[Tutor] obtaining values from a dictionary

[Benoit Dupire]

This is my try...
I did not read all of the previous messages so i don't know very well the
context of your question...
I assume the question is well formulated.

This should do what you want:

for  i in range(len(c)):            # for each element of your list c
    item1, item2 = c[i]               # this element is a tuple --> break it
into 2 parts.
    if d.has_key(item2):               # if the 2nd part  of the item is in the
dictionary d
            c[i][1]= d[item2]            # replace the 2nd part of the  tuple
with the entry in the dict
            if d.has_key(c[i]):                    # now there is a new tuple,
so we have to check whether it is or not, once again, in the dictionary
                    c[i] = d[c[i]]                    # if yes, we replace the
tuple with the entry in the dictionary.



A common pitfall for this algorithm is to do it like this:

for item1, item2 in c:               # This does exactly like my first 2 lines.
It take an element of c, and breaks it into 2 parts.1
        if d.has_key(item2):               # if the 2nd part of the item is in
the d ictionary d
            item2= d[item2]            #     replace the 2nd part of the tuple
------> Trap !!!!  this line does not change the list, but the variable item2

See the difference..?  i have to do something like c[index] to really change the
list.... IOW to refer to it explicitly.

Benoit



Julieta Rangel wrote:

> I have a list that looks like:
> c = [('a', ('a', 'a')), ('a', ('a', 'b')), ('a', ('b', 'a')), ('a', ('b',
> 'b')), ('b', ('a', 'a')), ('b', ('a', 'b')), ('b', ('b', 'a')), ('b', ('b',
> 'b'))]
>
> and I have a dictionary that looks like:
> d ={('b', 'a'): 'a', ('a', 'a'): 'a', ('a', 'b'): 'b', ('b', 'b'): 'b'}
>
> I want to replace the elements in my list according to my dictionary values.
>   In other words, I want to go element by element on my list and ask the
> computer to give me the corresponding value, according to the dictionary.
> For example, if I want to get the value of my second element in my list
> ('a',('a','b')), according to my dictionary, this is equal to ('a','b'),
> which is equal to b.  Can this be done?
>
> Julieta
>
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--
Benoit Dupire
Graduate Student
----------------
I'd like to buy a new Boomerang. How can i get rid of the old one?