[Tutor] Function for converting ints from base10 to base2?

Terry Carroll carroll at tjc.com
Thu Feb 22 03:34:12 CET 2007

On Wed, 21 Feb 2007, Dick Moores wrote:

> But there's syntax(?) there I've never seen before. "['','-'][n<0]". 
> I see it works:
>  >>> n = -6
>  >>> ['','-'][n<0]
> '-'
>  >>> n = 98
>  >>> ['','-'][n<0]
> ''
> What's this called? I'd like to look it up.

It's nothing special; it's just a lookup in a list.

First, there's a list of strings; the two strings are an empty string ''
and a minus sign '-'.  That's this part: ['','-']

If n is less than zero, I want to pick up the minus sign; otherwise, I 
want to pick up the empty string.

The expression n<0 evaluates to the boolean True for the first case, and
False otherwise.  When using a boolean as an index in a list, True is
treated as 1;  False is treated as 0.  This is the only arguably tricky
part, if you've never played with booleans;  it's documented at
http://docs.python.org/ref/types.html: "Boolean values [False and True]
behave like the values 0 and 1, respectively, in almost all contexts...".

That's this part: [n<0]


sign = ['','-'][n<0]

is more or less the equivalent of:

signlist = ['','-']
if n < 0:
  sign = signlist[1]
  sign = signlist[0]

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