[Tutor] Python String Help

[Walter Prins]

Hi,


On 28 October 2013 20:55, Alex Tenno <alex.tenno at gmail.com> wrote:

> Hey everyone,
>
> I'm encountering a problem with a python function that I am supposed to
> create.
>

OK.  You should try to actually write the function yourself first, then
give concrete details about what you've tried, and how you're stuck.


> I want my function to look at a string, and then replace each letter in
> the string with its relative position in the alphabet. for example, 'abcde'
> would return '12345', 'zabd' would return '4123', and 'xpft' would return
> '4213'.
>

Is this the exact problem statement or are you paraphrasing?  I'd like to
make sure I understand the question.


> I have been given hints that tell me "You may want to iterate over the
> letters ch in s as in the for loop above, and inside that for loop, count
> the number of letters that are in s and come before the loop variable ch.
> You will also need an accumulator to build the permutation the function
> will return." any help would be greatly appreciated.
>

OK so you've been given some high level idea of what to do. Try to break
this down to more concrete steps, perhaps firstly still only pseudocode,
and eventually translate that to Python.

The idea embodied in the suggestion is based on the observation that you
can determine the relative position in the alphabet, of each character in
the word by, looking the letter in the word, and then counting how many
/distinct/ letters in the word are smaller than or equal to the letter
under scrutiny.  For the smallest letter, this will by definition only
count the letter itself (if you compare with every distinct letter in the
word), resulting in a count of 1.  For the second smallest letter, this
will therefore result in the smallest letter and the 2nd smallest letter
being counted, resulting in a count of 2.  And so on.  These counts then
give the relative position of each letter in the alphabet.  Amit's
suggestion boils down to the same idea -- sort the set of distinct letters
in the string, and then look up the relative position of each letter in
your original string in this sorted set.  Next questions to think about:
How do you calculate the set of distinct letters in the string? (Hint: Look
at set() function.) How do you calculate the count of smaller letters from
this distinct set?  (Hint: Loop over the set and count as you go.) Or
convert this set to a sorted list to then lookup against.  (Hint: Look at
the list() function and the sorted() function.)

Walter
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