integer[] Update Questions

Greeting Hackers! Had some questions about integer array type...

If I have a integer[] field in a table, which already has the values
{1,2,3,6,17} and I additionally want to add the value '45' to this
array, is there any easier way than SELECTing the current values and
then updating the entire record?

I see that I could do:
UPDATE table SET array_field[6] = 45 WHERE ...

But how do I know that 6 is the correct index without first SELECTing
all the data?

Is there an array push/pop function? Is there a method of counting the
number of integers in an array?

Thanks for any help you can provide - indexable integer arrays are an
incredible feature, thanks for the hard work!

Ryan Mahoney