Mapping a dictionary key and its corresponding values from a list?

Question

I've managed to read in a .txt file and read in each line as a different element of a list; pretty easy. Next I stripped all the non-numerical characters from each element (leaving just 0-9 and '.'), also pretty easily done.

However, then I'm left with a list with 90 total elements. Starting from element 0, I want every 5th element to be the key to my dictionary (0, 5, 10, 15, etc). Then I want the 4 value in between to correspond to the values of the previous key (as a list length 4).

For example: my_list[0] is the key with corresponding values my_list[1:5] my_list[5] is the key with corresponding values my_list[6:10] etc, etc

I've managed to take every 5th element out and make it it's own list; just holding the keys. And I've even managed to create another list which holds all the in between values as seperate lists of length 4.

I've tried:

my_dict = dict()

for i in my_keys:
    my_dict[i] = [x for x in every_four]

AND

my_dict = dict()

for i in my_keys:
    for x in every_four:
        my_dict[i] = x

The first one gives me the correct keys but each value is just a the whole every_four list, not just every element. The second one gives me the correct keys but each value is just the first element of the every_four list, it's not iterating through.

Can someone help me get it working this way? And is there any easier way I can do this from the original array? Just set every 5th element to a key and the four elements in between as the corresponding values for that key?

Answer 1

If you want every fifth element as key of your dictionary and list of next consecutive four elements as values, then you can try dictionary comprehension.

my_dict = {my_list[i] : my_list[i+1 : i+5] for i in range(0, len(my_list), 5)}

For sample input -

my_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

output will be -

{0: [1, 2, 3, 4], 5: [6, 7, 8, 9]}

But this code will generate an empty list as value if for a key no next element is present.

Input -

my_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Output -

{0: [1, 2, 3, 4], 10: [], 5: [6, 7, 8, 9]}

As for your code, assuming you get every_four and my_keys correct, there are some problems. In your second attempt, you are looping over every element in the my_keys list and adding that as key of the dictionary. And in the inner loop you are adding x as the value of the dictionary. So, for every iteration in your inner loop, the value for key i is getting overwritten. What you could have done is -

my_dict = dict()

for i in my_keys:
    for x in every_four:
        my_dict.setdefault(i, []).append(x)

For first attempt, I really need to see how you get my_keys and every_four to point out the error.