9.18. Optional parametersΒΆ
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:
The call 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
understand how find2
works. Try it.
Better still, we can combine find
and find2
using an
optional parameter.
The call find3('banana', 'a', 2)
to this version of find
behaves just
like find2
, while in the call find3('banana', 'a')
, start
will be
set to the default value of 0
.
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 end
is
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 start
and end
in this function are precisely the same as they are in
the range
function.