping_"+i).text('value') sets the text of an html element.
$("#new_grouping_"+i).html('value') sets the html of an html element.

$("#new_grouping_"+i).append('value') prepends something at the beginning of an element $("#new_grouping_"+i).append('value') appends something at end of an element

$("#new_grouping_"+i).before('value') places something before an element $("#new_grouping_"+i).after('value') places something after an element.

See More: jQuery Manipulation

# is important for selector. Here I am setting value to span.

var i="spanId";
$("#"+ i).html("hello").show();