Loop through python array, set specific values to a specified column

The problem seems to stem from the way my_array was initialized. Each row is 'nested' with the other rows, by that I mean if you try to modify my_array[0], you will end up modify all the remaining rows. Here is an example to illustrate that:

my_array[0].insert(0, 'hello_world')
# this prints:
# [['hello_world', 0, 0, 0],
   ['hello_world', 0, 0, 0],
   ....
   ['hello_world', 0, 0, 0]]

To get around, this you can use numpy to initialize your array of zeros or do something like this:

my_array = [[0, 0, 0, 0] for i in range(18)]

To make the code slight more concise and print the expected output, you can then do the following:

for i in range(len(person_list)):
    # person1 : idx 0, 1, 2
    # person2 : idx 3, 4, 5
    # ...
    end_idx = (i + 1) * 3
    start_idx = end_idx - 3

    for sub_arr in arr[start_idx:end_idx]:
        sub_arr.insert(0, person_list[i])

Hope this helped!

You could use the modulo % to increment at certain intervals. For example, row%3==0 will return True at multiples of 3. FYI in your above code you were not resetting the row variable every interval of 3.

person_list = ["person1", "person2","person3","person4","person5","person6"]
my_array = [[0]*4]*len(person_list)*3

j=0
i=0
for row in my_array:
    if j%3==0:
        row[0] = person_list[i]
        i+=1
    j+=1 # increment the row here

I think everything is good in your code, you are just multiplying it by 3 :

person_list = ["person1", "person2","person3","person4","person5","person6"]
my_array = [[0]*4]*len(person_list)

i=0
j=1
for row in my_array:
    while j <= 3:
        row[0] = person_list[i]
       j+=1
    j=1
    i+=1

I think its this like which needs to change from my_array = [[0]*4]*len(person_list)*3 to my_array = [[0]*4]*len(person_list)

So I think NeuralX was helpful in pointing out the way I create the array is not going to work for what I intend to do to it. He is absolutely correct when I run a change it will update every entry in the final array.

In more searching with working with nested lists, I have found how I should create the array so that I can update nested lists individually by looping through it. The final results are going to be below.

person_list = ["person1", "person2","person3","person4","person5","person6"]

# The way I create the array appears the be the main root cause here. I need to create it like this
my_array = [[0 for x in range(4)] for y in range(sum(person_assessments))]

i=0
j=0
for person in person_list:

    for assessment in range(1, 4):
        my_array[i][0] = person
        # print(person, ", ", assessment)
        # print(i,", ", j,", ", person)

        i+=1

    j += 1

print(my_array)