Question

In the following code, when I print the row in the first loop after doing a bunch of manipulations with it I see the results that I want. However, after I exit the first loop, I find that I get a different result in the variable Dataset. I know this is a scoping issue but I cannot figure out what the problem is and how to get my desired result which is shown with the first “print” statement. Thanks for your help

import random
random.seed(1234567)
Key=[[.5,.5]]
Dataset=[[0]+[0]*1]*int(10/2) +[[1]+[0]*1]*int(10/2)

print "results I need"
for row in Dataset:
    response=row[0]
    for i in xrange(len(Key)):
        if random.random() < Key[i][response]: 
            row[i+1]=response
        else: 
            row[i+1]=1-response
    print row

print "Results I get"
for row in Dataset:
    print row

Answer 1

• Python has reference semantics. Your original Dataset contains multiple references to the common [0, 0] and [1, 0] sublists. (I’m also at a loss to understand why you’re making the initialization so complex.)

You could fix this by using [[0, 0] for i in xrange(5)] + [[1, 0] for i in xrange(5)].

• The rest of your logic is also needlessly complex. We can use a conditional expression to merge the two assignments together, and we should be using direct iteration instead of iterating over an artificial list of indices (xrange).

That looks like:

for row in Dataset:
    response = row[0]
    for k in Key:
        row[i + 1] = response if random.random() < k[response] else 1 - response

• However, this is still not the solution we’re looking for. There’s really no reason to put all those zeros in the initialization data that will be overwritten and never read in the loop. The [i + 1] indexing is also clumsy. A simpler solution: start with a list of just the first values for the sublists, and within the loop, construct the rest of each sublist (we can do this with a list comprehension) and prepend the first element. Instead of replacing the elements of the Dataset in place, we can use another list comprehension to bind them together.

That gives us:

import random
random.seed(1234567)
Key=[[.5,.5]]
Initial_Conditions=[0]*int(10/2) +[1]*int(10/2)

Dataset = [
    [i] + [
        i if random.random() < k[i] else 1 - i
        for k in Key
    ]
    for i in Initial_Conditions
]