delete an instance of subform at runtime

If the script is being executed from inside the instance you want to delete, the instance number can be acquire directly from the current object.

For example, if the script is run from a click event on a button inside the repeated subform (i.e. a delete button)

The subform index is:

var subIndex = this.parent.index;

If you want to use a button that is outside the repeated subform you'll need a way to id the particular subform that you want to delete, like a check box in each of the repeated subforms.

i have used this.parent.index;
now i have to find out which subform is being deleted,

var subIndex = this.parent.index;
when I am printing it in the console using
console.println("index: " + subIndex);
it gives '0' for every subform i delete.

If your delete button is part of the "row", then you can use code similar to:

Table1.resolveNode("row[" + this.parent.index + "]").instanceManager.removeInstance(1)

Otherwise, like Thom said, there is no context to specify the instance you wish to remove.