Python - nested for loops and index out of range

Question

I'm rather new to Python, and am running into the following error when using a nested for loop

IndexError: list index out of range

Here is my code

count = 0
count2 = 1
rlength = range(len(records))
for i in rlength:
    ex = records[count].id
    for bla in rlength:
        if re.search(ex, records[count2].id) != None:
            print records[count2].id
        count2 += 1
    count += 1
    count2 = count + 1

Edit:

Fixed with the following code

rlength = range(len(records))
for i in rlength:
    ex = records[i].id
    for bla in rlength:
        if bla + 1 < len(rlength) and re.search(ex, records[bla + 1].id) != None:
            print records[bla].id

I think here is problem records[count2].id), you are moving one step forward in it. — Rakesh Kumar
– Rakesh Kumar, Commented Jul 20, 2016 at 9:49
I think you have to change this line from if re.search(ex, records[count2].id) != None: to if count2 in records and re.search(ex, records[count2].id) != None: — Rakesh Kumar
– Rakesh Kumar, Commented Jul 20, 2016 at 9:50
I don't think you need count and count2 at all. You can just use i and bla instead and your for loops will always remain within the range. — Chris_Rands
– Chris_Rands, Commented Jul 20, 2016 at 10:00

Chris_Rands · Accepted Answer · 2016-07-20 10:24:13Z

1

If I understand what you are trying to do, I'm not sure you need the count and count2 at all. I think you can just use the numbers generated by your loop. I suggest using enumerate() instead of range(len()).

for i1,rlength1 in enumerate(records):
    ex = rlength1.id
    for i2,rlength2 in enumerate(records):
        if re.search(ex, rlength2.id) != None:
            print rlength2.id

edited Jul 20, 2016 at 10:24

answered Jul 20, 2016 at 10:05

Chris_Rands

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2 Comments

Thistle19 Over a year ago

This has helped, although the reason I implemented counts was because I wanted to compare record x with x+1, x+2, ... rather than itself. But this has pointed me in the right direction. Thanks

Chris_Rands Over a year ago

@Thistle19 You can of course do something like records[i] and records[i+1], records[i+2], but if you do that you need to stop the loop before you go past it to avoid the IndexError, either by using something like range(len(records)-2) or by doing an if i+1 == len(records): break

Rakesh Kumar · Accepted Answer · 2016-07-20 09:58:58Z

0

The loop should fail for i=1. Why ?
When i=1, count2 = 2 (= count + 1, and count = 0+1 = 1)
In the inner loop, count2 goes from 2 to 2+len(records)-1-1 (since we increment after looking at the values) = len(records)
But there is no records[len(records)] (and an index out of bound is NOT equivalent to None in python !)

edited Jul 20, 2016 at 9:58

Rakesh Kumar

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answered Jul 20, 2016 at 9:55

HolyDanna

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