遍历python列表索引

Question

I Have a list, and every time I enter "N" in my program I want the list to print the contents of the next index.

categories = ["Produce", "Meat", "Dairy" ,"Misc"]
 ...
elif item == "N":
    for d in categories[:1]:
        d = categories[0]
        d += 1
        print(d)

I understand that the above code is trying to add an integer to a string and throwing the error. What I haven't been able to figure out is how to increment the index.

I have looked at a few other posts concerning similar problems but it the solutions aren't clicking for me out of context.

Sample output of what it should look like

Add Item to list

Produce
>>Tomatoes
>>Grapes
Meat
>>Hamburger
Dairy
>>

Entire program

def add_item():
    exit_list = ["F", "Finished"]
    lists = []

    start = input("Would you like to add an item to the list? Y/N")
    print("Add Item to list")
    categories = ["Produce", "Meat", "dairy","snacks/boxed goods", "Cans", "Misc"]

    print(categories[0])
    while start in ('y', 'Y'):
        item = input(">>")
        if item in exit_list:
            break
        elif item == "N":
            for d in categories[:1]:
                i = 0
                i +=1
                d = categories[i]
                print(d)

        elif item:
            lists.append(item)
    else:
        print("ok")
    print(lists)
    return lists

add_item()

Answer 1

def add_item():
    exit_list = ["F", "Finished"]
    lists = []

    start = input("Would you like to add an item to the list? Y/N")
    print("Add Item to list")
    categories = ["Produce", "Meat", "dairy","snacks/boxed goods", "Cans", "Misc"]

    print(categories[0])
    i=0
    while start in ('y', 'Y'):
        item = input(">>")
        if item in exit_list:
            break
        elif item == "N":
                i +=1
                print(categories[i])

        elif item:
            lists.append(item)
    else:
        print("ok")
    print(lists)
    return lists

add_item()

Answer 2

This code will keep track of an index and increment it each time it's used:

i = 0
categories = ["hey", "1", "2" ,"3"] 
...
elif item == "N":
    print(categories[i])
    i += 1

And note that this code will eventually go out of bounds. If you want to wrap around you can do eg % len(categories) on i .

Answer 3

Most of the time it is possible, if not better, to avoid using index in Python. One way in your case would be to use a generator on your list. The following code will quit when the user enters q , print the next item when the user enters n , and do nothing if the input is something else.

categories = ["hey", "1", "2" ,"3"]                                             
user_input = None                                                               

# Transform your list into an iterator                                                                
categories = iter(categories)                                                   

# While the user doesn't want to quit                                                                                    
while user_input != "q":             
    # The user's input is transformed to lower case                                           
    user_input = input("What do you want to do ? ").lower()                     
    if user_input == "n":                                                       
        try:                                       
            # We print the next value on the iterator i.e. the next 
            # value in your list                             
            print(next(categories))                                             
        except StopIteration:                                                   
            # Raised when the iterator reached the end i.e. when we
            # have printed all the values in the list
            print("You've printed all the list!")                                  
            break         

One possible output is:

What do you want to do ? hello # Nothing happens here 
What do you want to do ? n
hey
What do you want to do ? n
1
What do you want to do ? n
2
What do you want to do ? n
3
What do you want to do ? n
You've printed all the list!

Note that this example is using Python3+