Python: Is There a builtin that works similar but opposite to .index()?

Question

Just a forewarning: I just recently started programming and Python is my first language and only language so far.

Is there a builtin that works in the opposite way of .index()? I'm looking for this because I made a bool function where I have a list of ints and I want to return True if the given list of ints is a list of powers of some int x of the form [x^0, x^1, x^2, x^3, ...] and `False' otherwise.

What I want to say in code is along the lines of:

n >= 1  
while the position(n+1) = position(1)*position(n) 
    for the length of the list
    return True
otherwise 
    False.

Is there a builtin I could use to input the position and return the item in the list?

list = [1,2,4,8,16]
position(4)

returns the int 16.

EDIT: sorry I don't know how to format on here ok ill show what I mean:

def powers(base):
''' (list of str) -> bool
Return True if the given list of ints is a list of powers of 
some int x of the form [x^0, x^1, x^2, x^3, ...] and False 
otherwise.
>>> powers([1, 2, 4, 8]) 
True
>>> powers([1, 5, 25, 75])
False
'''

FINAL EDIT:

I just went through all of the available list methods from here (https://docs.python.org/2/tutorial/datastructures.html) and read the descriptions. What I'm asking for, isn't available as a list method :(

sorry for any inconvenience.

How about `list[4]`? I would recommend reading through the basics, see e.g. http://sopython.com/wiki/What_tutorial_should_I_read%3F — jonrsharpe, Nov 02 '16 at 22:29
I tried that already but I can't make it general enough to apply for all items. — theoreticalmasterrace, Nov 02 '16 at 22:34
No. The literal answer to your question is `thing[index]` as shown above. If you've unsuccessfully *"tried that already"*, show a [mcve] of the code (not just the docstring) in which you did so. If you don't know how to format, read the help. — jonrsharpe, Nov 02 '16 at 22:47
@jonrsharpe My goal was to write the function using a while loop — theoreticalmasterrace, Nov 02 '16 at 22:51
...OK? I don't see how that's at all relevant. Please learn [ask] and actually provide what you've repeatedly been asked for. — jonrsharpe, Nov 02 '16 at 22:53

Moinuddin Quadri · Accepted Answer · 2016-11-02T23:01:14.463

-1

As an answer to:

Is there a builtin I could use to input the position and return the item in the list?

You just need to access the list with it's index as:

>>> my_list = [1,2,4,8,16]
>>> my_list[4]
16  # returns element at 4th index

And, this property is independent of language. All the languages supports this.

Based on your edit in the question, you may write your function as:

def check_value(my_list):
    # if len is less than 2
    if len(my_list) < 2:
        if my_list and my_list[0] == 1:
            return True
        else:
            return False 
    base_val = my_list[1] # as per the logic, it should be original number i.e num**1 
    for p, item in enumerate(my_list):
        if item != base_val ** p: 
            return False
    else:
        return True

Sample run:

>>> check_value([1, 2, 4, 8])
True
>>> check_value([1, 2, 4, 9])
False
>>> check_value([1, 5, 25, 75])
False

edited Nov 02 '16 at 23:01

answered Nov 02 '16 at 22:35

Moinuddin Quadri

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There's likely a lot more to this than just indexing into the list. – Makoto Nov 02 '16 at 22:36
1

But the question is specific to: *Is there a builtin I could use to input the position and return the item in the list?* Else is what OP is trying to achieve – Moinuddin Quadri Nov 02 '16 at 22:36
lol alot more huh? ... he says something along those lines in the comment but from what hes given us I would say this is the answer (although not really worthy of an answer) – Joran Beasley Nov 02 '16 at 22:37
`check_value([1])` – Joran Beasley Nov 02 '16 at 22:58
1

@JoranBeasley handled the edge scenario. – Moinuddin Quadri Nov 02 '16 at 23:01
Obviously it could be optimised but considering the skill-set of OP, I want to keep this simple : – Moinuddin Quadri Nov 02 '16 at 23:07
Thank you! I was able to make what I wanted, using this as a rough guide – theoreticalmasterrace Nov 03 '16 at 23:14

Joran Beasley · Answer 2 · 2016-11-02T22:53:15.197

-1

def powers(n):
    for i in itertools.count(0):
        yield n**i


def is_powers(li):
   if li[0] == 1:
      if len(li) > 1:
          return all(x==y for x,y in zip(li,powers(li[1])))
      return True
   return False

is_powers([1, 2, 4, 8])
is_powers([1, 5, 25, 75])

maybe ... its really not clear what you are asking... this assumes that it always must start with a 1 if it is valid...

edited Nov 02 '16 at 22:53

answered Nov 02 '16 at 22:40

Joran Beasley

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Python: Is There a builtin that works similar but opposite to .index()?

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