Question

Ok, this is bizarre. I don’t understand and it’s getting extremely frustrating.

1. I declare and array (list)
2. I initiate the first row with strings (stock symbols)
3. I populate the rest with zeros for the required number of trading days.

4. The I want to populate certain elements of the ‘positions’ matrix with a number for stocks to be purchased or sold on that day. And instead of populating only the one element, the entire column populates.

   # Initiating position matrix
   positions = []
   # Initiating a row of zeros (to fill position matrix later)
   empty_row = []
   # Symbols is a list of symbols that will be traded.
   for symbol in symbols:
       empty_row.append(0)
   
   # First row of positions will be symbols
   positions.append(symbols)
   
   # All other rows will be empty_rows
   for day in timestamps:
       positions.append(empty_row)
   

   Then I will determine what element of “positions” I need to populate. Let’s say the element on the 10th row and 1st column (indexing from zero). So i do:

   positions[10][1] = 100

   The result I get is that the ENTIRE column 1 is full of 100s. Not just the element. What am I doing wrong?

Answer 1

You’re appending the same empty list over and over:

for day in timestamps:
    positions.append(empty_row)

You need to create a new “empty” list for each new row:

for day in timestamps:
    positions.append([0]*len(symbols))

You can put all that into a list comprehension, so your entire code would become

positions = [symbols]
positions.extend([[0]*len(symbols) for day in timestamps])