2D array indexing error in Python

Question

I'm playing with 2D arrays to make a random sequence generator. I'm trying to generate 10 lists of 6 integers each, without replacement, inside a 2D array. The integers allowed are between 0 and 100. However, I keep getting list index out of range errors, and can't work out why. Here's my code:

import random

numbers = [i for i in range(100)]
picked = []
unpicked = []

lines = [[0 for x in range(6)] for j in range(10)]

for i in range(10):
    for j in range(6):
        unpicked = [x for x in numbers+picked if x not in picked]
        lines[i][j] = unpicked[random.randint(0,99)]
        picked.append(lines[i][j])

When picked grows its size, unpicked will get smaller and you use randint(0, 99) which might generate numbers that is bigger than unpicked's size. Thus, an error is thrown. — Tai
– Tai, Commented Jan 5, 2018 at 0:02

Tai · Accepted Answer · 2018-01-05 00:09:38Z

1

Your should shrink the size that random.randint can choose from. Based on your application, I think it should be

random.randint(0, len(unpicked)-1)

Another thing is that in the following line

unpicked = [x for x in numbers+picked if x not in picked]

you don't need numbers+picked. Just put numbers would be enough because numbers always have from 0 to 99.

answered Jan 5, 2018 at 0:09

Tai

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3 Comments

eddiewastaken Over a year ago

I've changed my unpicked to unpicked = [x for x in lines[i]+picked if x not in picked] as I want to generate sequences without replacement. Also, random.randint(0, len(unpicked)-1) gives me an empty range for randrange() error

Tai Over a year ago

Your unpacked should be unpicked = [x for x in numbers if x not in picked] unpicked means what is in numbers but not in picked. Thus, it should be this way. I think now the second error will be resolved.

eddiewastaken Over a year ago

Brilliant. Thanks a lot

zenwraight · Accepted Answer · 2018-01-04 23:59:21Z

1

I think you need something like this

import random

numbers = [i for i in range(100)]
picked = []
unpicked = []

lines = [[0 for x in range(7)] for j in range(11)]

for i in range(10):
    for j in range(6):
        #unpicked = [x for x in numbers+picked if x not in picked]
        x = random.randint(0,99)
        while(x in picked):
          x = random.randint(0,99)
        lines[i][j] = x
        picked.append(lines[i][j])

your logic of picking number is getting out of index.

Hope this helps!

answered Jan 4, 2018 at 23:59

zenwraight

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3 Comments

eddiewastaken Over a year ago

Hi, this works well but I need the 'unpicked' list as I need to do this without replacement.

zenwraight Over a year ago

What do you mean by replacement ? In the above example also you are not replacing anything ...

zenwraight Over a year ago

Also the above example is saving you space, which u did by creating a new array numbers

Hao Li · Accepted Answer · 2018-01-05 00:43:44Z

0

There is an easy way:

numbers = range(101)  # 0 ~ 100 (include 100)
random.shuffle(numbers)  # in place
pickup_num = 60
n = 6
picked = numbers[:pickup_num]
lines = [picked[i:i+n] for i in range(0, pickup_num, n)]

answered Jan 5, 2018 at 0:43

Hao Li

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