How to use variable in name in python

Question

I have 1000 variables, a1,a2,a3,a4,a5 and so on. Is there a way to initialize all to value 100 using loop

My attempt:

for i in range(1,1001):
    "a"i=100

I am missing basic syntax here.

http://stackoverflow.com/questions/521674/initializing-a-list-to-a-known-number-of-elements-in-python might be of use. — bentank, May 02 '15 at 05:48
@TanveerAlam..You can imagine a chess baord layout with each block having name as a1,a2..a8,b1.....h8. Now i dont want to write 64 different lines like a1=100,a2=100 etc. I want to do it by loop. — Saurabh Shrivastava, May 02 '15 at 05:55
@user3388005 use a 2D list for this. Or if you have to, a dict of lists. — Adam Smith, May 02 '15 at 05:56
This question has so many duplicates that I couldn't find which one is the canonical version, if there even is one. See [Why you don't want to dynamically create variables](http://stupidpythonideas.blogspot.com/2013/05/why-you-dont-want-to-dynamically-create.html) for links to 11 more dups. — abarnert, May 02 '15 at 06:29
As a side note, if you have non-trivial 2D arrays of things, you probably want to look at [NumPy](http://www.numpy.org/) or [Pandas](http://pandas.pydata.org/). — abarnert, May 02 '15 at 07:11

Adam Smith · Answer 1 · 2015-05-02T06:12:35.950

Don't ever ever do this. You gain absolutely nothing from doing it this way. How can you possibly REFER to these variables dynamically if you're setting them this way? Use a list or a dictionary instead.

a_list = [None] + [100] * 1000
# `a_list[n]` in place of `an`, here

a_dict = {num:100 for num in range(1, 1001)}
# `a_dict[n]` in place of `an`, here

edit for your chess board analogy:

import string

class ChessBoard(list):
    def __init__(self, side_length, initial_value=None):
        super().__init__()
        for _ in range(side_length):
            self.append([initial_value for _ in range(side_length)])

    def get(self, notation):
        """Maps chess notation to the 2D list"""

        indices = dict([(lett, num) for num,lett in 
                        enumerate(string.ascii_letters, start=1)])
        # {'a':1, 'b':2, ... 'Z':52}
        x, y = indices[notation[0]]-1, int(notation[1])-1
        return self[y][x]

board = ChessBoard(1000, 100)  # how do we index the 1000th...nevermind
board.get("a6")

Rather than sticking a `None` at the start of the list, it's a better idea to just get used to 0-indexing. `None`-padding creates awkward issues like `len` being one higher than the number of logical elements of the list. — user2357112, May 02 '15 at 06:21
@user2357112 agreed, which is why I implicitly converted from zero-indexing in the elegant solution. However if he's going to try and reference an element like you would a chess board, he needs to have 1-indexing. — Adam Smith, May 02 '15 at 07:45

score 1 · Answer 2 · answered May 02 '15 at 06:12

1

I'll reiterate that this is a bad practice but you could do this.

for i in xrange(1, 1001):
    globals()['a%s' % i] = 100

for i in xrange(1, 1001):
    name = 'a%d' % i
    print '%s = %d' % (name, globals()[name])

The second loop just prints them.

answered May 02 '15 at 06:12

Paul Rooney

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score 0 · Answer 3 · answered May 02 '15 at 05:57

To do something like this you really ought to use a list. Initializing a length 1000 list with each entry set to 100 can be done by

a = [100]  * 1000

You can set variables like this outside a list using exec, but that is not good practice unless you have some strange compelling reason to do so.

Note that the entries will be indexed from 0 to 999.

score 0 · Answer 4 · answered May 02 '15 at 06:01

You can do this using Python Arrays/Lists. For your case you could create a list of 1000 100s by doing the following:

my_array = [100]*1001

To find a certain value in that list you "index" into it. For example:

print(my_array[300])

The print statement would print the 301st value in the list (as array indexes start counting at 0).

How to use variable in name in python

4 Answers4