how to multiply each element in a string with python

Question

I have done some writing like below

new_list = ["000","122","121","230"]

for item in new_list:
   r = [np.matrix([[0.5,0.5],[0,1]]),
        np.matrix([[0.5,0.25],[0,1]]),
        np.matrix([[0.5,0.25],[0,1]]),
        np.matrix([[0.5,0],[0,1]])]

   (W0,W1,W2,W3) = r

   for i in item:
          i == 0
          if item[i] == 0:
             return W0
          elif item[i] == 1:
             return W1
          elif item[i] == 2:
             return W2
          else:
             return W3
          i = i + 1

list_new = [np.matrix([1,0]) * new_list * np.matrix([0.5],[1])]

print(list_new)

I want to create a program for example when the input from the list is "122", it will multiply each element inside "122" as matrix W1*W2*W2. The final result should be like

np.matrix([1,0]) * new_list2 * np.matrix([0.5],[1])

The

new_list2

here should be all multiplication result from the new_list. Meaning

new_list2 = [W0*W0*W0,W1*W2*W2,...]

I run this code and keep getting a syntax error saying

"return" outside function

I want to ask did I missed something here or there is other function/way to make it done using python. I am using Python 3.5 Thank you for the answer.

bruno desthuilliers · Accepted Answer · 2016-01-18 19:00:12Z

1

As the error message says, you are using "return" outside a function, which just doesn't make sense since "return" means "exit the function now and pass this value (or None if you don't specify a value) to the caller". If you're not inside function there is no caller to returns to...

answered Jan 18, 2016 at 19:00

bruno desthuilliers

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1 Comment

Ling Over a year ago

ok...thanks for your answer. i will try to search more about the use of return in python

AkiRoss · Accepted Answer · 2016-01-18 19:00:53Z

Something like this?

import numpy as np
from functools import reduce

new_list = ["000", "122", "121", "230"]

matrices = [
    np.matrix([[0.5,0.50],[0,1]]),
    np.matrix([[0.5,0.25],[0,1]]),
    np.matrix([[0.5,0.25],[0,1]]),
    np.matrix([[0.5,0.00],[0,1]])
]

def multiply(s):
    mt_seq = (matrices[i] for i in map(int, item))
    return reduce(np.multiply, mt_seq) # or reversed(mt_seq)

for item in new_list:
    print(multiply(item))

You can just convert each digit in the string with int(digit), e.g. int('2'), and do it for every element of the string with map(int, '01234').

Then, it's just a matter of taking those matrices and multiplying them. (Sorry, I am not familiar with np.multiply and np.matrix and maybe you need to reverse the order of multplication)

reduce(fun, seq) applies fun to seq, like this: fun(fun(fun(seq[0], seq[1]), seq[2]), ...)

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