[Tutor] Adding to a dict through a for loop confusion.

[Sibylle Koczian]

Hello,

Am 17.05.2016 um 10:28 schrieb Chris Kavanagh:
> Could someone tell me why this different behavior occurs between these 2
> code snippets, please. The 1st example has quotes around it ['item'] only
> adds the last item to the dict (cart). In the 2nd example the item does not
> have quotes around it [item] and every entry is added to the dict.
>
> Why?
>
>
> # Example #1
> cart_items = ['1','2','3','4','5']
>
> cart = {}
>
> for item in cart_items:
>     cart['item'] = item
>
> print cart
>
> #output
> {'item': 5}
>

Here you assign every item from your list to a dictionary entry with the 
same key 'item' - that's a string literal that hasn't got anything to do 
with the name item you give to each list element in turn.

Because a dictionary can only contain one entry for each key the value 
of that entry cart['item'] is overwritten in every step through the 
loop. Only the result of the last step is kept.

> ------------------------------------------------------------------------------------------------------------------------------------------------
> # Example #2
> cart_items = ['1','2','3','4','5']
>
> cart = {}
>
> for item in cart_items:
>     cart[item] = item
>
> print cart
>
> # output
> {'1': '1', '3': '3', '2': '2', '5': '5', '4': '4'}
>

Here you take the value from the list cart_items, called item, for the 
key and for the value of the dictionary entry. So every entry gets a 
different key and your dictionary has as many entries as the list has 
elements.

But did you really want to get a dictionary with the same values for key 
and value?

HTH
Sibylle