Indexing a list, the safe way

[bartonc]

Whenever a function retrieves a list element outside of loop, you should wrap the reference in a try block instead of testing the length of the list (because you will probably forget to subtract one in your test, introducing a but). For example:
These are safe:

 
# inside a loop
for item in someList:
    # use items from list sequentially
 
try:
    item = someList[index]
except IndexError:
    # handle out-ot-range index
 
if index < len(someList):
    item = someList[index]
else:
    # handle out-of-range index
 

This is a bug that will give an error sometimes:

 
if index > len(someList): # should be len(someList) - 1
    # handle out-ot-range index
else:
    item = someList[index]
 

As you can see, both the if block and the try block have the same number of lines and structure, but try reduces the number of references to someList, doesn't need to call len() and protects you from a simple mistake.

[kudos]

Hi,
its not a bug the indexing in languages like python, C, java indexes from 0 as the first. So, in a = [1,2,3] elements a[2] = 3, but is still contains 3 elements..
If I recall correctly pascal & matlab indexes from 1 (which actually is more "mathematical".

-kudos

Whenever a function retrieves a list element outside of loop, you should wrap the reference in a try block instead of testing the length of the list (because you will probably forget to subtract one in your test, introducing a but). For example:
These are safe:

 
# inside a loop
for item in someList:
    # use items from list sequentially
 
try:
    item = someList[index]
except IndexError:
    # handle out-ot-range index
 
if index < len(someList):
    item = someList[index]
else:
    # handle out-of-range index
 

This is a bug that will give an error sometimes:

 
if index > len(someList): # should be len(someList) - 1
    # handle out-ot-range index
else:
    item = someList[index]
 

As you can see, both the if block and the try block have the same number of lines and structure, but try reduces the number of references to someList, doesn't need to call len() and protects you from a simple mistake.

[bartonc]

Hi,
its not a bug the indexing in languages like python, C, java indexes from 0 as the first. So, in a = [1,2,3] elements a[2] = 3, but is still contains 3 elements..
If I recall correctly pascal & matlab indexes from 1 (which actually is more "mathematical".

-kudos

But len() returns the lenght of the object which IS one greater than the index of the last item: len([1,2,3]) = 3

[kudos]

yeah, I know. I guess it would make more sense if indexing started at 1, so one could get the last elements by doing a len, but for some reason indexing start at 0. I guess its just tradition, C index start at 0, so then everybody else do it too...(but its true, ALOT of bugs had been avoided we started at 1)

-kudos

But len() returns the lenght of the object which IS one greater than the index of the last item: len([1,2,3]) = 3