9.18. Optional parameters¶
To find the locations of the second or third occurrence of a character in a
string, we can modify the
find function, adding a third parameter for the
starting position in the search string:
find2('banana', 'a', 2) now returns
3, the index of the first
occurrence of ‘a’ in ‘banana’ after index 2. What does
find2('banana', 'n', 3) return? If you said, 4, there is a good chance you
find2 works. Try it.
Better still, we can combine
find2 using an
find3('banana', 'a', 2) to this version of
find behaves just
find2, while in the call
start will be
set to the default value of
Adding another optional parameter to
find makes it search from a starting
position, up to but not including the end position.
The optional value for
end is interesting. We give it a default value
None if the
caller does not supply any argument. In the body of the function we test what
and if the caller did not supply any argument, we reassign
end to be the length of the string.
If the caller has supplied an argument for
end, however, the caller’s value will be used in the loop.
The semantics of
end in this function are precisely the same as they are in