Help with this code

Peter Otten __peter__ at
Mon Jan 9 10:59:17 EST 2017

José Manuel Suárez Sierra wrote:

> This is the traceback:
> line 18, in <module>
> for transf2[j] in transf2:
> IndexError: list assignment index out of range
> If I have initialized j=0 (such as i) why does it not work?

A for loop

for x in y:

sequentually assigns every value in y to x. So with y = "ab" it executes

x = "a"
x = "b"

This is the case even when there is something more complex like transf2[j]:

for transf2[j] in y:

translates to

transf2[j] = "a"
...  # both transf2 and j may be rebound here and thus affect
transf2[j] = "b" # what this actually does

and thus can fail with an IndexError when j is bound to a value greater than 
the length of the transf2 list. 

But most likely you want to avoid this feature of Python -- even the experts 
never use it.

> I want the script to read sequence 1 and compares every element inside it
> with elements in sequence 2 no mattering where it matches.

That may be sufficient to explain the objective to a human who already has a 
rough idea of your goal, but to come up with an actual algorithm you need to 
think about -- and communicate -- a lot more detail.

There is a third party module that operates on strings, not lists which 
finds the edit operations

>>> Levenshtein.editops("abcdefgj", "axcdyezfg")
[('replace', 1, 1), ('insert', 4, 4), ('insert', 5, 6), ('delete', 7, 9)]

What should your function return given the equivalent lists

['a', 'b', 'c', 'd', 'e', 'f', 'g', 'j']
['a', 'x', 'c', 'd', 'y', 'e', 'z', 'f', 'g']

? Try to make your plain-english description as precise as possible before 
you start coding.

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