python中带有数组/列表的基本循环概念

Question

i know this will be a very novice question to ask here but i kind of blank out and need this to get resolved immediately.

I want to change values from S -> B and so on as mention in below code. For that i can create array or list "A = [S, B, N]" or whatever required. But need to traverse in this manner.

S --> B 
B --> S
S --> N
N --> B
B --> N
N --> S

I tried creating 2 lists A and B with same values [S, B, N] and traversing through both of them with

i=0;i<length.A;i++
   j=0;j<length.B;j++

and

i=0;i<length.A;i++
   j=i;j<length.B;j++

But in scenario 1: S changed to B, then it tries changes from S -> N. Which is not possible. In Scenario 2: We hit 4 out of 6 but not all 6 required outputs.

I could've researched more but due to lack of time, i couldn't.

Also I've written the code in pseudo manner.

Answer 1

As I understand, you have a specific sequence of states: S -> B -> S -> N -> B -> N -> [begin again]

This could be solved with a generator such as:

import itertools
state = itertools.cycle(['S', 'B', 'S', 'N', 'B', 'N'])

Subsequent calls of next(state) will yield the list in sequence, starting at the beginning when it reaches the end. Example:

>>> next(state)
'S'
>>> next(state)
'B'
>>> next(state)
'S'
>>> next(state)
'N'
>>> next(state)
'B'
>>> next(state)
'N'
>>> next(state)
'S'
>>> next(state)
'B'
>>> next(state)
'S'
>>> next(state)
'N'
>>> next(state)
'B'

Answer 2

I think this should answer your question, I know my list that changes the letters isn't exactly the same, but I think from here you should be able to implement it.

It changes each s to b, b to n and n to s.

lettersToChange = ['s','b','n']

randomLetters = ['k','l','m','n','s','s','b','n','b','n','s','b','n','b','s']
for i in range(len(randomLetters)):
    if randomLetters[i] in lettersToChange:
        pos = lettersToChange.index(randomLetters[i])
        randomLetters[i] = lettersToChange[(pos+1)%3]

So before and after of the randomLetters list:

 ['k', 'l', 'm', 'n', 's', 's', 'b', 'n', 'b', 'n', 's', 'b', 'n', 'b', 's']
 ['k', 'l', 'm', 's', 'b', 'b', 'n', 's', 'n', 's', 'b', 'n', 's', 'n', 'b']

Answer 3

If i understand correctly, you must have an index, because you have to tell apart, which S or B or N should advance to it's next state. Something like that:

TRAVERSE_ORDER = "SBSNBN"
EXAMPLE_INPUT = ["S", "B", "N"]

def traverse_single_item(state, state_index=0):
    next_state = TRAVERSE_ORDER.find(state, state_index) + 1
    if next_state == len(TRAVERSE_ORDER):
        next_state = 0
    return (TRAVERSE_ORDER[next_state], next_state)

new_state = [traverse_single_item(*elem) for elem in EXAMPLE_INPUT]
print(new_state)

newer_state = [traverse_single_item(*elem) for elem in new_state]
print(newer_state)

newest_state = [traverse_single_item(*elem) for elem in newer_state]
print(newest_state)

If you call the function without an index, it assumes to be the first occurence in the traverse order. Please note the asterisk: *elem. It unpacks the tuple for the function call.