Really weird python error

Question

...
def splitMunipulation(p,threshold=5000):
    runs=[];i=0
    while i<len(p):
        l=[];i+=1
        print i,p[i]
        while p[i]!=press(0,1,0):
            l.append(p[i]);i+=1
        else:       
            runs.append(l)#here i points to another (0,1,0)
    return runs
...

record=splitMunipulation(record)

'''

Output:

    1 <__main__.press instance at 0x046690A8>
      File "H:\mutate.py", line 28, in splitMunipulation
        while p[i]!=press(0,1,0):
    IndexError: list index out of range

pressis a class

and since print p[i] works well,why p[i] is considered out of range?

Really don't get what's going on

'''

The traceback you've posted doesn't match any line of your code. — user2357112
– user2357112, Commented Apr 6, 2014 at 4:43

Lily Mara · Accepted Answer · 2014-04-06 04:52:29Z

3

so, a few things..

Firstly, your code is very... unpythonic. This isn't C, so you don't need to use while loops for iteration, and don't use semicolons to separate multiple commands on one line in Python. Ever. Also, the while...else format is confusing and should be avoided.

If you look at the first few 'lines' of your while loop,

while i<len(p):
        l=[];i+=1

You keep i below the length of p, but you immediately increase i's value by one. As such, when i=len(p) - 1, you will make i one larger, len(p). So when you try to access p[i], you are trying to access a value that doesn't exist.

Fixing those issues, you would get:

...
def splitMunipulation(p,threshold=5000):
    runs=[]

    for i in p:
        l=[]
        print i
        if i != press(0,1,0):
            runs.append(i)
    return runs
...

record=splitMunipulation(record)

edited Apr 6, 2014 at 4:52

answered Apr 6, 2014 at 4:33

Lily Mara

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2 Comments

James King Over a year ago

I would add don't use the while . . . else construct. Even Guido has said it's a bad idea.

Lily Mara Over a year ago

I just realized that this would cause an infinite loop if i was not equal to press(0, 1, 0), so i substituted if for while. Should still give desired output... Looking at it now, I'm not really sure how this code is supposed to work.

James King · Accepted Answer · 2014-04-06 04:29:31Z

2

while p[i]!=press(0,1,0):
   l.append(p[i]);i+=1

The variable i gets incremented in this loop until p[i]!=press(0,1,0). Since nothing is happening to make p longer, or to test that i is not greater than the length of p, it is easy to see how the index could get out of range.

answered Apr 6, 2014 at 4:29

James King

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1 Comment

James King Over a year ago

Your print i, p[i] statement is outside the while loop. i is in range entering the loop, but seems to go out of range inside the while loop.

Novel · Accepted Answer · 2014-04-06 04:28:02Z

0

len returns the length, not the last index. If l=[1,2,3], then len(l) returns 3, but l[3] is out of range.

so you should use

while i<len(p)-1

or better yet:

for i in range(len(p)):

answered Apr 6, 2014 at 4:28

Novel

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3 Comments

Victory Over a year ago

while i < len(p): p[i] is fine while i <= len(p): p[i] would throw and error. But @sqd's error is increment i++ in between the two loops, so it would be like the above but with p[i++]

Lily Mara Over a year ago

This should be avoided in Python, if you really need to get at the numbers, use for i in enumerare(p)

sqd Over a year ago

well,if i=3,than i<3 is false.So the loop end

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