[Tutor] string indexing

[spir]

On 01/19/2014 02:59 PM, rahmad akbar wrote:> hey guys, super  noob here, i am 
trying to understand the following code
> from google tutorial which i failed to comprehend
>
> #code start
> # E. not_bad
> # Given a string, find the first appearance of the
> # substring 'not' and 'bad'. If the 'bad' follows
> # the 'not', replace the whole 'not'...'bad' substring
> # with 'good'.
> # Return the resulting string.
> # So 'This dinner is not that bad!' yields:
> # This dinner is good!
> def not_bad(s):
>    # +++your code here+++
>    # LAB(begin solution)
>    n = s.find('not')
>    b = s.find('bad')
>    if n != -1 and b != -1 and b > n:
>      s = s[:n] + 'good' + s[b+3:]
>    return s
> #code end
>
>   on the following lines, what is -1, is that index number? and i dont
> understand the entire second line

-1 is what find returns if the searched substring is not at all present in the 
bigger string: a trick meaning "could not find it". (-1 also means last index, 
but is not necessary for find, it would return the positive last index)

> if n != -1 and b != -1 and b > n:

a conditions for which 3 sub-conditions must be met at once

>      s = s[:n] + 'good' + s[b+3:]

Watch this:

     string:    This dinner is not that bad!
     indexes:   0              n        b  -1

s[:n]   = s[0:n]    = "This dinner is "
s[b+3:] = s[b+3:-1] = "!"

+ concatenates (glues together) bits of strings

denis