Variables in a loop, Newby question

Peter Otten __peter__ at web.de
Tue Dec 24 17:29:45 CET 2013


vanommen.robert at gmail.com wrote:

> Hello, for the first time I'm trying te create a little Python program.
> (on a raspberri Pi)
> 
> I don't understand the handling of variables in a loop with Python.
> 
> 
> Lets say i want something like this.
> 
> x = 1
> while x <> 10
> var x = x
> x = x + 1
> 
> The results must be:
> 
> var1 = 1
> var2 = 2
> 
> enz. until var9 = 9
> 
> How do i program this in python?

You are trying to generate a variable name programatically. While this is 
possible in Python

>>> x = 1
>>> while x != 10:
...     exec("var{} = x".format(x))
...     x = x + 1
... 
>>> var1
1
>>> var7
7

this is a really bad idea that you probably picked up from old code in a 
lesser language. Don't do it that way!

In Python you should use a dict or a list, for example:

>>> var = [] # an empty list
>>> x = 0
>>> while x < 10:
...     var.append(x) # append an item to the list
...     x += 1
... 
>>> var
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

This can be simplified to a single line:

>>> var = list(range(10))

A few usage examples:

>>> var[0]
0
>>> var[9]
9
>>> var[10]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

As you can see list indices are zero-based. A list of length 10 has items 0 
to 9.

>>> var[7] += 42
>>> var
[0, 1, 2, 3, 4, 5, 6, 49, 8, 9]
>>> sum(var)
87





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