targetted array element modification or delete without knowing the index?

Is there a way to modify or delete an element of an array with knowledge only of the element's value? Maybe an array index finder would help?

For example

create table foo (name,text, arr text[]);
insert into foo (name,arr) values ('joe',ARRAY['a','b','c']);

update foo set arr[indexfinder('b')] = 'z';

This would update arr[2], because that's the element with value 'b'

Thanks !