python for loop index

Question

i have two function below with two for loops. in the first one i need the index [i] for both inputs a[i] * b[i], while in the second I need it next to output as well as next to matrix[i].

Why, what is the logic behind these indexing reference [i]?

If I do not index matrix with [i] I get this

error

def w_sum(a,b):
      output = 0 
      assert(len(a) == len(b))
    
      for i in range(len(a)):
        output += (a[i] * b[i])
    
      return output


def vec_mat_mul(vector, matrix):
  output = [0, 0, 0]
  assert(len(vector) == len(matrix))

  for i in range(len(vector)):
    output[i] = w_sum(vector, matrix[i])
  
  return output

here are the input variables and dependent function w_sum:

#dataset at the beginning of a game
toes = [8.5, 9.5, 9.9, 9.0]
wlrec = [0.65, 0.8, 0.8, 0.9]
nfans = [1.2, 1.3, 0.5, 1.0]

#inserting one input datapoint of each variable
input = [toes[0], wlrec[0], nfans[0]]

#defining weights
weights = [[0.1, 0.2, -0.1],
           [-0.1, 0.1, 0.9],
           [0.1, .04, 0.1]]

Might be a very mundane question but I need to get the logic to move on.

Thanks!

If you want to multiply matrices, I suggest you use numpy arrays rather than Python lists — OneCricketeer
– OneCricketeer, Commented Apr 9, 2021 at 13:33
@OneCricketeer I want to create it without numpy first to understand the logic. I am calling vec_mat_mul here: def nn_mul_in_out(input, weights): pred = vec_mat_mul(input,weights) return pred neural_output = nn_mul_in_out(input, weights) print(neural_output) — jay_ee
– jay_ee, Commented Apr 9, 2021 at 14:05
@interjay no I am asking why i need to refer to [i] in the for loop of the vec_mat_mul function here: matrix[i] — jay_ee
– jay_ee, Commented Apr 9, 2021 at 14:08

dyabmo · Accepted Answer · 2021-04-09 17:52:17Z

3

TLDR; removing [i] from matrix means you multiplying a list (sequence) of lists with a list of floats, which gives the error message.

I am assuming you are calling vec_mat_mul(input, weights). You are multiplying a 1d vector (list of floats) by a 2d matrix (list of lists).

Now for the line output[i] = w_sum(vector, matrix[i]): If you remove the index [i] from matrix, it means that you are passing a matrix as a list of lists.

For the line output += (a[i] * b[i]) inside def w_sum(a,b): It will perform an element-wise multiplication between floats of the first list and lists of the second list which is undefined, and therefore you will get the error: TypeError: can't multiply sequence by non-int of type 'float'. The sequence here is the matrix which is a sequence of lists.

Now for the correct behavior, if you pass matrix[i] in output[i] = w_sum(vector, matrix[i]) it means you are passing only an element of the matrix which is a list of floats, therefore you are doing an element-wise multiplication of a list of floats with another list of floats which is what we expect.

edited Apr 9, 2021 at 17:52

answered Apr 9, 2021 at 14:25

dyabmo

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1 Comment

jay_ee Over a year ago

"it means you are passing only an element of the matrix which is a list of floats, therefore you are doing an element-wise multiplication of a list of floats with a list of floats which is what expected." thank you so much, this clears is up perfectly in my head!

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