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])

• Tai: 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.

Answer 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.

• eddiewastaken: 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: 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: Brilliant. Thanks a lot

Answer 2

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!

• eddiewastaken: Hi, this works well but I need the 'unpicked' list as I need to do this without replacement.
• zenwraight: What do you mean by replacement ? In the above example also you are not replacing anything ...
• zenwraight: Also the above example is saving you space, which u did by creating a new array numbers

Answer 3

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)]