Changing Value of One Index in a Nested List (Python)

Question

I am new to Python (and coding) and thought I had a reasonable grasp on the structure but I have been stuck on this one. I want to change the first value of a nested list and then update the value for the next position in the list (e.g., creating grid coordinates as nested lists).

p_d = 3

passes = 1
grid = []
row = []
column = [0, 0, 0]

while passes <= p_d:
    row.append(column)
    grid.append(row)
    passes += 1

for i in range(len(row)):
    column[i] = -(p_d - 1) / 2 + i

print(row)

The result is this:

[[-1.0, 0.0, 1.0], [-1.0, 0.0, 1.0], [-1.0, 0.0, 1.0]]

But what I really need SHOULD be something like this:

[[-1.0, 0, 0], [0.0, 0, 0], [1.0, 0, 0]]

No, it shouldn't. You've appended the same list, column, to the outer list row, three times. So you should expect to see the same list three times... — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jun 13, 2017 at 4:55

BrenBarn · Accepted Answer · 2017-06-13 04:03:45Z

1

By doing row.append(column) and grid.append(row), you are putting the same row and column objects into your matrix several times.

Instead, move the creation of row and column (e.g., the row = ... and column = ...) lines inside the loop so that you create new values on each iteration.

answered Jun 13, 2017 at 4:03

BrenBarn

253k39 gold badges421 silver badges392 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ishaan · Accepted Answer · 2017-06-13 04:20:10Z

0

You are appending the same list object to row when you do row.append(column) because column has been created only once globally. So changing any one column will change all the columns because they are the same list object. Same goes for rows

Move the line column = [0,0,0] and row = [] inside the for loop:

p_d = 3

passes = 1
grid = []

while passes <= p_d:
    row = []
    column = [0, 0, 0]
    row.append(column)
    grid.append(row)
    passes += 1


for i in range(len(grid)):
    grid[i][0][0] = -(p_d - 1) / 2 + i

print(grid)

edited Jun 13, 2017 at 4:20

answered Jun 13, 2017 at 4:10

Ishaan

8866 silver badges12 bronze badges

2 Comments

BrenBarn Over a year ago

You also need to move row = [] inside the loop, or you'll have the same problem with the rows.

broncosaurus Over a year ago

thanks much, saves me tons of time banging my head against the wall!

Collectives™ on Stack Overflow

Changing Value of One Index in a Nested List (Python)

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related