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Python Python Collections (Retired) Lists Redux Shopping List Take Three

Posted by Laurence Peberdy
Laurence Peberdy
Courses Plus Student 467 Points

Why "spot =+ 1"?

Hi, two questions about the usage of "spot =+ 1":

Firstly, if the purpose of the "spot" variable is to capture a new location within which the user wants to index an item every time it's looped through, why do we want to add a 1 to the variable every time?

Secondly, the way I interpret this code, every time the script loops through, it should reset the value of spot so that it equals the number entered as an index:

"spot = int(index) - 1"

e.g. First loop: spot = "given index location" - 1 to provide "desired index location". spot is referenced to place the item in the list 1 is added to spot

e.g. Second loop: spot = "given index location" - 1 to provide "desired index location" etc..

Why is the value written at the end of the first loop not overwritten when spot is given a new value in the second loop?

Apologies if this is a stupid question, I'm just struggling to get my head around it.

Russell Holz
Russell Holz
4,811 Points
Russell Holz
Russell Holz
4,811 Points

Ok so first off, the syntax is spot += 1, which means that each time through the loop the amount you will be adding to spot is 1. The spot variable has to change otherwise, you would have an index error when you tried to add two things to a single index in the list. I'm not quite sure what you were driving at with the second question, but I'll take a stab the value is not overridden because the "spot" variable is increasing changing the index at which you are adding an item to the list. If you didn't increase spot by 1 each time you looped through then you would be continually overriding the value at position 1 (index 0). Hope that helps.

Laurence Peberdy
Laurence Peberdy
Courses Plus Student 467 Points

Hi, thanks for the answer.

With regards to the second question, apologies if it was unclear. My perspective is that of a beginner, which is probably hard to make sense of if you have more experience.

For some reason I did not realise that the for loop placed all items from new_list in to shopping_list (I thought it only placed items if an index value was given and then other items were placed in to shopping_list in another loop).

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

Given the following code:

spot = int(index) - 1
for item in new_list:
    shopping_list.insert(spot, item.strip())
    spot += 1

spot is set to the the integer value of index minus 1. Because users like to interact in a list indexes starting at 1, 2, 3... and the spot will hold the "minus 1" value to interact with the shopping_list indexes starting at 0, 1, 2....

For the first iteration of the loop spot equals the user's desired insert location. The .insert() method puts the first item from new_list at that index and all following shopping list elements are shifted over. Before the second loop starts spot is incremented by 1 so that the insertion point is move along to follow the item just inserted.

For the second loop, the next item from new_list is assigned to item and the loop begins. The insertion takes place at the location following the previous insertion. spot is incremented again in preparation for the third loop.

If spot was not incremented, all insertions would happen at "index minus 1", which would effectively reverse the order the user listed in the cases where more than one item in new_list.

spot does not get reinitialized to "index - 1" because that statement is previous to and not in the loop.

Laurence Peberdy
Laurence Peberdy
Courses Plus Student 467 Points

Hi, thanks very much, I understand now.

For some reason I did not realise that the for loop placed all items from new_list in to shopping_list (I thought it only placed items if an index value was given and then other items were placed in to shopping_list in another loop).